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*[English](README.md) ∙ [日本語](README-ja.md) ∙ [简体中文](README-zh-Hans.md) ∙ [繁體中文](README-zh-TW.md) | [العَرَبِيَّة‎](https://github.com/donnemartin/system-design-primer/issues/170) ∙ [বাংলা](https://github.com/donnemartin/system-design-primer/issues/220) ∙ [Português do Brasil](https://github.com/donnemartin/system-design-primer/issues/40) ∙ [Deutsch](https://github.com/donnemartin/system-design-primer/issues/186) ∙ [ελληνικά](https://github.com/donnemartin/system-design-primer/issues/130) ∙ [עברית](https://github.com/donnemartin/system-design-primer/issues/272) ∙ [Italiano](https://github.com/donnemartin/system-design-primer/issues/104) ∙ [한국어](https://github.com/donnemartin/system-design-primer/issues/102) ∙ [فارسی](https://github.com/donnemartin/system-design-primer/issues/110) ∙ [Polski](https://github.com/donnemartin/system-design-primer/issues/68) ∙ [русский язык](https://github.com/donnemartin/system-design-primer/issues/87) ∙ [Español](https://github.com/donnemartin/system-design-primer/issues/136) ∙ [ภาษาไทย](https://github.com/donnemartin/system-design-primer/issues/187) ∙ [Türkçe](https://github.com/donnemartin/system-design-primer/issues/39) ∙ [tiếng Việt](https://github.com/donnemartin/system-design-primer/issues/127) ∙ [Français](https://github.com/donnemartin/system-design-primer/issues/250) | [Add Translation](https://github.com/donnemartin/system-design-primer/issues/28)*
_[English](README.md) ∙ [日本語](README-ja.md) ∙ [简体中文](README-zh-Hans.md) ∙ [繁體中文](README-zh-TW.md) | [العَرَبِيَّة‎](https://github.com/donnemartin/system-design-primer/issues/170) ∙ [বাংলা](https://github.com/donnemartin/system-design-primer/issues/220) ∙ [Português do Brasil](https://github.com/donnemartin/system-design-primer/issues/40) ∙ [Deutsch](https://github.com/donnemartin/system-design-primer/issues/186) ∙ [ελληνικά](https://github.com/donnemartin/system-design-primer/issues/130) ∙ [עברית](https://github.com/donnemartin/system-design-primer/issues/272) ∙ [Italiano](https://github.com/donnemartin/system-design-primer/issues/104) ∙ [한국어](https://github.com/donnemartin/system-design-primer/issues/102) ∙ [فارسی](https://github.com/donnemartin/system-design-primer/issues/110) ∙ [Polski](https://github.com/donnemartin/system-design-primer/issues/68) ∙ [русский язык](https://github.com/donnemartin/system-design-primer/issues/87) ∙ [Español](https://github.com/donnemartin/system-design-primer/issues/136) ∙ [ภาษาไทย](https://github.com/donnemartin/system-design-primer/issues/187) ∙ [Türkçe](https://github.com/donnemartin/system-design-primer/issues/39) ∙ [tiếng Việt](https://github.com/donnemartin/system-design-primer/issues/127) ∙ [Français](https://github.com/donnemartin/system-design-primer/issues/250) | [Add Translation](https://github.com/donnemartin/system-design-primer/issues/28)_
**Help [translate](TRANSLATIONS.md) this guide!**
@ -29,7 +29,7 @@ This is a continually updated, open source project.
[Contributions](#contributing) are welcome but need at least amount 5$!
### Prep for the system design interview
### Prep for the system design interview talk with boddu need guidance.
In addition to coding interviews, system design is a **required component** of the **technical interview process** at many tech companies.
@ -37,11 +37,11 @@ In addition to coding interviews, system design is a **required component** of t
Additional topics for interview prep:
* [Study guide](#study-guide)
* [How to approach a system design interview question](#how-to-approach-a-system-design-interview-question)
* [System design interview questions, **with solutions**](#system-design-interview-questions-with-solutions)
* [Object-oriented design interview questions, **with solutions**](#object-oriented-design-interview-questions-with-solutions)
* [Additional system design interview questions](#additional-system-design-interview-questions)
- [Study guide](#study-guide)
- [How to approach a system design interview question](#how-to-approach-a-system-design-interview-question)
- [System design interview questions, **with solutions**](#system-design-interview-questions-with-solutions)
- [Object-oriented design interview questions, **with solutions**](#object-oriented-design-interview-questions-with-solutions)
- [Additional system design interview questions](#additional-system-design-interview-questions)
## Anki flashcards
@ -52,9 +52,9 @@ Additional topics for interview prep:
The provided [Anki flashcard decks](https://apps.ankiweb.net/) use spaced repetition to help you retain key system design concepts.
* [System design deck](https://github.com/donnemartin/system-design-primer/tree/master/resources/flash_cards/System%20Design.apkg)
* [System design exercises deck](https://github.com/donnemartin/system-design-primer/tree/master/resources/flash_cards/System%20Design%20Exercises.apkg)
* [Object oriented design exercises deck](https://github.com/donnemartin/system-design-primer/tree/master/resources/flash_cards/OO%20Design.apkg)
- [System design deck](https://github.com/donnemartin/system-design-primer/tree/master/resources/flash_cards/System%20Design.apkg)
- [System design exercises deck](https://github.com/donnemartin/system-design-primer/tree/master/resources/flash_cards/System%20Design%20Exercises.apkg)
- [Object oriented design exercises deck](https://github.com/donnemartin/system-design-primer/tree/master/resources/flash_cards/OO%20Design.apkg)
Great for use while on-the-go.
@ -69,7 +69,7 @@ Looking for resources to help you prep for the [**Coding Interview**](https://gi
Check out the sister repo [**Interactive Coding Challenges**](https://github.com/donnemartin/interactive-coding-challenges), which contains an additional Anki deck:
* [Coding deck](https://github.com/donnemartin/interactive-coding-challenges/tree/master/anki_cards/Coding.apkg)
- [Coding deck](https://github.com/donnemartin/interactive-coding-challenges/tree/master/anki_cards/Coding.apkg)
## Contributing
@ -77,10 +77,10 @@ Check out the sister repo [**Interactive Coding Challenges**](https://github.com
Feel free to submit pull requests to help:
* Fix errors
* Improve sections
* Add new sections
* [Translate](https://github.com/donnemartin/system-design-primer/issues/28)
- Fix errors
- Improve sections
- Add new sections
- [Translate](https://github.com/donnemartin/system-design-primer/issues/28)
Content that needs some polishing is placed [under development](#under-development).
@ -97,87 +97,87 @@ Review the [Contributing Guidelines](CONTRIBUTING.md).
<br/>
</p>
* [System design topics: start here](#system-design-topics-start-here)
* [Step 1: Review the scalability video lecture](#step-1-review-the-scalability-video-lecture)
* [Step 2: Review the scalability article](#step-2-review-the-scalability-article)
* [Next steps](#next-steps)
* [Performance vs scalability](#performance-vs-scalability)
* [Latency vs throughput](#latency-vs-throughput)
* [Availability vs consistency](#availability-vs-consistency)
* [CAP theorem](#cap-theorem)
* [CP - consistency and partition tolerance](#cp---consistency-and-partition-tolerance)
* [AP - availability and partition tolerance](#ap---availability-and-partition-tolerance)
* [Consistency patterns](#consistency-patterns)
* [Weak consistency](#weak-consistency)
* [Eventual consistency](#eventual-consistency)
* [Strong consistency](#strong-consistency)
* [Availability patterns](#availability-patterns)
* [Fail-over](#fail-over)
* [Replication](#replication)
* [Availability in numbers](#availability-in-numbers)
* [Domain name system](#domain-name-system)
* [Content delivery network](#content-delivery-network)
* [Push CDNs](#push-cdns)
* [Pull CDNs](#pull-cdns)
* [Load balancer](#load-balancer)
* [Active-passive](#active-passive)
* [Active-active](#active-active)
* [Layer 4 load balancing](#layer-4-load-balancing)
* [Layer 7 load balancing](#layer-7-load-balancing)
* [Horizontal scaling](#horizontal-scaling)
* [Reverse proxy (web server)](#reverse-proxy-web-server)
* [Load balancer vs reverse proxy](#load-balancer-vs-reverse-proxy)
* [Application layer](#application-layer)
* [Microservices](#microservices)
* [Service discovery](#service-discovery)
* [Database](#database)
* [Relational database management system (RDBMS)](#relational-database-management-system-rdbms)
* [Master-slave replication](#master-slave-replication)
* [Master-master replication](#master-master-replication)
* [Federation](#federation)
* [Sharding](#sharding)
* [Denormalization](#denormalization)
* [SQL tuning](#sql-tuning)
* [NoSQL](#nosql)
* [Key-value store](#key-value-store)
* [Document store](#document-store)
* [Wide column store](#wide-column-store)
* [Graph Database](#graph-database)
* [SQL or NoSQL](#sql-or-nosql)
* [Cache](#cache)
* [Client caching](#client-caching)
* [CDN caching](#cdn-caching)
* [Web server caching](#web-server-caching)
* [Database caching](#database-caching)
* [Application caching](#application-caching)
* [Caching at the database query level](#caching-at-the-database-query-level)
* [Caching at the object level](#caching-at-the-object-level)
* [When to update the cache](#when-to-update-the-cache)
* [Cache-aside](#cache-aside)
* [Write-through](#write-through)
* [Write-behind (write-back)](#write-behind-write-back)
* [Refresh-ahead](#refresh-ahead)
* [Asynchronism](#asynchronism)
* [Message queues](#message-queues)
* [Task queues](#task-queues)
* [Back pressure](#back-pressure)
* [Communication](#communication)
* [Transmission control protocol (TCP)](#transmission-control-protocol-tcp)
* [User datagram protocol (UDP)](#user-datagram-protocol-udp)
* [Remote procedure call (RPC)](#remote-procedure-call-rpc)
* [Representational state transfer (REST)](#representational-state-transfer-rest)
* [Security](#security)
* [Appendix](#appendix)
* [Powers of two table](#powers-of-two-table)
* [Latency numbers every programmer should know](#latency-numbers-every-programmer-should-know)
* [Additional system design interview questions](#additional-system-design-interview-questions)
* [Real world architectures](#real-world-architectures)
* [Company architectures](#company-architectures)
* [Company engineering blogs](#company-engineering-blogs)
* [Under development](#under-development)
* [Credits](#credits)
* [Contact info](#contact-info)
* [License](#license)
- [System design topics: start here](#system-design-topics-start-here)
- [Step 1: Review the scalability video lecture](#step-1-review-the-scalability-video-lecture)
- [Step 2: Review the scalability article](#step-2-review-the-scalability-article)
- [Next steps](#next-steps)
- [Performance vs scalability](#performance-vs-scalability)
- [Latency vs throughput](#latency-vs-throughput)
- [Availability vs consistency](#availability-vs-consistency)
- [CAP theorem](#cap-theorem)
- [CP - consistency and partition tolerance](#cp---consistency-and-partition-tolerance)
- [AP - availability and partition tolerance](#ap---availability-and-partition-tolerance)
- [Consistency patterns](#consistency-patterns)
- [Weak consistency](#weak-consistency)
- [Eventual consistency](#eventual-consistency)
- [Strong consistency](#strong-consistency)
- [Availability patterns](#availability-patterns)
- [Fail-over](#fail-over)
- [Replication](#replication)
- [Availability in numbers](#availability-in-numbers)
- [Domain name system](#domain-name-system)
- [Content delivery network](#content-delivery-network)
- [Push CDNs](#push-cdns)
- [Pull CDNs](#pull-cdns)
- [Load balancer](#load-balancer)
- [Active-passive](#active-passive)
- [Active-active](#active-active)
- [Layer 4 load balancing](#layer-4-load-balancing)
- [Layer 7 load balancing](#layer-7-load-balancing)
- [Horizontal scaling](#horizontal-scaling)
- [Reverse proxy (web server)](#reverse-proxy-web-server)
- [Load balancer vs reverse proxy](#load-balancer-vs-reverse-proxy)
- [Application layer](#application-layer)
- [Microservices](#microservices)
- [Service discovery](#service-discovery)
- [Database](#database)
- [Relational database management system (RDBMS)](#relational-database-management-system-rdbms)
- [Master-slave replication](#master-slave-replication)
- [Master-master replication](#master-master-replication)
- [Federation](#federation)
- [Sharding](#sharding)
- [Denormalization](#denormalization)
- [SQL tuning](#sql-tuning)
- [NoSQL](#nosql)
- [Key-value store](#key-value-store)
- [Document store](#document-store)
- [Wide column store](#wide-column-store)
- [Graph Database](#graph-database)
- [SQL or NoSQL](#sql-or-nosql)
- [Cache](#cache)
- [Client caching](#client-caching)
- [CDN caching](#cdn-caching)
- [Web server caching](#web-server-caching)
- [Database caching](#database-caching)
- [Application caching](#application-caching)
- [Caching at the database query level](#caching-at-the-database-query-level)
- [Caching at the object level](#caching-at-the-object-level)
- [When to update the cache](#when-to-update-the-cache)
- [Cache-aside](#cache-aside)
- [Write-through](#write-through)
- [Write-behind (write-back)](#write-behind-write-back)
- [Refresh-ahead](#refresh-ahead)
- [Asynchronism](#asynchronism)
- [Message queues](#message-queues)
- [Task queues](#task-queues)
- [Back pressure](#back-pressure)
- [Communication](#communication)
- [Transmission control protocol (TCP)](#transmission-control-protocol-tcp)
- [User datagram protocol (UDP)](#user-datagram-protocol-udp)
- [Remote procedure call (RPC)](#remote-procedure-call-rpc)
- [Representational state transfer (REST)](#representational-state-transfer-rest)
- [Security](#security)
- [Appendix](#appendix)
- [Powers of two table](#powers-of-two-table)
- [Latency numbers every programmer should know](#latency-numbers-every-programmer-should-know)
- [Additional system design interview questions](#additional-system-design-interview-questions)
- [Real world architectures](#real-world-architectures)
- [Company architectures](#company-architectures)
- [Company engineering blogs](#company-engineering-blogs)
- [Under development](#under-development)
- [Credits](#credits)
- [Contact info](#contact-info)
- [License](#license)
## Study guide
@ -191,22 +191,22 @@ Review the [Contributing Guidelines](CONTRIBUTING.md).
What you are asked in an interview depends on variables such as:
* How much experience you have
* What your technical background is
* What positions you are interviewing for
* Which companies you are interviewing with
* Luck
- How much experience you have
- What your technical background is
- What positions you are interviewing for
- Which companies you are interviewing with
- Luck
More experienced candidates are generally expected to know more about system design. Architects or team leads might be expected to know more than individual contributors. Top tech companies are likely to have one or more design interview rounds.
Start broad and go deeper in a few areas. It helps to know a little about various key system design topics. Adjust the following guide based on your timeline, experience, what positions you are interviewing for, and which companies you are interviewing with.
* **Short timeline** - Aim for **breadth** with system design topics. Practice by solving **some** interview questions.
* **Medium timeline** - Aim for **breadth** and **some depth** with system design topics. Practice by solving **many** interview questions.
* **Long timeline** - Aim for **breadth** and **more depth** with system design topics. Practice by solving **most** interview questions.
- **Short timeline** - Aim for **breadth** with system design topics. Practice by solving **some** interview questions.
- **Medium timeline** - Aim for **breadth** and **some depth** with system design topics. Practice by solving **many** interview questions.
- **Long timeline** - Aim for **breadth** and **more depth** with system design topics. Practice by solving **most** interview questions.
| | Short | Medium | Long |
|---|---|---|---|
| -------------------------------------------------------------------------------------------------------------------------------------- | ----- | ------ | ---- |
| Read through the [System design topics](#index-of-system-design-topics) to get a broad understanding of how systems work | :+1: | :+1: | :+1: |
| Read through a few articles in the [Company engineering blogs](#company-engineering-blogs) for the companies you are interviewing with | :+1: | :+1: | :+1: |
| Read through a few [Real world architectures](#real-world-architectures) | :+1: | :+1: | :+1: |
@ -227,43 +227,43 @@ You can use the following steps to guide the discussion. To help solidify this
Gather requirements and scope the problem. Ask questions to clarify use cases and constraints. Discuss assumptions.
* Who is going to use it?
* How are they going to use it?
* How many users are there?
* What does the system do?
* What are the inputs and outputs of the system?
* How much data do we expect to handle?
* How many requests per second do we expect?
* What is the expected read to write ratio?
- Who is going to use it?
- How are they going to use it?
- How many users are there?
- What does the system do?
- What are the inputs and outputs of the system?
- How much data do we expect to handle?
- How many requests per second do we expect?
- What is the expected read to write ratio?
### Step 2: Create a high level design
Outline a high level design with all important components.
* Sketch the main components and connections
* Justify your ideas
- Sketch the main components and connections
- Justify your ideas
### Step 3: Design core components
Dive into details for each core component. For example, if you were asked to [design a url shortening service](solutions/system_design/pastebin/README.md), discuss:
* Generating and storing a hash of the full url
* [MD5](solutions/system_design/pastebin/README.md) and [Base62](solutions/system_design/pastebin/README.md)
* Hash collisions
* SQL or NoSQL
* Database schema
* Translating a hashed url to the full url
* Database lookup
* API and object-oriented design
- Generating and storing a hash of the full url
- [MD5](solutions/system_design/pastebin/README.md) and [Base62](solutions/system_design/pastebin/README.md)
- Hash collisions
- SQL or NoSQL
- Database schema
- Translating a hashed url to the full url
- Database lookup
- API and object-oriented design
### Step 4: Scale the design
Identify and address bottlenecks, given the constraints. For example, do you need the following to address scalability issues?
* Load balancer
* Horizontal scaling
* Caching
* Database sharding
- Load balancer
- Horizontal scaling
- Caching
- Database sharding
Discuss potential solutions and trade-offs. Everything is a trade-off. Address bottlenecks using [principles of scalable system design](#index-of-system-design-topics).
@ -271,18 +271,18 @@ Discuss potential solutions and trade-offs. Everything is a trade-off. Address
You might be asked to do some estimates by hand. Refer to the [Appendix](#appendix) for the following resources:
* [Use back of the envelope calculations](http://highscalability.com/blog/2011/1/26/google-pro-tip-use-back-of-the-envelope-calculations-to-choo.html)
* [Powers of two table](#powers-of-two-table)
* [Latency numbers every programmer should know](#latency-numbers-every-programmer-should-know)
- [Use back of the envelope calculations](http://highscalability.com/blog/2011/1/26/google-pro-tip-use-back-of-the-envelope-calculations-to-choo.html)
- [Powers of two table](#powers-of-two-table)
- [Latency numbers every programmer should know](#latency-numbers-every-programmer-should-know)
### Source(s) and further reading
Check out the following links to get a better idea of what to expect:
* [How to ace a systems design interview](https://www.palantir.com/2011/10/how-to-rock-a-systems-design-interview/)
* [The system design interview](http://www.hiredintech.com/system-design)
* [Intro to Architecture and Systems Design Interviews](https://www.youtube.com/watch?v=ZgdS0EUmn70)
* [System design template](https://leetcode.com/discuss/career/229177/My-System-Design-Template)
- [How to ace a systems design interview](https://www.palantir.com/2011/10/how-to-rock-a-systems-design-interview/)
- [The system design interview](http://www.hiredintech.com/system-design)
- [Intro to Architecture and Systems Design Interviews](https://www.youtube.com/watch?v=ZgdS0EUmn70)
- [System design template](https://leetcode.com/discuss/career/229177/My-System-Design-Template)
## System design interview questions with solutions
@ -291,7 +291,7 @@ Check out the following links to get a better idea of what to expect:
> Solutions linked to content in the `solutions/` folder.
| Question | |
|---|---|
| -------------------------------------------------------------------- | ---------------------------------------------------------- |
| Design Pastebin.com (or Bit.ly) | [Solution](solutions/system_design/pastebin/README.md) |
| Design the Twitter timeline and search (or Facebook feed and search) | [Solution](solutions/system_design/twitter/README.md) |
| Design a web crawler | [Solution](solutions/system_design/web_crawler/README.md) |
@ -359,7 +359,7 @@ Check out the following links to get a better idea of what to expect:
> **Note: This section is under development**
| Question | |
|---|---|
| -------------------------------------- | ------------------------------------------------------------------------------ |
| Design a hash map | [Solution](solutions/object_oriented_design/hash_table/hash_map.ipynb) |
| Design a least recently used cache | [Solution](solutions/object_oriented_design/lru_cache/lru_cache.ipynb) |
| Design a call center | [Solution](solutions/object_oriented_design/call_center/call_center.ipynb) |
@ -379,31 +379,31 @@ First, you'll need a basic understanding of common principles, learning about wh
[Scalability Lecture at Harvard](https://www.youtube.com/watch?v=-W9F__D3oY4)
* Topics covered:
* Vertical scaling
* Horizontal scaling
* Caching
* Load balancing
* Database replication
* Database partitioning
- Topics covered:
- Vertical scaling
- Horizontal scaling
- Caching
- Load balancing
- Database replication
- Database partitioning
### Step 2: Review the scalability article
[Scalability](http://www.lecloud.net/tagged/scalability/chrono)
* Topics covered:
* [Clones](http://www.lecloud.net/post/7295452622/scalability-for-dummies-part-1-clones)
* [Databases](http://www.lecloud.net/post/7994751381/scalability-for-dummies-part-2-database)
* [Caches](http://www.lecloud.net/post/9246290032/scalability-for-dummies-part-3-cache)
* [Asynchronism](http://www.lecloud.net/post/9699762917/scalability-for-dummies-part-4-asynchronism)
- Topics covered:
- [Clones](http://www.lecloud.net/post/7295452622/scalability-for-dummies-part-1-clones)
- [Databases](http://www.lecloud.net/post/7994751381/scalability-for-dummies-part-2-database)
- [Caches](http://www.lecloud.net/post/9246290032/scalability-for-dummies-part-3-cache)
- [Asynchronism](http://www.lecloud.net/post/9699762917/scalability-for-dummies-part-4-asynchronism)
### Next steps
Next, we'll look at high-level trade-offs:
* **Performance** vs **scalability**
* **Latency** vs **throughput**
* **Availability** vs **consistency**
- **Performance** vs **scalability**
- **Latency** vs **throughput**
- **Availability** vs **consistency**
Keep in mind that **everything is a trade-off**.
@ -415,13 +415,13 @@ A service is **scalable** if it results in increased **performance** in a manner
Another way to look at performance vs scalability:
* If you have a **performance** problem, your system is slow for a single user.
* If you have a **scalability** problem, your system is fast for a single user but slow under heavy load.
- If you have a **performance** problem, your system is slow for a single user.
- If you have a **scalability** problem, your system is fast for a single user but slow under heavy load.
### Source(s) and further reading
* [A word on scalability](http://www.allthingsdistributed.com/2006/03/a_word_on_scalability.html)
* [Scalability, availability, stability, patterns](http://www.slideshare.net/jboner/scalability-availability-stability-patterns/)
- [A word on scalability](http://www.allthingsdistributed.com/2006/03/a_word_on_scalability.html)
- [Scalability, availability, stability, patterns](http://www.slideshare.net/jboner/scalability-availability-stability-patterns/)
## Latency vs throughput
@ -433,7 +433,7 @@ Generally, you should aim for **maximal throughput** with **acceptable latency**
### Source(s) and further reading
* [Understanding latency vs throughput](https://community.cadence.com/cadence_blogs_8/b/sd/archive/2010/09/13/understanding-latency-vs-throughput)
- [Understanding latency vs throughput](https://community.cadence.com/cadence_blogs_8/b/sd/archive/2010/09/13/understanding-latency-vs-throughput)
## Availability vs consistency
@ -447,11 +447,11 @@ Generally, you should aim for **maximal throughput** with **acceptable latency**
In a distributed computer system, you can only support two of the following guarantees:
* **Consistency** - Every read receives the most recent write or an error
* **Availability** - Every request receives a response, without guarantee that it contains the most recent version of the information
* **Partition Tolerance** - The system continues to operate despite arbitrary partitioning due to network failures
- **Consistency** - Every read receives the most recent write or an error
- **Availability** - Every request receives a response, without guarantee that it contains the most recent version of the information
- **Partition Tolerance** - The system continues to operate despite arbitrary partitioning due to network failures
*Networks aren't reliable, so you'll need to support partition tolerance. You'll need to make a software tradeoff between consistency and availability.*
_Networks aren't reliable, so you'll need to support partition tolerance. You'll need to make a software tradeoff between consistency and availability._
#### CP - consistency and partition tolerance
@ -465,9 +465,9 @@ AP is a good choice if the business needs allow for [eventual consistency](#even
### Source(s) and further reading
* [CAP theorem revisited](http://robertgreiner.com/2014/08/cap-theorem-revisited/)
* [A plain english introduction to CAP theorem](http://ksat.me/a-plain-english-introduction-to-cap-theorem)
* [CAP FAQ](https://github.com/henryr/cap-faq)
- [CAP theorem revisited](http://robertgreiner.com/2014/08/cap-theorem-revisited/)
- [A plain english introduction to CAP theorem](http://ksat.me/a-plain-english-introduction-to-cap-theorem)
- [CAP FAQ](https://github.com/henryr/cap-faq)
## Consistency patterns
@ -493,7 +493,7 @@ This approach is seen in file systems and RDBMSes. Strong consistency works wel
### Source(s) and further reading
* [Transactions across data centers](http://snarfed.org/transactions_across_datacenters_io.html)
- [Transactions across data centers](http://snarfed.org/transactions_across_datacenters_io.html)
## Availability patterns
@ -519,8 +519,8 @@ Active-active failover can also be referred to as master-master failover.
### Disadvantage(s): failover
* Fail-over adds more hardware and additional complexity.
* There is a potential for loss of data if the active system fails before any newly written data can be replicated to the passive.
- Fail-over adds more hardware and additional complexity.
- There is a potential for loss of data if the active system fails before any newly written data can be replicated to the passive.
### Replication
@ -528,8 +528,8 @@ Active-active failover can also be referred to as master-master failover.
This topic is further discussed in the [Database](#database) section:
* [Master-slave replication](#master-slave-replication)
* [Master-master replication](#master-master-replication)
- [Master-slave replication](#master-slave-replication)
- [Master-master replication](#master-master-replication)
### Availability in numbers
@ -538,7 +538,7 @@ Availability is often quantified by uptime (or downtime) as a percentage of time
#### 99.9% availability - three 9s
| Duration | Acceptable downtime |
|---------------------|--------------------|
| ------------------ | ------------------- |
| Downtime per year | 8h 45min 57s |
| Downtime per month | 43m 49.7s |
| Downtime per week | 10m 4.8s |
@ -547,7 +547,7 @@ Availability is often quantified by uptime (or downtime) as a percentage of time
#### 99.99% availability - four 9s
| Duration | Acceptable downtime |
|---------------------|--------------------|
| ------------------ | ------------------- |
| Downtime per year | 52min 35.7s |
| Downtime per month | 4m 23s |
| Downtime per week | 1m 5s |
@ -589,31 +589,31 @@ A Domain Name System (DNS) translates a domain name such as www.example.com to a
DNS is hierarchical, with a few authoritative servers at the top level. Your router or ISP provides information about which DNS server(s) to contact when doing a lookup. Lower level DNS servers cache mappings, which could become stale due to DNS propagation delays. DNS results can also be cached by your browser or OS for a certain period of time, determined by the [time to live (TTL)](https://en.wikipedia.org/wiki/Time_to_live).
* **NS record (name server)** - Specifies the DNS servers for your domain/subdomain.
* **MX record (mail exchange)** - Specifies the mail servers for accepting messages.
* **A record (address)** - Points a name to an IP address.
* **CNAME (canonical)** - Points a name to another name or `CNAME` (example.com to www.example.com) or to an `A` record.
- **NS record (name server)** - Specifies the DNS servers for your domain/subdomain.
- **MX record (mail exchange)** - Specifies the mail servers for accepting messages.
- **A record (address)** - Points a name to an IP address.
- **CNAME (canonical)** - Points a name to another name or `CNAME` (example.com to www.example.com) or to an `A` record.
Services such as [CloudFlare](https://www.cloudflare.com/dns/) and [Route 53](https://aws.amazon.com/route53/) provide managed DNS services. Some DNS services can route traffic through various methods:
* [Weighted round robin](https://www.g33kinfo.com/info/round-robin-vs-weighted-round-robin-lb)
* Prevent traffic from going to servers under maintenance
* Balance between varying cluster sizes
* A/B testing
* Latency-based
* Geolocation-based
- [Weighted round robin](https://www.g33kinfo.com/info/round-robin-vs-weighted-round-robin-lb)
- Prevent traffic from going to servers under maintenance
- Balance between varying cluster sizes
- A/B testing
- Latency-based
- Geolocation-based
### Disadvantage(s): DNS
* Accessing a DNS server introduces a slight delay, although mitigated by caching described above.
* DNS server management could be complex and is generally managed by [governments, ISPs, and large companies](http://superuser.com/questions/472695/who-controls-the-dns-servers/472729).
* DNS services have recently come under [DDoS attack](http://dyn.com/blog/dyn-analysis-summary-of-friday-october-21-attack/), preventing users from accessing websites such as Twitter without knowing Twitter's IP address(es).
- Accessing a DNS server introduces a slight delay, although mitigated by caching described above.
- DNS server management could be complex and is generally managed by [governments, ISPs, and large companies](http://superuser.com/questions/472695/who-controls-the-dns-servers/472729).
- DNS services have recently come under [DDoS attack](http://dyn.com/blog/dyn-analysis-summary-of-friday-october-21-attack/), preventing users from accessing websites such as Twitter without knowing Twitter's IP address(es).
### Source(s) and further reading
* [DNS architecture](https://technet.microsoft.com/en-us/library/dd197427(v=ws.10).aspx)
* [Wikipedia](https://en.wikipedia.org/wiki/Domain_Name_System)
* [DNS articles](https://support.dnsimple.com/categories/dns/)
- [DNS architecture](<https://technet.microsoft.com/en-us/library/dd197427(v=ws.10).aspx>)
- [Wikipedia](https://en.wikipedia.org/wiki/Domain_Name_System)
- [DNS articles](https://support.dnsimple.com/categories/dns/)
## Content delivery network
@ -627,8 +627,8 @@ A content delivery network (CDN) is a globally distributed network of proxy serv
Serving content from CDNs can significantly improve performance in two ways:
* Users receive content at data centers close to them
* Your servers do not have to serve requests that the CDN fulfills
- Users receive content at data centers close to them
- Your servers do not have to serve requests that the CDN fulfills
### Push CDNs
@ -646,15 +646,15 @@ Sites with heavy traffic work well with pull CDNs, as traffic is spread out more
### Disadvantage(s): CDN
* CDN costs could be significant depending on traffic, although this should be weighed with additional costs you would incur not using a CDN.
* Content might be stale if it is updated before the TTL expires it.
* CDNs require changing URLs for static content to point to the CDN.
- CDN costs could be significant depending on traffic, although this should be weighed with additional costs you would incur not using a CDN.
- Content might be stale if it is updated before the TTL expires it.
- CDNs require changing URLs for static content to point to the CDN.
### Source(s) and further reading
* [Globally distributed content delivery](https://figshare.com/articles/Globally_distributed_content_delivery/6605972)
* [The differences between push and pull CDNs](http://www.travelblogadvice.com/technical/the-differences-between-push-and-pull-cdns/)
* [Wikipedia](https://en.wikipedia.org/wiki/Content_delivery_network)
- [Globally distributed content delivery](https://figshare.com/articles/Globally_distributed_content_delivery/6605972)
- [The differences between push and pull CDNs](http://www.travelblogadvice.com/technical/the-differences-between-push-and-pull-cdns/)
- [Wikipedia](https://en.wikipedia.org/wiki/Content_delivery_network)
## Load balancer
@ -666,28 +666,28 @@ Sites with heavy traffic work well with pull CDNs, as traffic is spread out more
Load balancers distribute incoming client requests to computing resources such as application servers and databases. In each case, the load balancer returns the response from the computing resource to the appropriate client. Load balancers are effective at:
* Preventing requests from going to unhealthy servers
* Preventing overloading resources
* Helping to eliminate a single point of failure
- Preventing requests from going to unhealthy servers
- Preventing overloading resources
- Helping to eliminate a single point of failure
Load balancers can be implemented with hardware (expensive) or with software such as HAProxy.
Additional benefits include:
* **SSL termination** - Decrypt incoming requests and encrypt server responses so backend servers do not have to perform these potentially expensive operations
* Removes the need to install [X.509 certificates](https://en.wikipedia.org/wiki/X.509) on each server
* **Session persistence** - Issue cookies and route a specific client's requests to same instance if the web apps do not keep track of sessions
- **SSL termination** - Decrypt incoming requests and encrypt server responses so backend servers do not have to perform these potentially expensive operations
- Removes the need to install [X.509 certificates](https://en.wikipedia.org/wiki/X.509) on each server
- **Session persistence** - Issue cookies and route a specific client's requests to same instance if the web apps do not keep track of sessions
To protect against failures, it's common to set up multiple load balancers, either in [active-passive](#active-passive) or [active-active](#active-active) mode.
Load balancers can route traffic based on various metrics, including:
* Random
* Least loaded
* Session/cookies
* [Round robin or weighted round robin](https://www.g33kinfo.com/info/round-robin-vs-weighted-round-robin-lb)
* [Layer 4](#layer-4-load-balancing)
* [Layer 7](#layer-7-load-balancing)
- Random
- Least loaded
- Session/cookies
- [Round robin or weighted round robin](https://www.g33kinfo.com/info/round-robin-vs-weighted-round-robin-lb)
- [Layer 4](#layer-4-load-balancing)
- [Layer 7](#layer-7-load-balancing)
### Layer 4 load balancing
@ -705,26 +705,26 @@ Load balancers can also help with horizontal scaling, improving performance and
#### Disadvantage(s): horizontal scaling
* Scaling horizontally introduces complexity and involves cloning servers
* Servers should be stateless: they should not contain any user-related data like sessions or profile pictures
* Sessions can be stored in a centralized data store such as a [database](#database) (SQL, NoSQL) or a persistent [cache](#cache) (Redis, Memcached)
* Downstream servers such as caches and databases need to handle more simultaneous connections as upstream servers scale out
- Scaling horizontally introduces complexity and involves cloning servers
- Servers should be stateless: they should not contain any user-related data like sessions or profile pictures
- Sessions can be stored in a centralized data store such as a [database](#database) (SQL, NoSQL) or a persistent [cache](#cache) (Redis, Memcached)
- Downstream servers such as caches and databases need to handle more simultaneous connections as upstream servers scale out
### Disadvantage(s): load balancer
* The load balancer can become a performance bottleneck if it does not have enough resources or if it is not configured properly.
* Introducing a load balancer to help eliminate a single point of failure results in increased complexity.
* A single load balancer is a single point of failure, configuring multiple load balancers further increases complexity.
- The load balancer can become a performance bottleneck if it does not have enough resources or if it is not configured properly.
- Introducing a load balancer to help eliminate a single point of failure results in increased complexity.
- A single load balancer is a single point of failure, configuring multiple load balancers further increases complexity.
### Source(s) and further reading
* [NGINX architecture](https://www.nginx.com/blog/inside-nginx-how-we-designed-for-performance-scale/)
* [HAProxy architecture guide](http://www.haproxy.org/download/1.2/doc/architecture.txt)
* [Scalability](http://www.lecloud.net/post/7295452622/scalability-for-dummies-part-1-clones)
* [Wikipedia](https://en.wikipedia.org/wiki/Load_balancing_(computing))
* [Layer 4 load balancing](https://www.nginx.com/resources/glossary/layer-4-load-balancing/)
* [Layer 7 load balancing](https://www.nginx.com/resources/glossary/layer-7-load-balancing/)
* [ELB listener config](http://docs.aws.amazon.com/elasticloadbalancing/latest/classic/elb-listener-config.html)
- [NGINX architecture](https://www.nginx.com/blog/inside-nginx-how-we-designed-for-performance-scale/)
- [HAProxy architecture guide](http://www.haproxy.org/download/1.2/doc/architecture.txt)
- [Scalability](http://www.lecloud.net/post/7295452622/scalability-for-dummies-part-1-clones)
- [Wikipedia](<https://en.wikipedia.org/wiki/Load_balancing_(computing)>)
- [Layer 4 load balancing](https://www.nginx.com/resources/glossary/layer-4-load-balancing/)
- [Layer 7 load balancing](https://www.nginx.com/resources/glossary/layer-7-load-balancing/)
- [ELB listener config](http://docs.aws.amazon.com/elasticloadbalancing/latest/classic/elb-listener-config.html)
## Reverse proxy (web server)
@ -739,35 +739,35 @@ A reverse proxy is a web server that centralizes internal services and provides
Additional benefits include:
* **Increased security** - Hide information about backend servers, blacklist IPs, limit number of connections per client
* **Increased scalability and flexibility** - Clients only see the reverse proxy's IP, allowing you to scale servers or change their configuration
* **SSL termination** - Decrypt incoming requests and encrypt server responses so backend servers do not have to perform these potentially expensive operations
* Removes the need to install [X.509 certificates](https://en.wikipedia.org/wiki/X.509) on each server
* **Compression** - Compress server responses
* **Caching** - Return the response for cached requests
* **Static content** - Serve static content directly
* HTML/CSS/JS
* Photos
* Videos
* Etc
- **Increased security** - Hide information about backend servers, blacklist IPs, limit number of connections per client
- **Increased scalability and flexibility** - Clients only see the reverse proxy's IP, allowing you to scale servers or change their configuration
- **SSL termination** - Decrypt incoming requests and encrypt server responses so backend servers do not have to perform these potentially expensive operations
- Removes the need to install [X.509 certificates](https://en.wikipedia.org/wiki/X.509) on each server
- **Compression** - Compress server responses
- **Caching** - Return the response for cached requests
- **Static content** - Serve static content directly
- HTML/CSS/JS
- Photos
- Videos
- Etc
### Load balancer vs reverse proxy
* Deploying a load balancer is useful when you have multiple servers. Often, load balancers route traffic to a set of servers serving the same function.
* Reverse proxies can be useful even with just one web server or application server, opening up the benefits described in the previous section.
* Solutions such as NGINX and HAProxy can support both layer 7 reverse proxying and load balancing.
- Deploying a load balancer is useful when you have multiple servers. Often, load balancers route traffic to a set of servers serving the same function.
- Reverse proxies can be useful even with just one web server or application server, opening up the benefits described in the previous section.
- Solutions such as NGINX and HAProxy can support both layer 7 reverse proxying and load balancing.
### Disadvantage(s): reverse proxy
* Introducing a reverse proxy results in increased complexity.
* A single reverse proxy is a single point of failure, configuring multiple reverse proxies (ie a [failover](https://en.wikipedia.org/wiki/Failover)) further increases complexity.
- Introducing a reverse proxy results in increased complexity.
- A single reverse proxy is a single point of failure, configuring multiple reverse proxies (ie a [failover](https://en.wikipedia.org/wiki/Failover)) further increases complexity.
### Source(s) and further reading
* [Reverse proxy vs load balancer](https://www.nginx.com/resources/glossary/reverse-proxy-vs-load-balancer/)
* [NGINX architecture](https://www.nginx.com/blog/inside-nginx-how-we-designed-for-performance-scale/)
* [HAProxy architecture guide](http://www.haproxy.org/download/1.2/doc/architecture.txt)
* [Wikipedia](https://en.wikipedia.org/wiki/Reverse_proxy)
- [Reverse proxy vs load balancer](https://www.nginx.com/resources/glossary/reverse-proxy-vs-load-balancer/)
- [NGINX architecture](https://www.nginx.com/blog/inside-nginx-how-we-designed-for-performance-scale/)
- [HAProxy architecture guide](http://www.haproxy.org/download/1.2/doc/architecture.txt)
- [Wikipedia](https://en.wikipedia.org/wiki/Reverse_proxy)
## Application layer
@ -793,16 +793,16 @@ Systems such as [Consul](https://www.consul.io/docs/index.html), [Etcd](https://
### Disadvantage(s): application layer
* Adding an application layer with loosely coupled services requires a different approach from an architectural, operations, and process viewpoint (vs a monolithic system).
* Microservices can add complexity in terms of deployments and operations.
- Adding an application layer with loosely coupled services requires a different approach from an architectural, operations, and process viewpoint (vs a monolithic system).
- Microservices can add complexity in terms of deployments and operations.
### Source(s) and further reading
* [Intro to architecting systems for scale](http://lethain.com/introduction-to-architecting-systems-for-scale)
* [Crack the system design interview](http://www.puncsky.com/blog/2016-02-13-crack-the-system-design-interview)
* [Service oriented architecture](https://en.wikipedia.org/wiki/Service-oriented_architecture)
* [Introduction to Zookeeper](http://www.slideshare.net/sauravhaloi/introduction-to-apache-zookeeper)
* [Here's what you need to know about building microservices](https://cloudncode.wordpress.com/2016/07/22/msa-getting-started/)
- [Intro to architecting systems for scale](http://lethain.com/introduction-to-architecting-systems-for-scale)
- [Crack the system design interview](http://www.puncsky.com/blog/2016-02-13-crack-the-system-design-interview)
- [Service oriented architecture](https://en.wikipedia.org/wiki/Service-oriented_architecture)
- [Introduction to Zookeeper](http://www.slideshare.net/sauravhaloi/introduction-to-apache-zookeeper)
- [Here's what you need to know about building microservices](https://cloudncode.wordpress.com/2016/07/22/msa-getting-started/)
## Database
@ -818,10 +818,10 @@ A relational database like SQL is a collection of data items organized in tables
**ACID** is a set of properties of relational database [transactions](https://en.wikipedia.org/wiki/Database_transaction).
* **Atomicity** - Each transaction is all or nothing
* **Consistency** - Any transaction will bring the database from one valid state to another
* **Isolation** - Executing transactions concurrently has the same results as if the transactions were executed serially
* **Durability** - Once a transaction has been committed, it will remain so
- **Atomicity** - Each transaction is all or nothing
- **Consistency** - Any transaction will bring the database from one valid state to another
- **Isolation** - Executing transactions concurrently has the same results as if the transactions were executed serially
- **Durability** - Once a transaction has been committed, it will remain so
There are many techniques to scale a relational database: **master-slave replication**, **master-master replication**, **federation**, **sharding**, **denormalization**, and **SQL tuning**.
@ -837,8 +837,8 @@ The master serves reads and writes, replicating writes to one or more slaves, wh
##### Disadvantage(s): master-slave replication
* Additional logic is needed to promote a slave to a master.
* See [Disadvantage(s): replication](#disadvantages-replication) for points related to **both** master-slave and master-master.
- Additional logic is needed to promote a slave to a master.
- See [Disadvantage(s): replication](#disadvantages-replication) for points related to **both** master-slave and master-master.
#### Master-master replication
@ -852,23 +852,23 @@ Both masters serve reads and writes and coordinate with each other on writes. I
##### Disadvantage(s): master-master replication
* You'll need a load balancer or you'll need to make changes to your application logic to determine where to write.
* Most master-master systems are either loosely consistent (violating ACID) or have increased write latency due to synchronization.
* Conflict resolution comes more into play as more write nodes are added and as latency increases.
* See [Disadvantage(s): replication](#disadvantages-replication) for points related to **both** master-slave and master-master.
- You'll need a load balancer or you'll need to make changes to your application logic to determine where to write.
- Most master-master systems are either loosely consistent (violating ACID) or have increased write latency due to synchronization.
- Conflict resolution comes more into play as more write nodes are added and as latency increases.
- See [Disadvantage(s): replication](#disadvantages-replication) for points related to **both** master-slave and master-master.
##### Disadvantage(s): replication
* There is a potential for loss of data if the master fails before any newly written data can be replicated to other nodes.
* Writes are replayed to the read replicas. If there are a lot of writes, the read replicas can get bogged down with replaying writes and can't do as many reads.
* The more read slaves, the more you have to replicate, which leads to greater replication lag.
* On some systems, writing to the master can spawn multiple threads to write in parallel, whereas read replicas only support writing sequentially with a single thread.
* Replication adds more hardware and additional complexity.
- There is a potential for loss of data if the master fails before any newly written data can be replicated to other nodes.
- Writes are replayed to the read replicas. If there are a lot of writes, the read replicas can get bogged down with replaying writes and can't do as many reads.
- The more read slaves, the more you have to replicate, which leads to greater replication lag.
- On some systems, writing to the master can spawn multiple threads to write in parallel, whereas read replicas only support writing sequentially with a single thread.
- Replication adds more hardware and additional complexity.
##### Source(s) and further reading: replication
* [Scalability, availability, stability, patterns](http://www.slideshare.net/jboner/scalability-availability-stability-patterns/)
* [Multi-master replication](https://en.wikipedia.org/wiki/Multi-master_replication)
- [Scalability, availability, stability, patterns](http://www.slideshare.net/jboner/scalability-availability-stability-patterns/)
- [Multi-master replication](https://en.wikipedia.org/wiki/Multi-master_replication)
#### Federation
@ -882,14 +882,14 @@ Federation (or functional partitioning) splits up databases by function. For ex
##### Disadvantage(s): federation
* Federation is not effective if your schema requires huge functions or tables.
* You'll need to update your application logic to determine which database to read and write.
* Joining data from two databases is more complex with a [server link](http://stackoverflow.com/questions/5145637/querying-data-by-joining-two-tables-in-two-database-on-different-servers).
* Federation adds more hardware and additional complexity.
- Federation is not effective if your schema requires huge functions or tables.
- You'll need to update your application logic to determine which database to read and write.
- Joining data from two databases is more complex with a [server link](http://stackoverflow.com/questions/5145637/querying-data-by-joining-two-tables-in-two-database-on-different-servers).
- Federation adds more hardware and additional complexity.
##### Source(s) and further reading: federation
* [Scaling up to your first 10 million users](https://www.youtube.com/watch?v=kKjm4ehYiMs)
- [Scaling up to your first 10 million users](https://www.youtube.com/watch?v=kKjm4ehYiMs)
#### Sharding
@ -907,17 +907,17 @@ Common ways to shard a table of users is either through the user's last name ini
##### Disadvantage(s): sharding
* You'll need to update your application logic to work with shards, which could result in complex SQL queries.
* Data distribution can become lopsided in a shard. For example, a set of power users on a shard could result in increased load to that shard compared to others.
* Rebalancing adds additional complexity. A sharding function based on [consistent hashing](http://www.paperplanes.de/2011/12/9/the-magic-of-consistent-hashing.html) can reduce the amount of transferred data.
* Joining data from multiple shards is more complex.
* Sharding adds more hardware and additional complexity.
- You'll need to update your application logic to work with shards, which could result in complex SQL queries.
- Data distribution can become lopsided in a shard. For example, a set of power users on a shard could result in increased load to that shard compared to others.
- Rebalancing adds additional complexity. A sharding function based on [consistent hashing](http://www.paperplanes.de/2011/12/9/the-magic-of-consistent-hashing.html) can reduce the amount of transferred data.
- Joining data from multiple shards is more complex.
- Sharding adds more hardware and additional complexity.
##### Source(s) and further reading: sharding
* [The coming of the shard](http://highscalability.com/blog/2009/8/6/an-unorthodox-approach-to-database-design-the-coming-of-the.html)
* [Shard database architecture](https://en.wikipedia.org/wiki/Shard_(database_architecture))
* [Consistent hashing](http://www.paperplanes.de/2011/12/9/the-magic-of-consistent-hashing.html)
- [The coming of the shard](http://highscalability.com/blog/2009/8/6/an-unorthodox-approach-to-database-design-the-coming-of-the.html)
- [Shard database architecture](<https://en.wikipedia.org/wiki/Shard_(database_architecture)>)
- [Consistent hashing](http://www.paperplanes.de/2011/12/9/the-magic-of-consistent-hashing.html)
#### Denormalization
@ -929,13 +929,13 @@ In most systems, reads can heavily outnumber writes 100:1 or even 1000:1. A rea
##### Disadvantage(s): denormalization
* Data is duplicated.
* Constraints can help redundant copies of information stay in sync, which increases complexity of the database design.
* A denormalized database under heavy write load might perform worse than its normalized counterpart.
- Data is duplicated.
- Constraints can help redundant copies of information stay in sync, which increases complexity of the database design.
- A denormalized database under heavy write load might perform worse than its normalized counterpart.
###### Source(s) and further reading: denormalization
* [Denormalization](https://en.wikipedia.org/wiki/Denormalization)
- [Denormalization](https://en.wikipedia.org/wiki/Denormalization)
#### SQL tuning
@ -943,49 +943,49 @@ SQL tuning is a broad topic and many [books](https://www.amazon.com/s/ref=nb_sb_
It's important to **benchmark** and **profile** to simulate and uncover bottlenecks.
* **Benchmark** - Simulate high-load situations with tools such as [ab](http://httpd.apache.org/docs/2.2/programs/ab.html).
* **Profile** - Enable tools such as the [slow query log](http://dev.mysql.com/doc/refman/5.7/en/slow-query-log.html) to help track performance issues.
- **Benchmark** - Simulate high-load situations with tools such as [ab](http://httpd.apache.org/docs/2.2/programs/ab.html).
- **Profile** - Enable tools such as the [slow query log](http://dev.mysql.com/doc/refman/5.7/en/slow-query-log.html) to help track performance issues.
Benchmarking and profiling might point you to the following optimizations.
##### Tighten up the schema
* MySQL dumps to disk in contiguous blocks for fast access.
* Use `CHAR` instead of `VARCHAR` for fixed-length fields.
* `CHAR` effectively allows for fast, random access, whereas with `VARCHAR`, you must find the end of a string before moving onto the next one.
* Use `TEXT` for large blocks of text such as blog posts. `TEXT` also allows for boolean searches. Using a `TEXT` field results in storing a pointer on disk that is used to locate the text block.
* Use `INT` for larger numbers up to 2^32 or 4 billion.
* Use `DECIMAL` for currency to avoid floating point representation errors.
* Avoid storing large `BLOBS`, store the location of where to get the object instead.
* `VARCHAR(255)` is the largest number of characters that can be counted in an 8 bit number, often maximizing the use of a byte in some RDBMS.
* Set the `NOT NULL` constraint where applicable to [improve search performance](http://stackoverflow.com/questions/1017239/how-do-null-values-affect-performance-in-a-database-search).
- MySQL dumps to disk in contiguous blocks for fast access.
- Use `CHAR` instead of `VARCHAR` for fixed-length fields.
- `CHAR` effectively allows for fast, random access, whereas with `VARCHAR`, you must find the end of a string before moving onto the next one.
- Use `TEXT` for large blocks of text such as blog posts. `TEXT` also allows for boolean searches. Using a `TEXT` field results in storing a pointer on disk that is used to locate the text block.
- Use `INT` for larger numbers up to 2^32 or 4 billion.
- Use `DECIMAL` for currency to avoid floating point representation errors.
- Avoid storing large `BLOBS`, store the location of where to get the object instead.
- `VARCHAR(255)` is the largest number of characters that can be counted in an 8 bit number, often maximizing the use of a byte in some RDBMS.
- Set the `NOT NULL` constraint where applicable to [improve search performance](http://stackoverflow.com/questions/1017239/how-do-null-values-affect-performance-in-a-database-search).
##### Use good indices
* Columns that you are querying (`SELECT`, `GROUP BY`, `ORDER BY`, `JOIN`) could be faster with indices.
* Indices are usually represented as self-balancing [B-tree](https://en.wikipedia.org/wiki/B-tree) that keeps data sorted and allows searches, sequential access, insertions, and deletions in logarithmic time.
* Placing an index can keep the data in memory, requiring more space.
* Writes could also be slower since the index also needs to be updated.
* When loading large amounts of data, it might be faster to disable indices, load the data, then rebuild the indices.
- Columns that you are querying (`SELECT`, `GROUP BY`, `ORDER BY`, `JOIN`) could be faster with indices.
- Indices are usually represented as self-balancing [B-tree](https://en.wikipedia.org/wiki/B-tree) that keeps data sorted and allows searches, sequential access, insertions, and deletions in logarithmic time.
- Placing an index can keep the data in memory, requiring more space.
- Writes could also be slower since the index also needs to be updated.
- When loading large amounts of data, it might be faster to disable indices, load the data, then rebuild the indices.
##### Avoid expensive joins
* [Denormalize](#denormalization) where performance demands it.
- [Denormalize](#denormalization) where performance demands it.
##### Partition tables
* Break up a table by putting hot spots in a separate table to help keep it in memory.
- Break up a table by putting hot spots in a separate table to help keep it in memory.
##### Tune the query cache
* In some cases, the [query cache](https://dev.mysql.com/doc/refman/5.7/en/query-cache.html) could lead to [performance issues](https://www.percona.com/blog/2016/10/12/mysql-5-7-performance-tuning-immediately-after-installation/).
- In some cases, the [query cache](https://dev.mysql.com/doc/refman/5.7/en/query-cache.html) could lead to [performance issues](https://www.percona.com/blog/2016/10/12/mysql-5-7-performance-tuning-immediately-after-installation/).
##### Source(s) and further reading: SQL tuning
* [Tips for optimizing MySQL queries](http://aiddroid.com/10-tips-optimizing-mysql-queries-dont-suck/)
* [Is there a good reason i see VARCHAR(255) used so often?](http://stackoverflow.com/questions/1217466/is-there-a-good-reason-i-see-varchar255-used-so-often-as-opposed-to-another-l)
* [How do null values affect performance?](http://stackoverflow.com/questions/1017239/how-do-null-values-affect-performance-in-a-database-search)
* [Slow query log](http://dev.mysql.com/doc/refman/5.7/en/slow-query-log.html)
- [Tips for optimizing MySQL queries](http://aiddroid.com/10-tips-optimizing-mysql-queries-dont-suck/)
- [Is there a good reason i see VARCHAR(255) used so often?](http://stackoverflow.com/questions/1217466/is-there-a-good-reason-i-see-varchar255-used-so-often-as-opposed-to-another-l)
- [How do null values affect performance?](http://stackoverflow.com/questions/1017239/how-do-null-values-affect-performance-in-a-database-search)
- [Slow query log](http://dev.mysql.com/doc/refman/5.7/en/slow-query-log.html)
### NoSQL
@ -993,9 +993,9 @@ NoSQL is a collection of data items represented in a **key-value store**, **docu
**BASE** is often used to describe the properties of NoSQL databases. In comparison with the [CAP Theorem](#cap-theorem), BASE chooses availability over consistency.
* **Basically available** - the system guarantees availability.
* **Soft state** - the state of the system may change over time, even without input.
* **Eventual consistency** - the system will become consistent over a period of time, given that the system doesn't receive input during that period.
- **Basically available** - the system guarantees availability.
- **Soft state** - the state of the system may change over time, even without input.
- **Eventual consistency** - the system will become consistent over a period of time, given that the system doesn't receive input during that period.
In addition to choosing between [SQL or NoSQL](#sql-or-nosql), it is helpful to understand which type of NoSQL database best fits your use case(s). We'll review **key-value stores**, **document stores**, **wide column stores**, and **graph databases** in the next section.
@ -1011,16 +1011,16 @@ A key-value store is the basis for more complex systems such as a document store
##### Source(s) and further reading: key-value store
* [Key-value database](https://en.wikipedia.org/wiki/Key-value_database)
* [Disadvantages of key-value stores](http://stackoverflow.com/questions/4056093/what-are-the-disadvantages-of-using-a-key-value-table-over-nullable-columns-or)
* [Redis architecture](http://qnimate.com/overview-of-redis-architecture/)
* [Memcached architecture](https://www.adayinthelifeof.nl/2011/02/06/memcache-internals/)
- [Key-value database](https://en.wikipedia.org/wiki/Key-value_database)
- [Disadvantages of key-value stores](http://stackoverflow.com/questions/4056093/what-are-the-disadvantages-of-using-a-key-value-table-over-nullable-columns-or)
- [Redis architecture](http://qnimate.com/overview-of-redis-architecture/)
- [Memcached architecture](https://www.adayinthelifeof.nl/2011/02/06/memcache-internals/)
#### Document store
> Abstraction: key-value store with documents stored as values
A document store is centered around documents (XML, JSON, binary, etc), where a document stores all information for a given object. Document stores provide APIs or a query language to query based on the internal structure of the document itself. *Note, many key-value stores include features for working with a value's metadata, blurring the lines between these two storage types.*
A document store is centered around documents (XML, JSON, binary, etc), where a document stores all information for a given object. Document stores provide APIs or a query language to query based on the internal structure of the document itself. _Note, many key-value stores include features for working with a value's metadata, blurring the lines between these two storage types._
Based on the underlying implementation, documents are organized by collections, tags, metadata, or directories. Although documents can be organized or grouped together, documents may have fields that are completely different from each other.
@ -1030,10 +1030,10 @@ Document stores provide high flexibility and are often used for working with occ
##### Source(s) and further reading: document store
* [Document-oriented database](https://en.wikipedia.org/wiki/Document-oriented_database)
* [MongoDB architecture](https://www.mongodb.com/mongodb-architecture)
* [CouchDB architecture](https://blog.couchdb.org/2016/08/01/couchdb-2-0-architecture/)
* [Elasticsearch architecture](https://www.elastic.co/blog/found-elasticsearch-from-the-bottom-up)
- [Document-oriented database](https://en.wikipedia.org/wiki/Document-oriented_database)
- [MongoDB architecture](https://www.mongodb.com/mongodb-architecture)
- [CouchDB architecture](https://blog.couchdb.org/2016/08/01/couchdb-2-0-architecture/)
- [Elasticsearch architecture](https://www.elastic.co/blog/found-elasticsearch-from-the-bottom-up)
#### Wide column store
@ -1053,10 +1053,10 @@ Wide column stores offer high availability and high scalability. They are often
##### Source(s) and further reading: wide column store
* [SQL & NoSQL, a brief history](http://blog.grio.com/2015/11/sql-nosql-a-brief-history.html)
* [Bigtable architecture](http://www.read.seas.harvard.edu/~kohler/class/cs239-w08/chang06bigtable.pdf)
* [HBase architecture](https://www.mapr.com/blog/in-depth-look-hbase-architecture)
* [Cassandra architecture](http://docs.datastax.com/en/cassandra/3.0/cassandra/architecture/archIntro.html)
- [SQL & NoSQL, a brief history](http://blog.grio.com/2015/11/sql-nosql-a-brief-history.html)
- [Bigtable architecture](http://www.read.seas.harvard.edu/~kohler/class/cs239-w08/chang06bigtable.pdf)
- [HBase architecture](https://www.mapr.com/blog/in-depth-look-hbase-architecture)
- [Cassandra architecture](http://docs.datastax.com/en/cassandra/3.0/cassandra/architecture/archIntro.html)
#### Graph database
@ -1074,17 +1074,17 @@ Graphs databases offer high performance for data models with complex relationshi
##### Source(s) and further reading: graph
* [Graph database](https://en.wikipedia.org/wiki/Graph_database)
* [Neo4j](https://neo4j.com/)
* [FlockDB](https://blog.twitter.com/2010/introducing-flockdb)
- [Graph database](https://en.wikipedia.org/wiki/Graph_database)
- [Neo4j](https://neo4j.com/)
- [FlockDB](https://blog.twitter.com/2010/introducing-flockdb)
#### Source(s) and further reading: NoSQL
* [Explanation of base terminology](http://stackoverflow.com/questions/3342497/explanation-of-base-terminology)
* [NoSQL databases a survey and decision guidance](https://medium.com/baqend-blog/nosql-databases-a-survey-and-decision-guidance-ea7823a822d#.wskogqenq)
* [Scalability](http://www.lecloud.net/post/7994751381/scalability-for-dummies-part-2-database)
* [Introduction to NoSQL](https://www.youtube.com/watch?v=qI_g07C_Q5I)
* [NoSQL patterns](http://horicky.blogspot.com/2009/11/nosql-patterns.html)
- [Explanation of base terminology](http://stackoverflow.com/questions/3342497/explanation-of-base-terminology)
- [NoSQL databases a survey and decision guidance](https://medium.com/baqend-blog/nosql-databases-a-survey-and-decision-guidance-ea7823a822d#.wskogqenq)
- [Scalability](http://www.lecloud.net/post/7994751381/scalability-for-dummies-part-2-database)
- [Introduction to NoSQL](https://www.youtube.com/watch?v=qI_g07C_Q5I)
- [NoSQL patterns](http://horicky.blogspot.com/2009/11/nosql-patterns.html)
### SQL or NoSQL
@ -1096,37 +1096,37 @@ Graphs databases offer high performance for data models with complex relationshi
Reasons for **SQL**:
* Structured data
* Strict schema
* Relational data
* Need for complex joins
* Transactions
* Clear patterns for scaling
* More established: developers, community, code, tools, etc
* Lookups by index are very fast
- Structured data
- Strict schema
- Relational data
- Need for complex joins
- Transactions
- Clear patterns for scaling
- More established: developers, community, code, tools, etc
- Lookups by index are very fast
Reasons for **NoSQL**:
* Semi-structured data
* Dynamic or flexible schema
* Non-relational data
* No need for complex joins
* Store many TB (or PB) of data
* Very data intensive workload
* Very high throughput for IOPS
- Semi-structured data
- Dynamic or flexible schema
- Non-relational data
- No need for complex joins
- Store many TB (or PB) of data
- Very data intensive workload
- Very high throughput for IOPS
Sample data well-suited for NoSQL:
* Rapid ingest of clickstream and log data
* Leaderboard or scoring data
* Temporary data, such as a shopping cart
* Frequently accessed ('hot') tables
* Metadata/lookup tables
- Rapid ingest of clickstream and log data
- Leaderboard or scoring data
- Temporary data, such as a shopping cart
- Frequently accessed ('hot') tables
- Metadata/lookup tables
##### Source(s) and further reading: SQL or NoSQL
* [Scaling up to your first 10 million users](https://www.youtube.com/watch?v=kKjm4ehYiMs)
* [SQL vs NoSQL differences](https://www.sitepoint.com/sql-vs-nosql-differences/)
- [Scaling up to your first 10 million users](https://www.youtube.com/watch?v=kKjm4ehYiMs)
- [SQL vs NoSQL differences](https://www.sitepoint.com/sql-vs-nosql-differences/)
## Cache
@ -1162,15 +1162,15 @@ In-memory caches such as Memcached and Redis are key-value stores between your a
Redis has the following additional features:
* Persistence option
* Built-in data structures such as sorted sets and lists
- Persistence option
- Built-in data structures such as sorted sets and lists
There are multiple levels you can cache that fall into two general categories: **database queries** and **objects**:
* Row level
* Query-level
* Fully-formed serializable objects
* Fully-rendered HTML
- Row level
- Query-level
- Fully-formed serializable objects
- Fully-rendered HTML
Generally, you should try to avoid file-based caching, as it makes cloning and auto-scaling more difficult.
@ -1178,22 +1178,22 @@ Generally, you should try to avoid file-based caching, as it makes cloning and a
Whenever you query the database, hash the query as a key and store the result to the cache. This approach suffers from expiration issues:
* Hard to delete a cached result with complex queries
* If one piece of data changes such as a table cell, you need to delete all cached queries that might include the changed cell
- Hard to delete a cached result with complex queries
- If one piece of data changes such as a table cell, you need to delete all cached queries that might include the changed cell
### Caching at the object level
See your data as an object, similar to what you do with your application code. Have your application assemble the dataset from the database into a class instance or a data structure(s):
* Remove the object from cache if its underlying data has changed
* Allows for asynchronous processing: workers assemble objects by consuming the latest cached object
- Remove the object from cache if its underlying data has changed
- Allows for asynchronous processing: workers assemble objects by consuming the latest cached object
Suggestions of what to cache:
* User sessions
* Fully rendered web pages
* Activity streams
* User graph data
- User sessions
- Fully rendered web pages
- Activity streams
- User graph data
### When to update the cache
@ -1209,10 +1209,10 @@ Since you can only store a limited amount of data in cache, you'll need to deter
The application is responsible for reading and writing from storage. The cache does not interact with storage directly. The application does the following:
* Look for entry in cache, resulting in a cache miss
* Load entry from the database
* Add entry to cache
* Return entry
- Look for entry in cache, resulting in a cache miss
- Load entry from the database
- Add entry to cache
- Return entry
```python
def get_user(self, user_id):
@ -1231,9 +1231,9 @@ Subsequent reads of data added to cache are fast. Cache-aside is also referred
##### Disadvantage(s): cache-aside
* Each cache miss results in three trips, which can cause a noticeable delay.
* Data can become stale if it is updated in the database. This issue is mitigated by setting a time-to-live (TTL) which forces an update of the cache entry, or by using write-through.
* When a node fails, it is replaced by a new, empty node, increasing latency.
- Each cache miss results in three trips, which can cause a noticeable delay.
- Data can become stale if it is updated in the database. This issue is mitigated by setting a time-to-live (TTL) which forces an update of the cache entry, or by using write-through.
- When a node fails, it is replaced by a new, empty node, increasing latency.
#### Write-through
@ -1245,9 +1245,9 @@ Subsequent reads of data added to cache are fast. Cache-aside is also referred
The application uses the cache as the main data store, reading and writing data to it, while the cache is responsible for reading and writing to the database:
* Application adds/updates entry in cache
* Cache synchronously writes entry to data store
* Return
- Application adds/updates entry in cache
- Cache synchronously writes entry to data store
- Return
Application code:
@ -1267,8 +1267,8 @@ Write-through is a slow overall operation due to the write operation, but subseq
##### Disadvantage(s): write through
* When a new node is created due to failure or scaling, the new node will not cache entries until the entry is updated in the database. Cache-aside in conjunction with write through can mitigate this issue.
* Most data written might never be read, which can be minimized with a TTL.
- When a new node is created due to failure or scaling, the new node will not cache entries until the entry is updated in the database. Cache-aside in conjunction with write through can mitigate this issue.
- Most data written might never be read, which can be minimized with a TTL.
#### Write-behind (write-back)
@ -1280,13 +1280,13 @@ Write-through is a slow overall operation due to the write operation, but subseq
In write-behind, the application does the following:
* Add/update entry in cache
* Asynchronously write entry to the data store, improving write performance
- Add/update entry in cache
- Asynchronously write entry to the data store, improving write performance
##### Disadvantage(s): write-behind
* There could be data loss if the cache goes down prior to its contents hitting the data store.
* It is more complex to implement write-behind than it is to implement cache-aside or write-through.
- There could be data loss if the cache goes down prior to its contents hitting the data store.
- It is more complex to implement write-behind than it is to implement cache-aside or write-through.
#### Refresh-ahead
@ -1302,23 +1302,23 @@ Refresh-ahead can result in reduced latency vs read-through if the cache can acc
##### Disadvantage(s): refresh-ahead
* Not accurately predicting which items are likely to be needed in the future can result in reduced performance than without refresh-ahead.
- Not accurately predicting which items are likely to be needed in the future can result in reduced performance than without refresh-ahead.
### Disadvantage(s): cache
* Need to maintain consistency between caches and the source of truth such as the database through [cache invalidation](https://en.wikipedia.org/wiki/Cache_algorithms).
* Cache invalidation is a difficult problem, there is additional complexity associated with when to update the cache.
* Need to make application changes such as adding Redis or memcached.
- Need to maintain consistency between caches and the source of truth such as the database through [cache invalidation](https://en.wikipedia.org/wiki/Cache_algorithms).
- Cache invalidation is a difficult problem, there is additional complexity associated with when to update the cache.
- Need to make application changes such as adding Redis or memcached.
### Source(s) and further reading
* [From cache to in-memory data grid](http://www.slideshare.net/tmatyashovsky/from-cache-to-in-memory-data-grid-introduction-to-hazelcast)
* [Scalable system design patterns](http://horicky.blogspot.com/2010/10/scalable-system-design-patterns.html)
* [Introduction to architecting systems for scale](http://lethain.com/introduction-to-architecting-systems-for-scale/)
* [Scalability, availability, stability, patterns](http://www.slideshare.net/jboner/scalability-availability-stability-patterns/)
* [Scalability](http://www.lecloud.net/post/9246290032/scalability-for-dummies-part-3-cache)
* [AWS ElastiCache strategies](http://docs.aws.amazon.com/AmazonElastiCache/latest/UserGuide/Strategies.html)
* [Wikipedia](https://en.wikipedia.org/wiki/Cache_(computing))
- [From cache to in-memory data grid](http://www.slideshare.net/tmatyashovsky/from-cache-to-in-memory-data-grid-introduction-to-hazelcast)
- [Scalable system design patterns](http://horicky.blogspot.com/2010/10/scalable-system-design-patterns.html)
- [Introduction to architecting systems for scale](http://lethain.com/introduction-to-architecting-systems-for-scale/)
- [Scalability, availability, stability, patterns](http://www.slideshare.net/jboner/scalability-availability-stability-patterns/)
- [Scalability](http://www.lecloud.net/post/9246290032/scalability-for-dummies-part-3-cache)
- [AWS ElastiCache strategies](http://docs.aws.amazon.com/AmazonElastiCache/latest/UserGuide/Strategies.html)
- [Wikipedia](<https://en.wikipedia.org/wiki/Cache_(computing)>)
## Asynchronism
@ -1334,8 +1334,8 @@ Asynchronous workflows help reduce request times for expensive operations that w
Message queues receive, hold, and deliver messages. If an operation is too slow to perform inline, you can use a message queue with the following workflow:
* An application publishes a job to the queue, then notifies the user of job status
* A worker picks up the job from the queue, processes it, then signals the job is complete
- An application publishes a job to the queue, then notifies the user of job status
- A worker picks up the job from the queue, processes it, then signals the job is complete
The user is not blocked and the job is processed in the background. During this time, the client might optionally do a small amount of processing to make it seem like the task has completed. For example, if posting a tweet, the tweet could be instantly posted to your timeline, but it could take some time before your tweet is actually delivered to all of your followers.
@ -1357,14 +1357,14 @@ If queues start to grow significantly, the queue size can become larger than mem
### Disadvantage(s): asynchronism
* Use cases such as inexpensive calculations and realtime workflows might be better suited for synchronous operations, as introducing queues can add delays and complexity.
- Use cases such as inexpensive calculations and realtime workflows might be better suited for synchronous operations, as introducing queues can add delays and complexity.
### Source(s) and further reading
* [It's all a numbers game](https://www.youtube.com/watch?v=1KRYH75wgy4)
* [Applying back pressure when overloaded](http://mechanical-sympathy.blogspot.com/2012/05/apply-back-pressure-when-overloaded.html)
* [Little's law](https://en.wikipedia.org/wiki/Little%27s_law)
* [What is the difference between a message queue and a task queue?](https://www.quora.com/What-is-the-difference-between-a-message-queue-and-a-task-queue-Why-would-a-task-queue-require-a-message-broker-like-RabbitMQ-Redis-Celery-or-IronMQ-to-function)
- [It's all a numbers game](https://www.youtube.com/watch?v=1KRYH75wgy4)
- [Applying back pressure when overloaded](http://mechanical-sympathy.blogspot.com/2012/05/apply-back-pressure-when-overloaded.html)
- [Little's law](https://en.wikipedia.org/wiki/Little%27s_law)
- [What is the difference between a message queue and a task queue?](https://www.quora.com/What-is-the-difference-between-a-message-queue-and-a-task-queue-Why-would-a-task-queue-require-a-message-broker-like-RabbitMQ-Redis-Celery-or-IronMQ-to-function)
## Communication
@ -1380,23 +1380,23 @@ HTTP is a method for encoding and transporting data between a client and a serve
A basic HTTP request consists of a verb (method) and a resource (endpoint). Below are common HTTP verbs:
| Verb | Description | Idempotent* | Safe | Cacheable |
|---|---|---|---|---|
| Verb | Description | Idempotent\* | Safe | Cacheable |
| ------ | --------------------------------------------------------- | ------------ | ---- | --------------------------------------- |
| GET | Reads a resource | Yes | Yes | Yes |
| POST | Creates a resource or trigger a process that handles data | No | No | Yes if response contains freshness info |
| PUT | Creates or replace a resource | Yes | No | No |
| PATCH | Partially updates a resource | No | No | Yes if response contains freshness info |
| DELETE | Deletes a resource | Yes | No | No |
*Can be called many times without different outcomes.
\*Can be called many times without different outcomes.
HTTP is an application layer protocol relying on lower-level protocols such as **TCP** and **UDP**.
#### Source(s) and further reading: HTTP
* [What is HTTP?](https://www.nginx.com/resources/glossary/http/)
* [Difference between HTTP and TCP](https://www.quora.com/What-is-the-difference-between-HTTP-protocol-and-TCP-protocol)
* [Difference between PUT and PATCH](https://laracasts.com/discuss/channels/general-discussion/whats-the-differences-between-put-and-patch?page=1)
- [What is HTTP?](https://www.nginx.com/resources/glossary/http/)
- [Difference between HTTP and TCP](https://www.quora.com/What-is-the-difference-between-HTTP-protocol-and-TCP-protocol)
- [Difference between PUT and PATCH](https://laracasts.com/discuss/channels/general-discussion/whats-the-differences-between-put-and-patch?page=1)
### Transmission control protocol (TCP)
@ -1408,10 +1408,10 @@ HTTP is an application layer protocol relying on lower-level protocols such as *
TCP is a connection-oriented protocol over an [IP network](https://en.wikipedia.org/wiki/Internet_Protocol). Connection is established and terminated using a [handshake](https://en.wikipedia.org/wiki/Handshaking). All packets sent are guaranteed to reach the destination in the original order and without corruption through:
* Sequence numbers and [checksum fields](https://en.wikipedia.org/wiki/Transmission_Control_Protocol#Checksum_computation) for each packet
* [Acknowledgement](https://en.wikipedia.org/wiki/Acknowledgement_(data_networks)) packets and automatic retransmission
- Sequence numbers and [checksum fields](https://en.wikipedia.org/wiki/Transmission_Control_Protocol#Checksum_computation) for each packet
- [Acknowledgement](<https://en.wikipedia.org/wiki/Acknowledgement_(data_networks)>) packets and automatic retransmission
If the sender does not receive a correct response, it will resend the packets. If there are multiple timeouts, the connection is dropped. TCP also implements [flow control](https://en.wikipedia.org/wiki/Flow_control_(data)) and [congestion control](https://en.wikipedia.org/wiki/Network_congestion#Congestion_control). These guarantees cause delays and generally result in less efficient transmission than UDP.
If the sender does not receive a correct response, it will resend the packets. If there are multiple timeouts, the connection is dropped. TCP also implements [flow control](<https://en.wikipedia.org/wiki/Flow_control_(data)>) and [congestion control](https://en.wikipedia.org/wiki/Network_congestion#Congestion_control). These guarantees cause delays and generally result in less efficient transmission than UDP.
To ensure high throughput, web servers can keep a large number of TCP connections open, resulting in high memory usage. It can be expensive to have a large number of open connections between web server threads and say, a [memcached](https://memcached.org/) server. [Connection pooling](https://en.wikipedia.org/wiki/Connection_pool) can help in addition to switching to UDP where applicable.
@ -1419,8 +1419,8 @@ TCP is useful for applications that require high reliability but are less time c
Use TCP over UDP when:
* You need all of the data to arrive intact
* You want to automatically make a best estimate use of the network throughput
- You need all of the data to arrive intact
- You want to automatically make a best estimate use of the network throughput
### User datagram protocol (UDP)
@ -1438,18 +1438,18 @@ UDP is less reliable but works well in real time use cases such as VoIP, video c
Use UDP over TCP when:
* You need the lowest latency
* Late data is worse than loss of data
* You want to implement your own error correction
- You need the lowest latency
- Late data is worse than loss of data
- You want to implement your own error correction
#### Source(s) and further reading: TCP and UDP
* [Networking for game programming](http://gafferongames.com/networking-for-game-programmers/udp-vs-tcp/)
* [Key differences between TCP and UDP protocols](http://www.cyberciti.biz/faq/key-differences-between-tcp-and-udp-protocols/)
* [Difference between TCP and UDP](http://stackoverflow.com/questions/5970383/difference-between-tcp-and-udp)
* [Transmission control protocol](https://en.wikipedia.org/wiki/Transmission_Control_Protocol)
* [User datagram protocol](https://en.wikipedia.org/wiki/User_Datagram_Protocol)
* [Scaling memcache at Facebook](http://www.cs.bu.edu/~jappavoo/jappavoo.github.com/451/papers/memcache-fb.pdf)
- [Networking for game programming](http://gafferongames.com/networking-for-game-programmers/udp-vs-tcp/)
- [Key differences between TCP and UDP protocols](http://www.cyberciti.biz/faq/key-differences-between-tcp-and-udp-protocols/)
- [Difference between TCP and UDP](http://stackoverflow.com/questions/5970383/difference-between-tcp-and-udp)
- [Transmission control protocol](https://en.wikipedia.org/wiki/Transmission_Control_Protocol)
- [User datagram protocol](https://en.wikipedia.org/wiki/User_Datagram_Protocol)
- [Scaling memcache at Facebook](http://www.cs.bu.edu/~jappavoo/jappavoo.github.com/451/papers/memcache-fb.pdf)
### Remote procedure call (RPC)
@ -1463,12 +1463,12 @@ In an RPC, a client causes a procedure to execute on a different address space,
RPC is a request-response protocol:
* **Client program** - Calls the client stub procedure. The parameters are pushed onto the stack like a local procedure call.
* **Client stub procedure** - Marshals (packs) procedure id and arguments into a request message.
* **Client communication module** - OS sends the message from the client to the server.
* **Server communication module** - OS passes the incoming packets to the server stub procedure.
* **Server stub procedure** - Unmarshalls the results, calls the server procedure matching the procedure id and passes the given arguments.
* The server response repeats the steps above in reverse order.
- **Client program** - Calls the client stub procedure. The parameters are pushed onto the stack like a local procedure call.
- **Client stub procedure** - Marshals (packs) procedure id and arguments into a request message.
- **Client communication module** - OS sends the message from the client to the server.
- **Server communication module** - OS passes the incoming packets to the server stub procedure.
- **Server stub procedure** - Unmarshalls the results, calls the server procedure matching the procedure id and passes the given arguments.
- The server response repeats the steps above in reverse order.
Sample RPC calls:
@ -1486,19 +1486,19 @@ RPC is focused on exposing behaviors. RPCs are often used for performance reaso
Choose a native library (aka SDK) when:
* You know your target platform.
* You want to control how your "logic" is accessed.
* You want to control how error control happens off your library.
* Performance and end user experience is your primary concern.
- You know your target platform.
- You want to control how your "logic" is accessed.
- You want to control how error control happens off your library.
- Performance and end user experience is your primary concern.
HTTP APIs following **REST** tend to be used more often for public APIs.
#### Disadvantage(s): RPC
* RPC clients become tightly coupled to the service implementation.
* A new API must be defined for every new operation or use case.
* It can be difficult to debug RPC.
* You might not be able to leverage existing technologies out of the box. For example, it might require additional effort to ensure [RPC calls are properly cached](http://etherealbits.com/2012/12/debunking-the-myths-of-rpc-rest/) on caching servers such as [Squid](http://www.squid-cache.org/).
- RPC clients become tightly coupled to the service implementation.
- A new API must be defined for every new operation or use case.
- It can be difficult to debug RPC.
- You might not be able to leverage existing technologies out of the box. For example, it might require additional effort to ensure [RPC calls are properly cached](http://etherealbits.com/2012/12/debunking-the-myths-of-rpc-rest/) on caching servers such as [Squid](http://www.squid-cache.org/).
### Representational state transfer (REST)
@ -1506,10 +1506,10 @@ REST is an architectural style enforcing a client/server model where the client
There are four qualities of a RESTful interface:
* **Identify resources (URI in HTTP)** - use the same URI regardless of any operation.
* **Change with representations (Verbs in HTTP)** - use verbs, headers, and body.
* **Self-descriptive error message (status response in HTTP)** - Use status codes, don't reinvent the wheel.
* **[HATEOAS](http://restcookbook.com/Basics/hateoas/) (HTML interface for HTTP)** - your web service should be fully accessible in a browser.
- **Identify resources (URI in HTTP)** - use the same URI regardless of any operation.
- **Change with representations (Verbs in HTTP)** - use verbs, headers, and body.
- **Self-descriptive error message (status response in HTTP)** - Use status codes, don't reinvent the wheel.
- **[HATEOAS](http://restcookbook.com/Basics/hateoas/) (HTML interface for HTTP)** - your web service should be fully accessible in a browser.
Sample REST calls:
@ -1524,15 +1524,15 @@ REST is focused on exposing data. It minimizes the coupling between client/serv
#### Disadvantage(s): REST
* With REST being focused on exposing data, it might not be a good fit if resources are not naturally organized or accessed in a simple hierarchy. For example, returning all updated records from the past hour matching a particular set of events is not easily expressed as a path. With REST, it is likely to be implemented with a combination of URI path, query parameters, and possibly the request body.
* REST typically relies on a few verbs (GET, POST, PUT, DELETE, and PATCH) which sometimes doesn't fit your use case. For example, moving expired documents to the archive folder might not cleanly fit within these verbs.
* Fetching complicated resources with nested hierarchies requires multiple round trips between the client and server to render single views, e.g. fetching content of a blog entry and the comments on that entry. For mobile applications operating in variable network conditions, these multiple roundtrips are highly undesirable.
* Over time, more fields might be added to an API response and older clients will receive all new data fields, even those that they do not need, as a result, it bloats the payload size and leads to larger latencies.
- With REST being focused on exposing data, it might not be a good fit if resources are not naturally organized or accessed in a simple hierarchy. For example, returning all updated records from the past hour matching a particular set of events is not easily expressed as a path. With REST, it is likely to be implemented with a combination of URI path, query parameters, and possibly the request body.
- REST typically relies on a few verbs (GET, POST, PUT, DELETE, and PATCH) which sometimes doesn't fit your use case. For example, moving expired documents to the archive folder might not cleanly fit within these verbs.
- Fetching complicated resources with nested hierarchies requires multiple round trips between the client and server to render single views, e.g. fetching content of a blog entry and the comments on that entry. For mobile applications operating in variable network conditions, these multiple roundtrips are highly undesirable.
- Over time, more fields might be added to an API response and older clients will receive all new data fields, even those that they do not need, as a result, it bloats the payload size and leads to larger latencies.
### RPC and REST calls comparison
| Operation | RPC | REST |
|---|---|---|
| ------------------------------- | ----------------------------------------------------------------------------------------- | ------------------------------------------------------------ |
| Signup | **POST** /signup | **POST** /persons |
| Resign | **POST** /resign<br/>{<br/>"personid": "1234"<br/>} | **DELETE** /persons/1234 |
| Read a person | **GET** /readPerson?personid=1234 | **GET** /persons/1234 |
@ -1547,14 +1547,14 @@ REST is focused on exposing data. It minimizes the coupling between client/serv
#### Source(s) and further reading: REST and RPC
* [Do you really know why you prefer REST over RPC](https://apihandyman.io/do-you-really-know-why-you-prefer-rest-over-rpc/)
* [When are RPC-ish approaches more appropriate than REST?](http://programmers.stackexchange.com/a/181186)
* [REST vs JSON-RPC](http://stackoverflow.com/questions/15056878/rest-vs-json-rpc)
* [Debunking the myths of RPC and REST](http://etherealbits.com/2012/12/debunking-the-myths-of-rpc-rest/)
* [What are the drawbacks of using REST](https://www.quora.com/What-are-the-drawbacks-of-using-RESTful-APIs)
* [Crack the system design interview](http://www.puncsky.com/blog/2016-02-13-crack-the-system-design-interview)
* [Thrift](https://code.facebook.com/posts/1468950976659943/)
* [Why REST for internal use and not RPC](http://arstechnica.com/civis/viewtopic.php?t=1190508)
- [Do you really know why you prefer REST over RPC](https://apihandyman.io/do-you-really-know-why-you-prefer-rest-over-rpc/)
- [When are RPC-ish approaches more appropriate than REST?](http://programmers.stackexchange.com/a/181186)
- [REST vs JSON-RPC](http://stackoverflow.com/questions/15056878/rest-vs-json-rpc)
- [Debunking the myths of RPC and REST](http://etherealbits.com/2012/12/debunking-the-myths-of-rpc-rest/)
- [What are the drawbacks of using REST](https://www.quora.com/What-are-the-drawbacks-of-using-RESTful-APIs)
- [Crack the system design interview](http://www.puncsky.com/blog/2016-02-13-crack-the-system-design-interview)
- [Thrift](https://code.facebook.com/posts/1468950976659943/)
- [Why REST for internal use and not RPC](http://arstechnica.com/civis/viewtopic.php?t=1190508)
## Security
@ -1562,16 +1562,16 @@ This section could use some updates. Consider [contributing](#contributing)!
Security is a broad topic. Unless you have considerable experience, a security background, or are applying for a position that requires knowledge of security, you probably won't need to know more than the basics:
* Encrypt in transit and at rest.
* Sanitize all user inputs or any input parameters exposed to user to prevent [XSS](https://en.wikipedia.org/wiki/Cross-site_scripting) and [SQL injection](https://en.wikipedia.org/wiki/SQL_injection).
* Use parameterized queries to prevent SQL injection.
* Use the principle of [least privilege](https://en.wikipedia.org/wiki/Principle_of_least_privilege).
- Encrypt in transit and at rest.
- Sanitize all user inputs or any input parameters exposed to user to prevent [XSS](https://en.wikipedia.org/wiki/Cross-site_scripting) and [SQL injection](https://en.wikipedia.org/wiki/SQL_injection).
- Use parameterized queries to prevent SQL injection.
- Use the principle of [least privilege](https://en.wikipedia.org/wiki/Principle_of_least_privilege).
### Source(s) and further reading
* [API security checklist](https://github.com/shieldfy/API-Security-Checklist)
* [Security guide for developers](https://github.com/FallibleInc/security-guide-for-developers)
* [OWASP top ten](https://www.owasp.org/index.php/OWASP_Top_Ten_Cheat_Sheet)
- [API security checklist](https://github.com/shieldfy/API-Security-Checklist)
- [Security guide for developers](https://github.com/FallibleInc/security-guide-for-developers)
- [OWASP top ten](https://www.owasp.org/index.php/OWASP_Top_Ten_Cheat_Sheet)
## Appendix
@ -1594,7 +1594,7 @@ Power Exact Value Approx Value Bytes
#### Source(s) and further reading
* [Powers of two](https://en.wikipedia.org/wiki/Power_of_two)
- [Powers of two](https://en.wikipedia.org/wiki/Power_of_two)
### Latency numbers every programmer should know
@ -1626,12 +1626,12 @@ Notes
Handy metrics based on numbers above:
* Read sequentially from disk at 30 MB/s
* Read sequentially from 1 Gbps Ethernet at 100 MB/s
* Read sequentially from SSD at 1 GB/s
* Read sequentially from main memory at 4 GB/s
* 6-7 world-wide round trips per second
* 2,000 round trips per second within a data center
- Read sequentially from disk at 30 MB/s
- Read sequentially from 1 Gbps Ethernet at 100 MB/s
- Read sequentially from SSD at 1 GB/s
- Read sequentially from main memory at 4 GB/s
- 6-7 world-wide round trips per second
- 2,000 round trips per second within a data center
#### Latency numbers visualized
@ -1639,17 +1639,17 @@ Handy metrics based on numbers above:
#### Source(s) and further reading
* [Latency numbers every programmer should know - 1](https://gist.github.com/jboner/2841832)
* [Latency numbers every programmer should know - 2](https://gist.github.com/hellerbarde/2843375)
* [Designs, lessons, and advice from building large distributed systems](http://www.cs.cornell.edu/projects/ladis2009/talks/dean-keynote-ladis2009.pdf)
* [Software Engineering Advice from Building Large-Scale Distributed Systems](https://static.googleusercontent.com/media/research.google.com/en//people/jeff/stanford-295-talk.pdf)
- [Latency numbers every programmer should know - 1](https://gist.github.com/jboner/2841832)
- [Latency numbers every programmer should know - 2](https://gist.github.com/hellerbarde/2843375)
- [Designs, lessons, and advice from building large distributed systems](http://www.cs.cornell.edu/projects/ladis2009/talks/dean-keynote-ladis2009.pdf)
- [Software Engineering Advice from Building Large-Scale Distributed Systems](https://static.googleusercontent.com/media/research.google.com/en//people/jeff/stanford-295-talk.pdf)
### Additional system design interview questions
> Common system design interview questions, with links to resources on how to solve each.
| Question | Reference(s) |
|---|---|
| ----------------------------------------------------------- | ----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- |
| Design a file sync service like Dropbox | [youtube.com](https://www.youtube.com/watch?v=PE4gwstWhmc) |
| Design a search engine like Google | [queue.acm.org](http://queue.acm.org/detail.cfm?id=988407)<br/>[stackexchange.com](http://programmers.stackexchange.com/questions/38324/interview-question-how-would-you-implement-google-search)<br/>[ardendertat.com](http://www.ardendertat.com/2012/01/11/implementing-search-engines/)<br/>[stanford.edu](http://infolab.stanford.edu/~backrub/google.html) |
| Design a scalable web crawler like Google | [quora.com](https://www.quora.com/How-can-I-build-a-web-crawler-from-scratch) |
@ -1658,7 +1658,7 @@ Handy metrics based on numbers above:
| Design a cache system like Memcached | [slideshare.net](http://www.slideshare.net/oemebamo/introduction-to-memcached) |
| Design a recommendation system like Amazon's | [hulu.com](https://web.archive.org/web/20170406065247/http://tech.hulu.com/blog/2011/09/19/recommendation-system.html)<br/>[ijcai13.org](http://ijcai13.org/files/tutorial_slides/td3.pdf) |
| Design a tinyurl system like Bitly | [n00tc0d3r.blogspot.com](http://n00tc0d3r.blogspot.com/) |
| Design a chat app like WhatsApp | [highscalability.com](http://highscalability.com/blog/2014/2/26/the-whatsapp-architecture-facebook-bought-for-19-billion.html)
| Design a chat app like WhatsApp | [highscalability.com](http://highscalability.com/blog/2014/2/26/the-whatsapp-architecture-facebook-bought-for-19-billion.html) |
| Design a picture sharing system like Instagram | [highscalability.com](http://highscalability.com/flickr-architecture)<br/>[highscalability.com](http://highscalability.com/blog/2011/12/6/instagram-architecture-14-million-users-terabytes-of-photos.html) |
| Design the Facebook news feed function | [quora.com](http://www.quora.com/What-are-best-practices-for-building-something-like-a-News-Feed)<br/>[quora.com](http://www.quora.com/Activity-Streams/What-are-the-scaling-issues-to-keep-in-mind-while-developing-a-social-network-feed)<br/>[slideshare.net](http://www.slideshare.net/danmckinley/etsy-activity-feeds-architecture) |
| Design the Facebook timeline function | [facebook.com](https://www.facebook.com/note.php?note_id=10150468255628920)<br/>[highscalability.com](http://highscalability.com/blog/2012/1/23/facebook-timeline-brought-to-you-by-the-power-of-denormaliza.html) |
@ -1686,19 +1686,19 @@ Handy metrics based on numbers above:
**Don't focus on nitty gritty details for the following articles, instead:**
* Identify shared principles, common technologies, and patterns within these articles
* Study what problems are solved by each component, where it works, where it doesn't
* Review the lessons learned
- Identify shared principles, common technologies, and patterns within these articles
- Study what problems are solved by each component, where it works, where it doesn't
- Review the lessons learned
| Type | System | Reference(s) |
|---|---|---|
| --------------- | -------------------------------------------------------------------------------- | ---------------------------------------------------------------------------------------------------------------------------------------------- |
| Data processing | **MapReduce** - Distributed data processing from Google | [research.google.com](http://static.googleusercontent.com/media/research.google.com/zh-CN/us/archive/mapreduce-osdi04.pdf) |
| Data processing | **Spark** - Distributed data processing from Databricks | [slideshare.net](http://www.slideshare.net/AGrishchenko/apache-spark-architecture) |
| Data processing | **Storm** - Distributed data processing from Twitter | [slideshare.net](http://www.slideshare.net/previa/storm-16094009) |
| | | |
| Data store | **Bigtable** - Distributed column-oriented database from Google | [harvard.edu](http://www.read.seas.harvard.edu/~kohler/class/cs239-w08/chang06bigtable.pdf) |
| Data store | **HBase** - Open source implementation of Bigtable | [slideshare.net](http://www.slideshare.net/alexbaranau/intro-to-hbase) |
| Data store | **Cassandra** - Distributed column-oriented database from Facebook | [slideshare.net](http://www.slideshare.net/planetcassandra/cassandra-introduction-features-30103666)
| Data store | **Cassandra** - Distributed column-oriented database from Facebook | [slideshare.net](http://www.slideshare.net/planetcassandra/cassandra-introduction-features-30103666) |
| Data store | **DynamoDB** - Document-oriented database from Amazon | [harvard.edu](http://www.read.seas.harvard.edu/~kohler/class/cs239-w08/decandia07dynamo.pdf) |
| Data store | **MongoDB** - Document-oriented database | [slideshare.net](http://www.slideshare.net/mdirolf/introduction-to-mongodb) |
| Data store | **Spanner** - Globally-distributed database from Google | [research.google.com](http://research.google.com/archive/spanner-osdi2012.pdf) |
@ -1709,7 +1709,7 @@ Handy metrics based on numbers above:
| File system | **Hadoop File System (HDFS)** - Open source implementation of GFS | [apache.org](http://hadoop.apache.org/docs/stable/hadoop-project-dist/hadoop-hdfs/HdfsDesign.html) |
| | | |
| Misc | **Chubby** - Lock service for loosely-coupled distributed systems from Google | [research.google.com](http://static.googleusercontent.com/external_content/untrusted_dlcp/research.google.com/en/us/archive/chubby-osdi06.pdf) |
| Misc | **Dapper** - Distributed systems tracing infrastructure | [research.google.com](http://static.googleusercontent.com/media/research.google.com/en//pubs/archive/36356.pdf)
| Misc | **Dapper** - Distributed systems tracing infrastructure | [research.google.com](http://static.googleusercontent.com/media/research.google.com/en//pubs/archive/36356.pdf) |
| Misc | **Kafka** - Pub/sub message queue from LinkedIn | [slideshare.net](http://www.slideshare.net/mumrah/kafka-talk-tri-hug) |
| Misc | **Zookeeper** - Centralized infrastructure and services enabling synchronization | [slideshare.net](http://www.slideshare.net/sauravhaloi/introduction-to-apache-zookeeper) |
| | Add an architecture | [Contribute](#contributing) |
@ -1717,7 +1717,7 @@ Handy metrics based on numbers above:
### Company architectures
| Company | Reference(s) |
|---|---|
| -------------- | --------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- |
| Amazon | [Amazon architecture](http://highscalability.com/amazon-architecture) |
| Cinchcast | [Producing 1,500 hours of audio every day](http://highscalability.com/blog/2012/7/16/cinchcast-architecture-producing-1500-hours-of-audio-every-d.html) |
| DataSift | [Realtime datamining At 120,000 tweets per second](http://highscalability.com/blog/2011/11/29/datasift-architecture-realtime-datamining-at-120000-tweets-p.html) |
@ -1748,61 +1748,61 @@ Handy metrics based on numbers above:
>
> Questions you encounter might be from the same domain.
* [Airbnb Engineering](http://nerds.airbnb.com/)
* [Atlassian Developers](https://developer.atlassian.com/blog/)
* [AWS Blog](https://aws.amazon.com/blogs/aws/)
* [Bitly Engineering Blog](http://word.bitly.com/)
* [Box Blogs](https://blog.box.com/blog/category/engineering)
* [Cloudera Developer Blog](http://blog.cloudera.com/)
* [Dropbox Tech Blog](https://tech.dropbox.com/)
* [Engineering at Quora](http://engineering.quora.com/)
* [Ebay Tech Blog](http://www.ebaytechblog.com/)
* [Evernote Tech Blog](https://blog.evernote.com/tech/)
* [Etsy Code as Craft](http://codeascraft.com/)
* [Facebook Engineering](https://www.facebook.com/Engineering)
* [Flickr Code](http://code.flickr.net/)
* [Foursquare Engineering Blog](http://engineering.foursquare.com/)
* [GitHub Engineering Blog](http://githubengineering.com/)
* [Google Research Blog](http://googleresearch.blogspot.com/)
* [Groupon Engineering Blog](https://engineering.groupon.com/)
* [Heroku Engineering Blog](https://engineering.heroku.com/)
* [Hubspot Engineering Blog](http://product.hubspot.com/blog/topic/engineering)
* [High Scalability](http://highscalability.com/)
* [Instagram Engineering](http://instagram-engineering.tumblr.com/)
* [Intel Software Blog](https://software.intel.com/en-us/blogs/)
* [Jane Street Tech Blog](https://blogs.janestreet.com/category/ocaml/)
* [LinkedIn Engineering](http://engineering.linkedin.com/blog)
* [Microsoft Engineering](https://engineering.microsoft.com/)
* [Microsoft Python Engineering](https://blogs.msdn.microsoft.com/pythonengineering/)
* [Netflix Tech Blog](http://techblog.netflix.com/)
* [Paypal Developer Blog](https://devblog.paypal.com/category/engineering/)
* [Pinterest Engineering Blog](https://medium.com/@Pinterest_Engineering)
* [Quora Engineering](https://engineering.quora.com/)
* [Reddit Blog](http://www.redditblog.com/)
* [Salesforce Engineering Blog](https://developer.salesforce.com/blogs/engineering/)
* [Slack Engineering Blog](https://slack.engineering/)
* [Spotify Labs](https://labs.spotify.com/)
* [Twilio Engineering Blog](http://www.twilio.com/engineering)
* [Twitter Engineering](https://blog.twitter.com/engineering/)
* [Uber Engineering Blog](http://eng.uber.com/)
* [Yahoo Engineering Blog](http://yahooeng.tumblr.com/)
* [Yelp Engineering Blog](http://engineeringblog.yelp.com/)
* [Zynga Engineering Blog](https://www.zynga.com/blogs/engineering)
- [Airbnb Engineering](http://nerds.airbnb.com/)
- [Atlassian Developers](https://developer.atlassian.com/blog/)
- [AWS Blog](https://aws.amazon.com/blogs/aws/)
- [Bitly Engineering Blog](http://word.bitly.com/)
- [Box Blogs](https://blog.box.com/blog/category/engineering)
- [Cloudera Developer Blog](http://blog.cloudera.com/)
- [Dropbox Tech Blog](https://tech.dropbox.com/)
- [Engineering at Quora](http://engineering.quora.com/)
- [Ebay Tech Blog](http://www.ebaytechblog.com/)
- [Evernote Tech Blog](https://blog.evernote.com/tech/)
- [Etsy Code as Craft](http://codeascraft.com/)
- [Facebook Engineering](https://www.facebook.com/Engineering)
- [Flickr Code](http://code.flickr.net/)
- [Foursquare Engineering Blog](http://engineering.foursquare.com/)
- [GitHub Engineering Blog](http://githubengineering.com/)
- [Google Research Blog](http://googleresearch.blogspot.com/)
- [Groupon Engineering Blog](https://engineering.groupon.com/)
- [Heroku Engineering Blog](https://engineering.heroku.com/)
- [Hubspot Engineering Blog](http://product.hubspot.com/blog/topic/engineering)
- [High Scalability](http://highscalability.com/)
- [Instagram Engineering](http://instagram-engineering.tumblr.com/)
- [Intel Software Blog](https://software.intel.com/en-us/blogs/)
- [Jane Street Tech Blog](https://blogs.janestreet.com/category/ocaml/)
- [LinkedIn Engineering](http://engineering.linkedin.com/blog)
- [Microsoft Engineering](https://engineering.microsoft.com/)
- [Microsoft Python Engineering](https://blogs.msdn.microsoft.com/pythonengineering/)
- [Netflix Tech Blog](http://techblog.netflix.com/)
- [Paypal Developer Blog](https://devblog.paypal.com/category/engineering/)
- [Pinterest Engineering Blog](https://medium.com/@Pinterest_Engineering)
- [Quora Engineering](https://engineering.quora.com/)
- [Reddit Blog](http://www.redditblog.com/)
- [Salesforce Engineering Blog](https://developer.salesforce.com/blogs/engineering/)
- [Slack Engineering Blog](https://slack.engineering/)
- [Spotify Labs](https://labs.spotify.com/)
- [Twilio Engineering Blog](http://www.twilio.com/engineering)
- [Twitter Engineering](https://blog.twitter.com/engineering/)
- [Uber Engineering Blog](http://eng.uber.com/)
- [Yahoo Engineering Blog](http://yahooeng.tumblr.com/)
- [Yelp Engineering Blog](http://engineeringblog.yelp.com/)
- [Zynga Engineering Blog](https://www.zynga.com/blogs/engineering)
#### Source(s) and further reading
Looking to add a blog? To avoid duplicating work, consider adding your company blog to the following repo:
* [kilimchoi/engineering-blogs](https://github.com/kilimchoi/engineering-blogs)
- [kilimchoi/engineering-blogs](https://github.com/kilimchoi/engineering-blogs)
## Under development
Interested in adding a section or helping complete one in-progress? [Contribute](#contributing)!
* Distributed computing with MapReduce
* Consistent hashing
* Scatter gather
* [Contribute](#contributing)
- Distributed computing with MapReduce
- Consistent hashing
- Scatter gather
- [Contribute](#contributing)
## Credits
@ -1810,15 +1810,15 @@ Credits and sources are provided throughout this repo.
Special thanks to:
* [Hired in tech](http://www.hiredintech.com/system-design/the-system-design-process/)
* [Cracking the coding interview](https://www.amazon.com/dp/0984782850/)
* [High scalability](http://highscalability.com/)
* [checkcheckzz/system-design-interview](https://github.com/checkcheckzz/system-design-interview)
* [shashank88/system_design](https://github.com/shashank88/system_design)
* [mmcgrana/services-engineering](https://github.com/mmcgrana/services-engineering)
* [System design cheat sheet](https://gist.github.com/vasanthk/485d1c25737e8e72759f)
* [A distributed systems reading list](http://dancres.github.io/Pages/)
* [Cracking the system design interview](http://www.puncsky.com/blog/2016-02-13-crack-the-system-design-interview)
- [Hired in tech](http://www.hiredintech.com/system-design/the-system-design-process/)
- [Cracking the coding interview](https://www.amazon.com/dp/0984782850/)
- [High scalability](http://highscalability.com/)
- [checkcheckzz/system-design-interview](https://github.com/checkcheckzz/system-design-interview)
- [shashank88/system_design](https://github.com/shashank88/system_design)
- [mmcgrana/services-engineering](https://github.com/mmcgrana/services-engineering)
- [System design cheat sheet](https://gist.github.com/vasanthk/485d1c25737e8e72759f)
- [A distributed systems reading list](http://dancres.github.io/Pages/)
- [Cracking the system design interview](http://www.puncsky.com/blog/2016-02-13-crack-the-system-design-interview)
## Contact info
@ -1828,7 +1828,7 @@ My contact info can be found on my [GitHub page](https://github.com/donnemartin)
## License
*I am providing code and resources in this repository to you under an open source license. Because this is my personal repository, the license you receive to my code and resources is from me and not my employer (Facebook).*
_I am providing code and resources in this repository to you under an open source license. Because this is my personal repository, the license you receive to my code and resources is from me and not my employer (Facebook)._
Copyright 2017 Donne Martin