* [Bộ card bài tập thiết kế hệ thống](https://github.com/donnemartin/system-design-primer/tree/master/resources/flash_cards/System%20Design%20Exercises.apkg)
### Tài liệu code: Tương tác (literal translation would be odd here)
Looking for resources to help you prep for the [**Coding Interview**](https://github.com/donnemartin/interactive-coding-challenges)?
Bạn đang tìm tài liệu để hỗ trợ cho [**Phỏng vấn Coding**](https://github.com/donnemartin/interactive-coding-challenges)
<palign="center">
<imgsrc="images/b4YtAEN.png">
<br/>
</p>
Check out the sister repo [**Interactive Coding Challenges**](https://github.com/donnemartin/interactive-coding-challenges), which contains an additional Anki deck:
Hãy xem qua repo "em" [**Coding Tương Tác**](https://github.com/donnemartin/interactive-coding-challenges), repo này chứa những bộ Anki bổ sung sau:
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.
Ứng cử viên càng có bề dày kinh nghiệm thì càng được mong dợi là sẽ biết nhiều hơn về thiết kế hệ thống. Architects hoặc team leads có được mong là biết nhiều hơn những cá nhân khác. Các công ty hàng đầu thì khả năng cao là có nhiều phỏng vấn về thiết kế hơn.
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.
Bắt đầu ở bề rộng và đi sâu hơn vào từng vùng. Biết một chút cho nhiều chủ đề khác nhau thì thường sẽ giúp ít bạn hơn. Điều chỉnh những hướng dẫn trong đây tùy vào thời gian, kinh nghiệm, và vị trí nào mà bạn đang phỏng vấn vào, và công ty nào mà bạn phỏng vấn với.
* **Thời gian ngắn** - Nhắm vào bề rộng của các chủ đề thiết kế hệ thống. Tập luyện bằng cách giải quyết **một số** câu hỏi phỏng vấn.
* **Thời gian trung bình** - Nhắm vào bề rộng và một chút phần sâu với các chủ đều thiết kế hệ thống. Tập luyện giải quyết **nhiều** câu hỏi phỏng vấn.
| 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: |
| Review [How to approach a system design interview question](#how-to-approach-a-system-design-interview-question) | :+1: | :+1: | :+1: |
| Work through [System design interview questions with solutions](#system-design-interview-questions-with-solutions) | Some | Many | Most |
| Work through [Object-oriented design interview questions with solutions](#object-oriented-design-interview-questions-with-solutions) | Some | Many | Most |
| Review [Additional system design interview questions](#additional-system-design-interview-questions) | Some | Many | Most |
## How to approach a system design interview question
You can use the following steps to guide the discussion. To help solidify this process, work through the [System design interview questions with solutions](#system-design-interview-questions-with-solutions) section using the following steps.
Bạn có thể dùng những bước sao để lái cuộc thảo luận. Để củng cố, hãy đi qua phần [Các câu hỏi phỏng vấn thiết kế hệ thống kèm lời giải](#system-design-interview-questions-with-solutions) sử dụng các bước sau.
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:
Đào sâu vào chi tiết của từng thành phần lõi. Ví dụ, nếu bạn được đề nghị [thiết kế một dịch vụ rút ngắn url](solutions/system_design/pastebin/README.md), hãy trình bày:
Discuss potential solutions and trade-offs. Everything is a trade-off. Address bottlenecks using [principles of scalable system design](#index-of-system-design-topics).
Thảo luận các phương án khả thi và đánh đổi. Mọi thứ đều là đánh đổi. Giải quyết cổ chai bằng cách dùng [các nguyên tắc của thiết kế hệ thống scalable](#index-of-system-design-topics)
Đầu tiên bạn sẽ cần một số hiểu biết cơ bản về các nguyên lý chung, học về định nghĩa, cách sử dụng, cùng với điểm điểm được và mất cả các nguyên lý này.
A service is **scalable** if it results in increased **performance** in a manner proportional to resources added. Generally, increasing performance means serving more units of work, but it can also be to handle larger units of work, such as when datasets grow.<sup><ahref=http://www.allthingsdistributed.com/2006/03/a_word_on_scalability.html>1</a></sup>
Một dịch vụ có tính mở rộng nếu đạt tăng được hiệu năng tỷ lệ thuận với tài nguyên được thêm vào. Tăng hiệu năng đồng nghĩa với phục vụ được nhiều đơn vị công việc hơn, nhưng cũng có nghĩa là xử lý được công việc lớn hơn, chẳng hạn như dataset tăng.<sup><ahref=http://www.allthingsdistributed.com/2006/03/a_word_on_scalability.html>1</a></sup>
*Networks aren't reliable, so you'll need to support partition tolerance. You'll need to make a software tradeoff between consistency and availability.*
Waiting for a response from the partitioned node might result in a timeout error. CP is a good choice if your business needs require atomic reads and writes.
Việc đợi phản hồi từ một node bị phân vùng (mạng) có thể mang lại kết quả lỗi timeout. CP là một lựa chọn tốt nếu nhu cầu đòi hỏi đọc và ghi phải atomic.
Responses return the most readily available version of the data available on any node, which might not be the latest. Writes might take some time to propagate when the partition is resolved.
Các câu trả lời trả về phiên bản đang có của dữ liệu trên node, có thể không phải là bản mới nhất. Việc ghi có thể mất thời gian để lan truyền khi các partition được phân giải. # FIXME: "phân giải" doesn't sound right.
AP is a good choice if the business needs allow for [eventual consistency](#eventual-consistency) or when the system needs to continue working despite external errors.
AP là một lựa chọn tốt cho các nhu cầu cần [eventual consistency](#eventual-consistency) hoặc các hệ thống cần tiếp tục vận hành khi đối mặt với các lỗi hỏng bên ngoài.
With multiple copies of the same data, we are faced with options on how to synchronize them so clients have a consistent view of the data. Recall the definition of consistency from the [CAP theorem](#cap-theorem) - Every read receives the most recent write or an error.
Với nhiều phiên bản của cùng một dữ liệu, chúng ta đối mặt với những cách thức đồng bộ để các client nhìn dữ liệu một cách nhất quán. Gợi nhớ lại định nghĩa của tính nhất quán [Định lý CAP](#cap-theorem) - Mọi hành vi đọc nhận được phiên bản ghi gần nhất hoặc là một lỗi.
This approach is seen in systems such as memcached. Weak consistency works well in real time use cases such as VoIP, video chat, and realtime multiplayer games. For example, if you are on a phone call and lose reception for a few seconds, when you regain connection you do not hear what was spoken during connection loss.
Phương cách này thấy áp dụng ở nhiều hệ thống như memcached. Nhất quán kém làm việc tốt trong cách nhu cầu cần thời gian thực như VoIP, video chat, và game thời gian thực nhiều người chơi. Ví dụ nếu trong một cuộc gọi, ta bị mất tín hiệu một vài giây, khi lấy lại được kết nối thì bạn không nghe lại cuộc nói chuyện trong quá trình đứt kết nối.
With active-passive fail-over, heartbeats are sent between the active and the passive server on standby. If the heartbeat is interrupted, the passive server takes over the active's IP address and resumes service.
Với fail-over chủ động - bị động, nhịp tim (heartbeat) sẽ được gửi giữa máy chủ chủ động và máy chủ bị động trực chờ. Nếu nhịp tim bị ngắt quãng, máy chủ bị động sẽ tiếp quản địa chỉ IP của máy chủ động và tiếp tục dịch vụ.
The length of downtime is determined by whether the passive server is already running in 'hot' standby or whether it needs to start up from 'cold' standby. Only the active server handles traffic.
Độ dài của thời gian dừng sẽ được quyết định bởi việc máy chủ bị động có đang chạy ở chế độ trực chờ sẵn sàng ('hot' standby) hay cần phải được khởi động ('cold' standby). Chỉ duy nhất máy chủ chủ động xử lí liên lạc. # FIXME: dull translation
If the servers are public-facing, the DNS would need to know about the public IPs of both servers. If the servers are internal-facing, application logic would need to know about both servers.
Nếu các server là public-facing, DNS sẽ cần phải biết public IP của cả hai. Nếu các server là internal-facing, thì application logic cần biết về cả hai server.
Availability is often quantified by uptime (or downtime) as a percentage of time the service is available. Availability is generally measured in number of 9s--a service with 99.99% availability is described as having four 9s.
Tính khả dụng thường được lượng hoá bởi uptime (hoặc downtime) bằng tỉ lệ phần trăm của thời gian dịch vụ ở trạng thái khả dụng. Tính khả dụng nhìn chung được đo lường với đơn vị số "9"--như một dịch vụ khả dụng với mức 99.99% sử dụng bốn số "9".
If a service consists of multiple components prone to failure, the service's overall availability depends on whether the components are in sequence or in parallel.
Nếu một dịch vụ bao gồm nhiều thành phần dễ mắc lỗi, thì tính khả dụng toàn phần của dịch vụ này phụ thuộc vào các thành phần nằm trong một trình tự hay nằm song song.
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).
DNS được phân cấp, với một vài máy chủ ở cấp cao nhất. Router hoặc ISP cung cấp thông tin về máy chủ DNS để liên lạc khi cần tra cứu tên miền. Máy chủ DNS cấp thấp sẽ lưu nhớ kết quả, kết quả này có thể trở nên lạc hậu do độ trễ lan truyền của DNS. Kết quả DNS cũng có thể được lưu nhớ ở trình duyệt hoặc hệ điều hành trong một khoảng thời gian xác định, được quyết định bởi ["thời gian tồn tại" (TTL)](https://en.wikipedia.org/wiki/Time_to_live) # FIXME: incomplete and bad translation
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:
* 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).
<i><ahref=https://www.creative-artworks.eu/why-use-a-content-delivery-network-cdn/>Source: Why use a CDN</a></i>
</p>
A content delivery network (CDN) is a globally distributed network of proxy servers, serving content from locations closer to the user. Generally, static files such as HTML/CSS/JS, photos, and videos are served from CDN, although some CDNs such as Amazon's CloudFront support dynamic content. The site's DNS resolution will tell clients which server to contact.
Serving content from CDNs can significantly improve performance in two ways:
* Users receive content from data centers close to them
* Your servers do not have to serve requests that the CDN fulfills
### Push CDNs
Push CDNs receive new content whenever changes occur on your server. You take full responsibility for providing content, uploading directly to the CDN and rewriting URLs to point to the CDN. You can configure when content expires and when it is updated. Content is uploaded only when it is new or changed, minimizing traffic, but maximizing storage.
Push CDN nhận nội dung mới khi thay đổi xày ra phía máy chủ của bạn. Bạn nhận hoàn toàn trách nhiệm của việc cung cấp nội dung, tải trực tiếp lên CDN và viết lại URL trỏ vvào CDN. Bạn có thể cấu hình khi nội dung hết hạn và khi nào được cập nhật. Nội dung được tải lên chỉ khi có nội dung mới hoặc được thay đổi, tối thiểu hoá lưu thông mạng, nhưng lại tối đa việc lưu trữ.
Sites with a small amount of traffic or sites with content that isn't often updated work well with push CDNs. Content is placed on the CDNs once, instead of being re-pulled at regular intervals.
Các trang với với lưu thông nhỏ hoặc với nội dung không được cập nhật thường xuyên thì thường làm việc tốt với push CDN. Nội dung được đặt lên CDN chỉ một lần, thay vì phải được kéo về đều đặn.
Pull CDNs grab new content from your server when the first user requests the content. You leave the content on your server and rewrite URLs to point to the CDN. This results in a slower request until the content is cached on the CDN.
Pull CDN kéo nội dung mới từ máy chủ của bạn khi có yêu cầu đầu tiên từ người dùng cho nội dung này. Bạn để nội dung trên máy chủ và viết lại URL để trỏ đến CDN. Việc này dẫn đến thời gian thực thi yêu cầu sẽ chậm, cho đến hki nội dung được cache trên CDN.
A [time-to-live (TTL)](https://en.wikipedia.org/wiki/Time_to_live) determines how long content is cached. Pull CDNs minimize storage space on the CDN, but can create redundant traffic if files expire and are pulled before they have actually changed.
TTL (time-to-live - thời gian tồn tại) xác định thời gian mà nội dung sẽ được cache. Pull CDN tối thiểu hoá dụng lượng chứa trên CDN, nhưng có thể tạo ra lưu thông dư thừa nếu các tệp tin bị quá thời hạn cache và được kéo về khi chưa thực sự thay đổi.
Các trang với lưu lượng cao sẽ hoạt động tốt với pull CDN, vì lẽ lưu thông sẽ được dàn trải cân bằng hơn với nội dung được yêu cầu gần nhất sẽ được lưu tồn trên CDN.
<i><ahref=http://horicky.blogspot.com/2010/10/scalable-system-design-patterns.html>Source: Scalable system design patterns</a></i>
</p>
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:
Cân bằng tải phân phối các yêu cầu từ máy khách đến các tài nguyên tính toán như là máy chủ ứng dụng và cơ sở dữ liệu. Trong mỗi trường hợp, các cân bằng tải trả hồi đáp từ các tài nguyên tính toán về cho máy khách thích hợp. Cân bằng tải hiệu quả cho:
* Tránh gửi yêu cầu đến các máy chủ không khoẻ mạnh # FIXME: although unhealthy is literally "không khoẻ**, but the translation in this context sounds unusual.
* **SSL termination** - Decrypt incoming requests and encrypt server responses so backend servers do not have to perform these potentially expensive operations
* **Triệt tiêu SSL** - Giải mã các requests đi đến và mã hoá responses từ máy chủ, để các máy chủ không phải thực hiện các thao tác chi phí (tính toán) cao này.
* **Lưu phiên** - Phát hành cookie và điều hướng yêu cầu từ máy khách đến đúng nơi (máy chủ, ứng dụng) mà máy khách đang làm việc nếu các ứng dụng web không theo dõi phiên.
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.
Để bảo vệ failure, thường sẽ nhiều cân bằng tải sẽ được dựng, hoặc dùng mô hình [active-passive](#active-passive) hoặc [active-active](#active-active).
Layer 4 load balancers look at info at the [transport layer](#communication) to decide how to distribute requests. Generally, this involves the source, destination IP addresses, and ports in the header, but not the contents of the packet. Layer 4 load balancers forward network packets to and from the upstream server, performing [Network Address Translation (NAT)](https://www.nginx.com/resources/glossary/layer-4-load-balancing/).
Cân bằng tải ở lớp 4 nhìn vào thông tin ở [lớp truyền tải (transport)](#communication) để quyết định cách phân phối yêu cầu. Nhìn chung thì điều này sẽ liên quan đến nguồn, đích của địa chỉ IP, và cổng (port) trong header, nhưng không dựa vào nội dung của gói tin (packet). Cân bằng tải lớp 4 truyền tải các gói tin mạng đi đến và từ máy trạm đầu nguồn, thực hiện [biên dịch địa chỉ mạng (Network Address Translation - NAT)](https://www.nginx.com/resources/glossary/layer-4-load-balancing/)
Layer 7 load balancers look at the [application layer](#communication) to decide how to distribute requests. This can involve contents of the header, message, and cookies. Layer 7 load balancers terminate network traffic, reads the message, makes a load-balancing decision, then opens a connection to the selected server. For example, a layer 7 load balancer can direct video traffic to servers that host videos while directing more sensitive user billing traffic to security-hardened servers.
Cân bằng tải lớp 7 nhìn vào [lớp ứng dụng (application)](#communication) quyết định cách phân phối yêu cầu. Việc này có thể liên quan đến nội dung của header, gói tin, và cookie. Cân bằng tải lớp 7 kết thúc liên lạc, đọc tin, thực hiện quyết định về cân bằng tải, sau đó mở một kết nối đến máy trạm được lựa chọn. Ví dụ một cân bằng tải lớp 7 có thể điều hướng liên lạc hình ảnh đến các máy trạm chứa dữ liệu hình ảnh, trong khi đó, các liên lạc nhạy cảm hơn về thông tin thanh toán của người dùng có thể được điều hướng đến các máy chủ đã được củng cố về an ninh. # FIXME: bad translation
At the cost of flexibility, layer 4 load balancing requires less time and computing resources than Layer 7, although the performance impact can be minimal on modern commodity hardware.
### Horizontal scaling
Load balancers can also help with horizontal scaling, improving performance and availability. Scaling out using commodity machines is more cost efficient and results in higher availability than scaling up a single server on more expensive hardware, called **Vertical Scaling**. It is also easier to hire for talent working on commodity hardware than it is for specialized enterprise systems.
#### 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
A reverse proxy is a web server that centralizes internal services and provides unified interfaces to the public. Requests from clients are forwarded to a server that can fulfill it before the reverse proxy returns the server's response to the client.
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
* 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.
* Việc triển khai một cân bằng tải là hữu dụng khi bạn có nhiều máy trạm. Thông thường thì cân bằng tải điều hướng lưu thông đến một tập máy trạm phục vụ chung một chức năng.
* Proxy đảo có thể hữu dụng ngay cả với chỉ một máy trạm web, hoặc một máy trạm ứng dung, mở ra những ưu thế được mô tả ở phần trước.
* 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/)
<i><ahref=http://lethain.com/introduction-to-architecting-systems-for-scale/#platform_layer>Source: Intro to architecting systems for scale</a></i>
</p>
Separating out the web layer from the application layer (also known as platform layer) allows you to scale and configure both layers independently. Adding a new API results in adding application servers without necessarily adding additional web servers. The **single responsibility principle** advocates for small and autonomous services that work together. Small teams with small services can plan more aggressively for rapid growth.
Workers in the application layer also help enable [asynchronism](#asynchronism).
### Microservices
Related to this discussion are [microservices](https://en.wikipedia.org/wiki/Microservices), which can be described as a suite of independently deployable, small, modular services. Each service runs a unique process and communicates through a well-defined, lightweight mechanism to serve a business goal. <sup><ahref=https://smartbear.com/learn/api-design/what-are-microservices>1</a></sup>
Liên quan đến chủ đề này là [microservices](https://en.wikipedia.org/wiki/Microservices), có thể được diễn đạt là một bộ của những service được deploy độc lập, nhỏ, tách rời. Mỗi service chạy một process duy nhất và liên lạc thông qua một cơ chế đơn giản, được định nghĩa rõ ràng. <sup><ahref=https://smartbear.com/learn/api-design/what-are-microservices>1</a></sup> # FIXME: incomplete translation
Systems such as [Consul](https://www.consul.io/docs/index.html), [Etcd](https://coreos.com/etcd/docs/latest), and [Zookeeper](http://www.slideshare.net/sauravhaloi/introduction-to-apache-zookeeper) can help services find each other by keeping track of registered names, addresses, and ports. [Health checks](https://www.consul.io/intro/getting-started/checks.html) help verify service integrity and are often done using an [HTTP](#hypertext-transfer-protocol-http) endpoint. Both Consul and Etcd have a built in [key-value store](#key-value-store) that can be useful for storing config values and other shared data.
### 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.
* **Atomicity** - Mỗi giao dịch hoặc là hoàn thành tất cả, hoặc là không gì cả
* **Consistency** - Bất kỳ giao dịch nào sẽ chuyển cơ sở dữ liệu từ một trạng thái đúng đắn này sang một trạng thái đúng đắn khác # FIXME: bad translation
* **Isolation** - Thực thi các giao dịch sẽ mang lại kết quả y như là nếu các giao dịch được thực thi một cách tuần tự
* **Durability** - Một khi giao dịch đã được cam kết (commit), nó sẽ tồn tại như nguyên
There are many techniques to scale a relational database: **master-slave replication**, **master-master replication**, **federation**, **sharding**, **denormalization**, and **SQL tuning**.
#### Master-slave replication
The master serves reads and writes, replicating writes to one or more slaves, which serve only reads. Slaves can also replicate to additional slaves in a tree-like fashion. If the master goes offline, the system can continue to operate in read-only mode until a slave is promoted to a master or a new master is provisioned.
* 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
Both masters serve reads and writes and coordinate with each other on writes. If either master goes down, the system can continue to operate with both reads and writes.
* 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.
* Write sẽ được replay trên các read replica. Nếu có quá nhiều thao tác write, các read replica có thể bị nghẽn cho việc ghi và không thể phục vụ cho việc đọc.
* 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.
* Trong một vài hệ thống, việc ghi vào master có thể tạo nhiều thread để ghi song song, trong khi đó thì read replica sẽ chỉ hỗ trợ ghi nối tiếp trong một thread đơn nhất.
Federation (or functional partitioning) splits up databases by function. For example, instead of a single, monolithic database, you could have three databases: **forums**, **users**, and **products**, resulting in less read and write traffic to each database and therefore less replication lag. Smaller databases result in more data that can fit in memory, which in turn results in more cache hits due to improved cache locality. With no single central master serializing writes you can write in parallel, increasing throughput.
Federation (hay còn gọi là functional partitioning) chia cắt database theo tính năng. Ví dụ thay vì một database đơn nhất (monolithic database), ta có thể có ba database: **forums**, **users**, và **products**, kết quả là sẽ có ít tác vụ đọc và ghi vào mỗi database và từ đó giảm thiểu độ trễ của replication. Database nhỏ hơn cũng đồng nghĩa với việc sẽ có nhiều dữ liệu có thể chứa vừa trên vùng nhớ hơn, đồng nghĩa với khả năng cache hit (dữ liệu tương ứng có tồn tại trên cache) cao hơn, bởi cache nằm ở cục bộ của database (cache locality). Với việc không có một master trung tâm để serialize (sắp xếp lại theo thứ tự) tác vụ ghi, ta có thể thực hiện việc ghi này song song, từ đó tăng được công suất (throughput).
* 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).
* Liên kết dữ liệu giữa hai CSDL là phức tạp hơn với [server link](http://stackoverflow.com/questions/5145637/querying-data-by-joining-two-tables-in-two-database-on-different-servers). # FIXME: probably wrong meaning?
Sharding distributes data across different databases such that each database can only manage a subset of the data. Taking a users database as an example, as the number of users increases, more shards are added to the cluster.
Similar to the advantages of [federation](#federation), sharding results in less read and write traffic, less replication, and more cache hits. Index size is also reduced, which generally improves performance with faster queries. If one shard goes down, the other shards are still operational, although you'll want to add some form of replication to avoid data loss. Like federation, there is no single central master serializing writes, allowing you to write in parallel with increased throughput.
Common ways to shard a table of users is either through the user's last name initial or the user's geographic location.
##### 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.
##### 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)
Denormalization attempts to improve read performance at the expense of some write performance. Redundant copies of the data are written in multiple tables to avoid expensive joins. Some RDBMS such as [PostgreSQL](https://en.wikipedia.org/wiki/PostgreSQL) and Oracle support [materialized views](https://en.wikipedia.org/wiki/Materialized_view) which handle the work of storing redundant information and keeping redundant copies consistent.
Once data becomes distributed with techniques such as [federation](#federation) and [sharding](#sharding), managing joins across data centers further increases complexity. Denormalization might circumvent the need for such complex joins.
In most systems, reads can heavily outnumber writes 100:1 or even 1000:1. A read resulting in a complex database join can be very expensive, spending a significant amount of time on disk operations.
##### 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.
###### Source(s) and further reading: denormalization
SQL tuning is a broad topic and many [books](https://www.amazon.com/s/ref=nb_sb_noss_2?url=search-alias%3Daps&field-keywords=sql+tuning) have been written as reference.
Điều chỉnh SQL là một chủ đề rộng và có rất nhiều [quyển sách](https://www.amazon.com/s/ref=nb_sb_noss_2?url=search-alias%3Daps&field-keywords=sql+tuning) đã viết về chủ đề này.
* **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.
* **Profile** - Bật các công cụ như là [slow query log](http://dev.mysql.com/doc/refman/5.7/en/slow-query-log.html) để theo dõi các vấn đề về hiệu năng.
* 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.
##### Avoid expensive joins
* [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.
##### 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/).
##### 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)
NoSQL is a collection of data items represented in a **key-value store**, **document store**, **wide column store**, or a **graph database**. Data is denormalized, and joins are generally done in the application code. Most NoSQL stores lack true ACID transactions and favor [eventual consistency](#eventual-consistency).
**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.
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.
A key-value store generally allows for O(1) reads and writes and is often backed by memory or SSD. Data stores can maintain keys in [lexicographic order](https://en.wikipedia.org/wiki/Lexicographical_order), allowing efficient retrieval of key ranges. Key-value stores can allow for storing of metadata with a value.
Một store khoá-trị thông thường sẽ chấp nhận đọc và ghi trong O(1) và thường được chứa trong bộ nhớ (memory) hoặc SSD. Các data stores có thể sắp xếp khoá với thứ tự [lexicographic](https://en.wikipedia.org/wiki/Lexicographical_order), việc này sẽ cho phép truy vấn dải khoá một cách hiệu quả. Store khoá-trị có thể chấp nhận chứa metadata kèm với giá trị.
Key-value stores provide high performance and are often used for simple data models or for rapidly-changing data, such as an in-memory cache layer. Since they offer only a limited set of operations, complexity is shifted to the application layer if additional operations are needed.
Store khoá trị cung cấp hiệu năng cao và thường được dụng trong các mô hình dữ liệu đơn giản hoặc cho dữ liệu thay đổi liên tục, như ở lớp in-memory cache. Do sẽ có giới hạn cho tập các tác vụ được hỗ trợ, độ phức tạp sẽ được chuyển sang lớp ứng dụng nếu cần thêm các tác vụ khác.
* [Disadvantages of key-value stores](http://stackoverflow.com/questions/4056093/what-are-the-disadvantages-of-using-a-key-value-table-over-nullable-columns-or)
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.*
Store tài liệu xoay quanh các loại tài liệu (XML, JSON, binary, vâng vâng), trong đó một tài liệu chứa tất các các thông tin liên quan cho một đối tượng. Store tài liệu cung cấp API hoặc một ngôn ngữ truy vấn để truy vấn dựa trên cấu trúc bên trong của bản thân tài liệu. *Lưu ý, có nhiều store khoá-trị đã bao gồm các tính năng cho thao tác với metadata của giá trị, làm mờ đi ranh giới giữa hai store này.**
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.
Dựa trên cách hiện thực bên dưới, các tài liệu được tổ chức theo bộ sưu tập (collection), tags, metadata, hoặc thư mục. Mặc dù tài liệu có thể được tổ chức hoặc gom nhóm với nhau, tài liệu có thể có các trường hoàn toàn khác biệt với nhau.
Some document stores like [MongoDB](https://www.mongodb.com/mongodb-architecture) and [CouchDB](https://blog.couchdb.org/2016/08/01/couchdb-2-0-architecture/) also provide a SQL-like language to perform complex queries. [DynamoDB](http://www.read.seas.harvard.edu/~kohler/class/cs239-w08/decandia07dynamo.pdf) supports both key-values and documents.
Một vài store tài liệu như là [MongoDB](https://www.mongodb.com/mongodb-architecture và [CouchDB](https://blog.couchdb.org/2016/08/01/couchdb-2-0-architecture/) còn cung cấp ngôn ngữ gần SQL để thực thi các câu truy vấn phức tạp. [DynamoDB](http://www.read.seas.harvard.edu/~kohler/class/cs239-w08/decandia07dynamo.pdf) hỗ trợ cả hai kiểu khoá-trị và tài liệu.
A wide column store's basic unit of data is a column (name/value pair). A column can be grouped in column families (analogous to a SQL table). Super column families further group column families. You can access each column independently with a row key, and columns with the same row key form a row. Each value contains a timestamp for versioning and for conflict resolution.
Đơn vị cơ bản của store wide column là cột (một cặp tên/giá trị). Một cột có thể được nhóm vào một column family (tương ứng với một bảng SQL). Super column family nhóm lại cái column family. Ta có thể truy cập vào từng cột một cách độc lập với một khoá hàng (row key), các cột với cùng một khoá hàng tạo thành một hàng. Mỗi giá trị chứa một timestamp cho việc giữ phiên bản và giải quyết xung đột (conflict resolution) # FIXME: "giải quyết xung đột" hardly yield the right Vietnamese interpretation.
Google introduced [Bigtable](http://www.read.seas.harvard.edu/~kohler/class/cs239-w08/chang06bigtable.pdf) as the first wide column store, which influenced the open-source [HBase](https://www.edureka.co/blog/hbase-architecture/) often-used in the Hadoop ecosystem, and [Cassandra](http://docs.datastax.com/en/cassandra/3.0/cassandra/architecture/archIntro.html) from Facebook. Stores such as BigTable, HBase, and Cassandra maintain keys in lexicographic order, allowing efficient retrieval of selective key ranges.
Wide column stores offer high availability and high scalability. They are often used for very large data sets.
##### 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)
In a graph database, each node is a record and each arc is a relationship between two nodes. Graph databases are optimized to represent complex relationships with many foreign keys or many-to-many relationships.
Graphs databases offer high performance for data models with complex relationships, such as a social network. They are relatively new and are not yet widely-used; it might be more difficult to find development tools and resources. Many graphs can only be accessed with [REST APIs](#representational-state-transfer-rest).
* [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)
Caching improves page load times and can reduce the load on your servers and databases. In this model, the dispatcher will first lookup if the request has been made before and try to find the previous result to return, in order to save the actual execution.
Cache cải thiện thời gian tải trang và giảm tải lên máy chủ và cơ sở dữ liệu. Trong mô hình này, bộ điều phối sẽ tra cứu nếu yêu cầu đã được làm trước đây và thử tìm kết quả trước đó để trả về, nhằm mục đích tiết kiệm thời gian thực thi.
Databases often benefit from a uniform distribution of reads and writes across its partitions. Popular items can skew the distribution, causing bottlenecks. Putting a cache in front of a database can help absorb uneven loads and spikes in traffic.
Cơ sở dữ liệu thường được lợi từ sự phân phối đồng đều giữa đọc và ghi giữa các phân vùng. Các mục phổ biến có thể xiên khả năng phân phối, gây nên tắc nghẽn. Đặt cache trước cơ sở dữ liệu có thể giúp hấp thu mất cân bằng tải và "gai" liên lạc thông tin. # FIXME: bad translation, I'm not sure if whether "gai" as "spike" is an acceptable analogy in Vietnamese.
[Reverse proxies](#reverse-proxy-web-server) and caches such as [Varnish](https://www.varnish-cache.org/) can serve static and dynamic content directly. Web servers can also cache requests, returning responses without having to contact application servers.
[Reverse proxy](#reverse-proxy-web-server) và cache như là [Varnish](https://www.varnish-cache.org/) có thể trực tiếp cung cấp nội dung tĩnh hoặc động. Máy chủ web có thể cache request, trả về response mà không cần liên lạc với máy chủ ứng dụng.
Your database usually includes some level of caching in a default configuration, optimized for a generic use case. Tweaking these settings for specific usage patterns can further boost performance.
Cơ sở dữ liệu có thể bao gồm một vài mức độ cache ở cấu hình mặc định, tối ưu hoá cho những mục chung. Chỉnh sửa những cấu hình này cho những kiểu sử dụng đặc thù có thể làm tăng được hiệu suất. # FIXME: bad translation
In-memory caches such as Memcached and Redis are key-value stores between your application and your data storage. Since the data is held in RAM, it is much faster than typical databases where data is stored on disk. RAM is more limited than disk, so [cache invalidation](https://en.wikipedia.org/wiki/Cache_algorithms) algorithms such as [least recently used (LRU)](https://en.wikipedia.org/wiki/Cache_replacement_policies#Least_recently_used_(LRU)) can help invalidate 'cold' entries and keep 'hot' data in RAM.
Redis has the following additional features:
* 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
Generally, you should try to avoid file-based caching, as it makes cloning and auto-scaling more difficult.
### Caching at the database query level
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
### 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
Suggestions of what to cache:
* User sessions
* Fully rendered web pages
* Activity streams
* User graph data
### When to update the cache
Since you can only store a limited amount of data in cache, you'll need to determine which cache update strategy works best for your use case.
#### Cache-aside
<palign="center">
<imgsrc="images/ONjORqk.png">
<br/>
<i><ahref=http://www.slideshare.net/tmatyashovsky/from-cache-to-in-memory-data-grid-introduction-to-hazelcast>Source: From cache to in-memory data grid</a></i>
</p>
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
```python
def get_user(self, user_id):
user = cache.get("user.{0}", user_id)
if user is None:
user = db.query("SELECT * FROM users WHERE user_id = {0}", user_id)
if user is not None:
key = "user.{0}".format(user_id)
cache.set(key, json.dumps(user))
return user
```
[Memcached](https://memcached.org/) is generally used in this manner.
Subsequent reads of data added to cache are fast. Cache-aside is also referred to as lazy loading. Only requested data is cached, which avoids filling up the cache with data that isn't requested.
##### 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.
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 code:
```python
set_user(12345, {"foo":"bar"})
```
Cache code:
```python
def set_user(user_id, values):
user = db.query("UPDATE Users WHERE id = {0}", user_id, values)
cache.set(user_id, user)
```
Write-through is a slow overall operation due to the write operation, but subsequent reads of just written data are fast. Users are generally more tolerant of latency when updating data than reading data. Data in the cache is not stale.
##### 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.
In write-behind, the application does the following:
* 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.
#### Refresh-ahead
<palign="center">
<imgsrc="images/kxtjqgE.png">
<br/>
<i><ahref=http://www.slideshare.net/tmatyashovsky/from-cache-to-in-memory-data-grid-introduction-to-hazelcast>Source: From cache to in-memory data grid</a></i>
</p>
You can configure the cache to automatically refresh any recently accessed cache entry prior to its expiration.
Refresh-ahead can result in reduced latency vs read-through if the cache can accurately predict which items are likely to be needed in the future.
##### 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.
### 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.
### 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/)
<i><ahref=http://lethain.com/introduction-to-architecting-systems-for-scale/#platform_layer>Source: Intro to architecting systems for scale</a></i>
</p>
Asynchronous workflows help reduce request times for expensive operations that would otherwise be performed in-line. They can also help by doing time-consuming work in advance, such as periodic aggregation of data.
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:
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.
**[Redis](https://redis.io/)** is useful as a simple message broker but messages can be lost.
**[RabbitMQ](https://www.rabbitmq.com/)** is popular but requires you to adapt to the 'AMQP' protocol and manage your own nodes.
**[Amazon SQS](https://aws.amazon.com/sqs/)** is hosted but can have high latency and has the possibility of messages being delivered twice.
Tasks queues receive tasks and their related data, runs them, then delivers their results. They can support scheduling and can be used to run computationally-intensive jobs in the background.
Hàng đợi nhận tác vụ và các dữ liệu liên quan, thực thi và giao kết quả. Hàng đợi có thể hỗ trợ lên thời gian biểu và có thể được dùng cho các công việc ngầm.
**[Celery](https://docs.celeryproject.org/en/stable/)** has support for scheduling and primarily has python support.
### Back pressure
If queues start to grow significantly, the queue size can become larger than memory, resulting in cache misses, disk reads, and even slower performance. [Back pressure](http://mechanical-sympathy.blogspot.com/2012/05/apply-back-pressure-when-overloaded.html) can help by limiting the queue size, thereby maintaining a high throughput rate and good response times for jobs already in the queue. Once the queue fills up, clients get a server busy or HTTP 503 status code to try again later. Clients can retry the request at a later time, perhaps with [exponential backoff](https://en.wikipedia.org/wiki/Exponential_backoff).
### 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.
* [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)
HTTP is a method for encoding and transporting data between a client and a server. It is a request/response protocol: clients issue requests and servers issue responses with relevant content and completion status info about the request. HTTP is self-contained, allowing requests and responses to flow through many intermediate routers and servers that perform load balancing, caching, encryption, and compression.
HTTP là một phương thức mã hoá và truyền tải dữ liệu giữa một client và một server. Là một giao thức yêu cầu/hồi đáp (request/response): client phát yêu cầu và server phát hồi đáp với nội dung tương ứng kèm theo thông tin về trạng thái hoàn tất của yêu cầu. HTTP là độc lập (self-contained), chấp nhận yêu cầu và hồi đáp đi qua nhiều router và server trung gian, cân bằng tải; cache, mã hoá, và nén có thể được thực hiện ở các chặng giữa này.
*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)
### Transmission control protocol (TCP)
<palign="center">
<imgsrc="images/JdAsdvG.jpg">
<br/>
<i><ahref=http://www.wildbunny.co.uk/blog/2012/10/09/how-to-make-a-multi-player-game-part-1/>Source: How to make a multiplayer game</a></i>
</p>
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
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.
TCP is useful for applications that require high reliability but are less time critical. Some examples include web servers, database info, SMTP, FTP, and SSH.
<i><ahref=http://www.wildbunny.co.uk/blog/2012/10/09/how-to-make-a-multi-player-game-part-1/>Source: How to make a multiplayer game</a></i>
</p>
UDP is connectionless. Datagrams (analogous to packets) are guaranteed only at the datagram level. Datagrams might reach their destination out of order or not at all. UDP does not support congestion control. Without the guarantees that TCP support, UDP is generally more efficient.
UDP can broadcast, sending datagrams to all devices on the subnet. This is useful with [DHCP](https://en.wikipedia.org/wiki/Dynamic_Host_Configuration_Protocol) because the client has not yet received an IP address, thus preventing a way for TCP to stream without the IP address.
UDP is less reliable but works well in real time use cases such as VoIP, video chat, streaming, and realtime multiplayer games.
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
#### 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)
In an RPC, a client causes a procedure to execute on a different address space, usually a remote server. The procedure is coded as if it were a local procedure call, abstracting away the details of how to communicate with the server from the client program. Remote calls are usually slower and less reliable than local calls so it is helpful to distinguish RPC calls from local calls. Popular RPC frameworks include [Protobuf](https://developers.google.com/protocol-buffers/), [Thrift](https://thrift.apache.org/), and [Avro](https://avro.apache.org/docs/current/).
Trong RPC, client làm cho một thủ tục được thực thi trên một vùng địa chỉ khác, thường là một server từ xa. Thủ tục được lập trình như là một thủ nội bộ, trừu tượng hoá đi chi tiết về cách trao đổi giữa chương trình trên server và chương trình client. Việc gọi từ xa thường là chậm và ít reliable hơn là gọi nội bộ, nên thường nên cần phân hoá giữa RPC và nội bộ. Các framework RPC phổ biết: [Protobuf](https://developers.google.com/protocol-buffers/), [Thrift](https://thrift.apache.org/), và [Avro](https://avro.apache.org/docs/current/).
RPC is focused on exposing behaviors. RPCs are often used for performance reasons with internal communications, as you can hand-craft native calls to better fit your use cases.
RPC chú trọng trên việc phơi bày hành vi. RPC thường được sử dụng cho lí do hiệu năng trong truyền đạt nội bộ, vì lẽ bạn có thể, một cách thủ công, tạo các cuộc gọi native cho phù hợp với nhu cầu.
(Ghi chú của người dịch: ý chính ở đây khi dịch ra tiếng Việt khó thoát nghĩa. Nhìn chung, khi so sánh với các mô hình không phải RPC, thì ta cần thiết kế một lớp trừu tượng hoá. Chẳng hạn với RESTful thì đối tượng trừu tượng chính sẽ là "resource". RPC mô hình hoá một cách trực tiếp hơn: thủ tục gọi thủ tục. Nên sẽ co giãn hơn vì bạn không cần phải bám theo một mô hình nào ở lớp trừu tượng trên. Nhưng đánh đổi bởi "thủ công" là ở việc phải duy trì những thủ tục đối ứng giữa hai bên client và server).
* 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/).
* Client RPC trở nên couple với implementation của service.
* Sẽ cần định nghĩa API mới cho cho mọi operation và use case mới.
* Debug RPC có thể sẽ khó khăn.
* Khó tận dụng các công nghệ có sẵn. Ví dụ, có thể phải bỏ công thêm để [cache RPC call](http://etherealbits.com/2012/12/debunking-the-myths-of-rpc-rest/) trên các cache server như là [Squid](http://www.squid-cache.org/).
REST is an architectural style enforcing a client/server model where the client acts on a set of resources managed by the server. The server provides a representation of resources and actions that can either manipulate or get a new representation of resources. All communication must be stateless and cacheable.
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.
Sample REST calls:
```
GET /someresources/anId
PUT /someresources/anId
{"anotherdata": "another value"}
```
REST is focused on exposing data. It minimizes the coupling between client/server and is often used for public HTTP APIs. REST uses a more generic and uniform method of exposing resources through URIs, [representation through headers](https://github.com/for-GET/know-your-http-well/blob/master/headers.md), and actions through verbs such as GET, POST, PUT, DELETE, and PATCH. Being stateless, REST is great for horizontal scaling and partitioning.
#### 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.
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:
An ninh là một chủ đề rộng. Trừ khi bạn có kinh nghiệm đáng kể, một nền tảng về an ninh, hoặc đang ứng tuyển vào một ví trí đòi hỏi kiến thức về an ninh, hầu như bạn sẽ không cần biết nhiều hơn ở mức cơ bản này:
* 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).
* [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
You'll sometimes be asked to do 'back-of-the-envelope' estimates. For example, you might need to determine how long it will take to generate 100 image thumbnails from disk or how much memory a data structure will take. The **Powers of two table** and **Latency numbers every programmer should know** are handy references.
* [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)
| 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) |
| Design Google docs | [code.google.com](https://code.google.com/p/google-mobwrite/)<br/>[neil.fraser.name](https://neil.fraser.name/writing/sync/) |
| Design a key-value store like Redis | [slideshare.net](http://www.slideshare.net/dvirsky/introduction-to-redis) |
| 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 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) |
| Design the Facebook chat function | [erlang-factory.com](http://www.erlang-factory.com/upload/presentations/31/EugeneLetuchy-ErlangatFacebook.pdf)<br/>[facebook.com](https://www.facebook.com/note.php?note_id=14218138919&id=9445547199&index=0) |
| Design a graph search function like Facebook's | [facebook.com](https://www.facebook.com/notes/facebook-engineering/under-the-hood-building-out-the-infrastructure-for-graph-search/10151347573598920)<br/>[facebook.com](https://www.facebook.com/notes/facebook-engineering/under-the-hood-indexing-and-ranking-in-graph-search/10151361720763920)<br/>[facebook.com](https://www.facebook.com/notes/facebook-engineering/under-the-hood-the-natural-language-interface-of-graph-search/10151432733048920) |
| Design a content delivery network like CloudFlare | [figshare.com](https://figshare.com/articles/Globally_distributed_content_delivery/6605972) |
| Design a trending topic system like Twitter's | [michael-noll.com](http://www.michael-noll.com/blog/2013/01/18/implementing-real-time-trending-topics-in-storm/)<br/>[snikolov .wordpress.com](http://snikolov.wordpress.com/2012/11/14/early-detection-of-twitter-trends/) |
| Design a random ID generation system | [blog.twitter.com](https://blog.twitter.com/2010/announcing-snowflake)<br/>[github.com](https://github.com/twitter/snowflake/) |
| Return the top k requests during a time interval | [cs.ucsb.edu](https://www.cs.ucsb.edu/sites/cs.ucsb.edu/files/docs/reports/2005-23.pdf)<br/>[wpi.edu](http://davis.wpi.edu/xmdv/docs/EDBT11-diyang.pdf) |
| Design a system that serves data from multiple data centers | [highscalability.com](http://highscalability.com/blog/2009/8/24/how-google-serves-data-from-multiple-datacenters.html) |
| Design an online multiplayer card game | [indieflashblog.com](https://web.archive.org/web/20180929181117/http://www.indieflashblog.com/how-to-create-an-asynchronous-multiplayer-game.html)<br/>[buildnewgames.com](http://buildnewgames.com/real-time-multiplayer/) |
| Design a garbage collection system | [stuffwithstuff.com](http://journal.stuffwithstuff.com/2013/12/08/babys-first-garbage-collector/)<br/>[washington.edu](http://courses.cs.washington.edu/courses/csep521/07wi/prj/rick.pdf) |
| Design an API rate limiter | [https://stripe.com/blog/](https://stripe.com/blog/rate-limiters) |
| Design a Stock Exchange (like NASDAQ or Binance) | [Jane Street](https://youtu.be/b1e4t2k2KJY)<br/>[Golang Implementation](https://around25.com/blog/building-a-trading-engine-for-a-crypto-exchange/)<br/>[Go Implemenation](http://bhomnick.net/building-a-simple-limit-order-in-go/) |
| Add a system design question | [Contribute](#contributing) |
| 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 | **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) |
| Data store | **Memcached** - Distributed memory caching system | [slideshare.net](http://www.slideshare.net/oemebamo/introduction-to-memcached) |
| Data store | **Redis** - Distributed memory caching system with persistence and value types | [slideshare.net](http://www.slideshare.net/dvirsky/introduction-to-redis) |
| | | |
| File system | **Google File System (GFS)** - Distributed file system | [research.google.com](http://static.googleusercontent.com/media/research.google.com/zh-CN/us/archive/gfs-sosp2003.pdf) |
| 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) |
| ESPN | [Operating At 100,000 duh nuh nuhs per second](http://highscalability.com/blog/2013/11/4/espns-architecture-at-scale-operating-at-100000-duh-nuh-nuhs.html) |
| Google | [Google architecture](http://highscalability.com/google-architecture) |
| Instagram | [14 million users, terabytes of photos](http://highscalability.com/blog/2011/12/6/instagram-architecture-14-million-users-terabytes-of-photos.html)<br/>[What powers Instagram](http://instagram-engineering.tumblr.com/post/13649370142/what-powers-instagram-hundreds-of-instances) |
| Justin.tv | [Justin.Tv's live video broadcasting architecture](http://highscalability.com/blog/2010/3/16/justintvs-live-video-broadcasting-architecture.html) |
| Facebook | [Scaling memcached at Facebook](https://cs.uwaterloo.ca/~brecht/courses/854-Emerging-2014/readings/key-value/fb-memcached-nsdi-2013.pdf)<br/>[TAO: Facebook’s distributed data store for the social graph](https://cs.uwaterloo.ca/~brecht/courses/854-Emerging-2014/readings/data-store/tao-facebook-distributed-datastore-atc-2013.pdf)<br/>[Facebook’s photo storage](https://www.usenix.org/legacy/event/osdi10/tech/full_papers/Beaver.pdf)<br/>[How Facebook Live Streams To 800,000 Simultaneous Viewers](http://highscalability.com/blog/2016/6/27/how-facebook-live-streams-to-800000-simultaneous-viewers.html) |
| Mailbox | [From 0 to one million users in 6 weeks](http://highscalability.com/blog/2013/6/18/scaling-mailbox-from-0-to-one-million-users-in-6-weeks-and-1.html) |
| Netflix | [A 360 Degree View Of The Entire Netflix Stack](http://highscalability.com/blog/2015/11/9/a-360-degree-view-of-the-entire-netflix-stack.html)<br/>[Netflix: What Happens When You Press Play?](http://highscalability.com/blog/2017/12/11/netflix-what-happens-when-you-press-play.html) |
| Pinterest | [From 0 To 10s of billions of page views a month](http://highscalability.com/blog/2013/4/15/scaling-pinterest-from-0-to-10s-of-billions-of-page-views-a.html)<br/>[18 million visitors, 10x growth, 12 employees](http://highscalability.com/blog/2012/5/21/pinterest-architecture-update-18-million-visitors-10x-growth.html) |
| Playfish | [50 million monthly users and growing](http://highscalability.com/blog/2010/9/21/playfishs-social-gaming-architecture-50-million-monthly-user.html) |
| Tumblr | [15 billion page views a month](http://highscalability.com/blog/2012/2/13/tumblr-architecture-15-billion-page-views-a-month-and-harder.html) |
| Twitter | [Making Twitter 10000 percent faster](http://highscalability.com/scaling-twitter-making-twitter-10000-percent-faster)<br/>[Storing 250 million tweets a day using MySQL](http://highscalability.com/blog/2011/12/19/how-twitter-stores-250-million-tweets-a-day-using-mysql.html)<br/>[150M active users, 300K QPS, a 22 MB/S firehose](http://highscalability.com/blog/2013/7/8/the-architecture-twitter-uses-to-deal-with-150m-active-users.html)<br/>[Timelines at scale](https://www.infoq.com/presentations/Twitter-Timeline-Scalability)<br/>[Big and small data at Twitter](https://www.youtube.com/watch?v=5cKTP36HVgI)<br/>[Operations at Twitter: scaling beyond 100 million users](https://www.youtube.com/watch?v=z8LU0Cj6BOU)<br/>[How Twitter Handles 3,000 Images Per Second](http://highscalability.com/blog/2016/4/20/how-twitter-handles-3000-images-per-second.html) |
| Uber | [How Uber scales their real-time market platform](http://highscalability.com/blog/2015/9/14/how-uber-scales-their-real-time-market-platform.html)<br/>[Lessons Learned From Scaling Uber To 2000 Engineers, 1000 Services, And 8000 Git Repositories](http://highscalability.com/blog/2016/10/12/lessons-learned-from-scaling-uber-to-2000-engineers-1000-ser.html) |
* [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
Feel free to contact me to discuss any issues, questions, or comments.
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).*
Copyright 2017 Donne Martin
Creative Commons Attribution 4.0 International License (CC BY 4.0)
- Partition tolerance: "dung sai phân vùng" (Typically, "dung sai" in Vietnamese is a scalar value, and often accompanied with an unit. So I'm not sure this is the right one.)
