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+# Design a web crawler
+
+*Note: This document links directly to relevant areas found in the [system design topics](https://github.com/donnemartin/system-design-primer-interview#index-of-system-design-topics-1) to avoid duplication.  Refer to the linked content for general talking points, tradeoffs, and alternatives.*
+
+## Step 1: Outline use cases and constraints
+
+> Gather requirements and scope the problem.
+> Ask questions to clarify use cases and constraints.
+> Discuss assumptions.
+
+Without an interviewer to address clarifying questions, we'll define some use cases and constraints.
+
+### Use cases
+
+#### We'll scope the problem to handle only the following use cases
+
+* **Service** crawls a list of urls:
+    * Generates reverse index of words to pages containing the search terms
+    * Generates titles and snippets for pages
+        * Title and snippets are static, they do not change based on search query
+* **User** inputs a search term and sees a list of relevant pages with titles and snippets  the crawler generated
+    * Only sketch high level components and interactions for this use case, no need to go into depth
+* **Service** has high availability
+
+#### Out of scope
+
+* Search analytics
+* Personalized search results
+* Page rank
+
+### Constraints and assumptions
+
+#### State assumptions
+
+* Traffic is not evenly distributed
+    * Some searches are very popular, while others are only executed once
+* Support only anonymous users
+* Generating search results should be fast
+* The web crawler should not get stuck in an infinite loop
+    * We get stuck in an infinite loop if the graph contains a cycle
+* 1 billion links to crawl
+    * Pages need to be crawled regularly to ensure freshness
+    * Average refresh rate of about once per week, more frequent for popular sites
+        * 4 billion links crawled each month
+    * Average stored size per web page: 500 KB
+        * For simplicity, count changes the same as new pages
+* 100 billion searches per month
+
+Exercise the use of more traditional systems - don't use existing systems such as [solr](http://lucene.apache.org/solr/) or [nutch](http://nutch.apache.org/).
+
+#### Calculate usage
+
+**Clarify with your interviewer if you should run back-of-the-envelope usage calculations.**
+
+* 2 PB of stored page content per month
+    * 500 KB per page * 4 billion links crawled per month
+    * 72 PB of stored page content in 3 years
+* 1,600 write requests per second
+* 40,000 search requests per second
+
+Handy conversion guide:
+
+* 2.5 million seconds per month
+* 1 request per second = 2.5 million requests per month
+* 40 requests per second = 100 million requests per month
+* 400 requests per second = 1 billion requests per month
+
+## Step 2: Create a high level design
+
+> Outline a high level design with all important components.
+
+![Imgur](http://i.imgur.com/xjdAAUv.png)
+
+## Step 3: Design core components
+
+> Dive into details for each core component.
+
+### Use case: Service crawls a list of urls
+
+We'll assume we have an initial list of `links_to_crawl` ranked initially based on overall site popularity.  If this is not a reasonable assumption, we can seed the crawler with popular sites that link to outside content such as [Yahoo](https://www.yahoo.com/), [DMOZ](http://www.dmoz.org/), etc
+
+We'll use a table `crawled_links` to store processed links and their page signatures.
+
+We could store `links_to_crawl` and `crawled_links` in a key-value **NoSQL Database**.  For the ranked links in `links_to_crawl`, we could use [Redis](https://redis.io/) with sorted sets to maintain a ranking of page links.  We should discuss the [use cases and tradeoffs between choosing SQL or NoSQL](https://github.com/donnemartin/system-design-primer-interview#sql-or-nosql).
+
+* The **Crawler Service** processes each page link by doing the following in a loop:
+    * Takes the top ranked page link to crawl
+        * Checks `crawled_links` in the **NoSQL Database** for an entry with a similar page signature
+            * If we have a similar page, reduces the priority of the page link
+                * This prevents us from getting into a cycle
+                * Continue
+            * Else, crawls the link
+                * Adds a job to the **Reverse Index Service** queue to generate a [reverse index](https://en.wikipedia.org/wiki/Search_engine_indexing)
+                * Adds a job to the **Document Service** queue to generate a static title and snippet
+                * Generates the page signature
+                * Removes the link from `links_to_crawl` in the **NoSQL Database**
+                * Inserts the page link and signature to `crawled_links` in the **NoSQL Database**
+
+**Clarify with your interviewer how much code you are expected to write**.
+
+`PagesDataStore` is an abstraction within the **Crawler Service** that uses the **NoSQL Database**:
+
+```
+class PagesDataStore(object):
+
+    def __init__(self, db);
+        self.db = db
+        ...
+
+    def add_link_to_crawl(self, url):
+        """Add the given link to `links_to_crawl`."""
+        ...
+
+    def remove_link_to_crawl(self, url):
+        """Remove the given link from `links_to_crawl`."""
+        ...
+
+    def reduce_priority_link_to_crawl(self, url)
+        """Reduce the priority of a link in `links_to_crawl` to avoid cycles."""
+        ...
+
+    def extract_max_priority_page(self):
+        """Return the highest priority link in `links_to_crawl`."""
+        ...
+
+    def insert_crawled_link(self, url, signature):
+        """Add the given link to `crawled_links`."""
+        ...
+
+    def crawled_similar(self, signature):
+        """Determine if we've already crawled a page matching the given signature"""
+        ...
+```
+
+`Page` is an abstraction within the **Crawler Service** that encapsulates a page, its contents, child urls, and signature:
+
+```
+class Page(object):
+
+    def __init__(self, url, contents, child_urls, signature):
+        self.url = url
+        self.contents = contents
+        self.child_urls = child_urls
+        self.signature = signature
+```
+
+`Crawler` is the main class within **Crawler Service**, composed of `Page` and `PagesDataStore`.
+
+```
+class Crawler(object):
+
+    def __init__(self, data_store, reverse_index_queue, doc_index_queue):
+        self.data_store = data_store
+        self.reverse_index_queue = reverse_index_queue
+        self.doc_index_queue = doc_index_queue
+
+    def create_signature(self, page):
+        """Create signature based on url and contents."""
+        ...
+
+    def crawl_page(self, page):
+        for url in page.child_urls:
+            self.data_store.add_link_to_crawl(url)
+        page.signature = self.create_signature(page)
+        self.data_store.remove_link_to_crawl(page.url)
+        self.data_store.insert_crawled_link(page.url, page.signature)
+
+    def crawl(self):
+        while True:
+            page = self.data_store.extract_max_priority_page()
+            if page is None:
+                break
+            if self.data_store.crawled_similar(page.signature):
+                self.data_store.reduce_priority_link_to_crawl(page.url)
+            else:
+                self.crawl_page(page)
+```
+
+### Handling duplicates
+
+We need to be careful the web crawler doesn't get stuck in an infinite loop, which happens when the graph contains a cycle.
+
+**Clarify with your interviewer how much code you are expected to write**.
+
+We'll want to remove duplicate urls:
+
+* For smaller lists we could use something like `sort | unique`
+* With 1 billion links to crawl, we could use **MapReduce** to output only entries that have a frequency of 1
+
+```
+class RemoveDuplicateUrls(MRJob):
+
+    def mapper(self, _, line):
+        yield line, 1
+
+    def reducer(self, key, values):
+        total = sum(values)
+        if total == 1:
+            yield key, total
+```
+
+Detecting duplicate content is more complex.  We could generate a signature based on the contents of the page and compare those two signatures for similarity.  Some potential algorithms are [Jaccard index](https://en.wikipedia.org/wiki/Jaccard_index) and [cosine similarity](https://en.wikipedia.org/wiki/Cosine_similarity).
+
+### Determining when to update the crawl results
+
+Pages need to be crawled regularly to ensure freshness.  Crawl results could have a `timestamp` field that indicates the last time a page was crawled.  After a default time period, say one week, all pages should be refreshed.  Frequently updated or more popular sites could be refreshed in shorter intervals.
+
+Although we won't dive into details on analytics, we could do some data mining to determine the mean time before a particular page is updated, and use that statistic to determine how often to re-crawl the page.
+
+We might also choose to support a `Robots.txt` file that gives webmasters control of crawl frequency.
+
+### Use case: User inputs a search term and sees a list of relevant pages with titles and snippets
+
+* The **Client** sends a request to the **Web Server**, running as a [reverse proxy](https://github.com/donnemartin/system-design-primer-interview#reverse-proxy-web-server)
+* The **Web Server** forwards the request to the **Query API** server
+* The **Query API** server does does the following:
+    * Parses the query
+        * Removes markup
+        * Breaks up the text into terms
+        * Fixes typos
+        * Normalizes capitalization
+        * Converts the query to use boolean operations
+    * Uses the **Reverse Index Service** to find documents matching the query
+        * The **Reverse Index Service** ranks the matching results and returns the top ones
+    * Uses the **Document Service** to return titles and snippets
+
+We'll use a public [**REST API**](https://github.com/donnemartin/system-design-primer-interview##representational-state-transfer-rest):
+
+```
+$ curl https://search.com/api/v1/search?query=hello+world
+```
+
+Response:
+
+```
+{
+    "title": "foo's title",
+    "snippet": "foo's snippet",
+    "link": "https://foo.com",
+},
+{
+    "title": "bar's title",
+    "snippet": "bar's snippet",
+    "link": "https://bar.com",
+},
+{
+    "title": "baz's title",
+    "snippet": "baz's snippet",
+    "link": "https://baz.com",
+},
+```
+
+For internal communications, we could use [Remote Procedure Calls](https://github.com/donnemartin/system-design-primer-interview#remote-procedure-call-rpc).
+
+## Step 4: Scale the design
+
+> Identify and address bottlenecks, given the constraints.
+
+![Imgur](http://i.imgur.com/bWxPtQA.png)
+
+**Important: Do not simply jump right into the final design from the initial design!**
+
+State you would 1) **Benchmark/Load Test**, 2) **Profile** for bottlenecks 3) address bottlenecks while evaluating alternatives and trade-offs, and 4) repeat.  See [Design a system that scales to millions of users on AWS]() as a sample on how to iteratively scale the initial design.
+
+It's important to discuss what bottlenecks you might encounter with the initial design and how you might address each of them.  For example, what issues are addressed by adding a **Load Balancer** with multiple **Web Servers**?  **CDN**?  **Master-Slave Replicas**?  What are the alternatives and **Trade-Offs** for each?
+
+We'll introduce some components to complete the design and to address scalability issues.  Internal load balancers are not shown to reduce clutter.
+
+*To avoid repeating discussions*, refer to the following [system design topics](https://github.com/donnemartin/system-design-primer-interview#) for main talking points, tradeoffs, and alternatives:
+
+* [DNS](https://github.com/donnemartin/system-design-primer-interview#domain-name-system)
+* [Load balancer](https://github.com/donnemartin/system-design-primer-interview#load-balancer)
+* [Horizontal scaling](https://github.com/donnemartin/system-design-primer-interview#horizontal-scaling)
+* [Web server (reverse proxy)](https://github.com/donnemartin/system-design-primer-interview#reverse-proxy-web-server)
+* [API server (application layer)](https://github.com/donnemartin/system-design-primer-interview#application-layer)
+* [Cache](https://github.com/donnemartin/system-design-primer-interview#cache)
+* [NoSQL](https://github.com/donnemartin/system-design-primer-interview#nosql)
+* [Consistency patterns](https://github.com/donnemartin/system-design-primer-interview#consistency-patterns)
+* [Availability patterns](https://github.com/donnemartin/system-design-primer-interview#availability-patterns)
+
+Some searches are very popular, while others are only executed once.  Popular queries can be served from a **Memory Cache** such as Redis or Memcached to reduce response times and to avoid overloading the **Reverse Index Service** and **Document Service**.  The **Memory Cache** is also useful for handling the unevenly distributed traffic and traffic spikes.  Reading 1 MB sequentially from memory takes about 250 microseconds, while reading from SSD takes 4x and from disk takes 80x longer.<sup><a href=https://github.com/donnemartin/system-design-primer-interview#latency-numbers-every-programmer-should-know>1</a></sup>
+
+Below are a few other optimizations to the **Crawling Service**:
+
+* To handle the data size and request load, the **Reverse Index Service** and **Document Service** will likely need to make heavy use sharding and replication.
+* DNS lookup can be a bottleneck, the **Crawler Service** can keep its own DNS lookup that is refreshed periodically
+* The **Crawler Service** can improve performance and reduce memory usage by keeping many open connections at a time, referred to as [connection pooling](https://en.wikipedia.org/wiki/Connection_pool)
+    * Switching to [UDP](https://github.com/donnemartin/system-design-primer-interview#user-datagram-protocol-udp) could also boost performance
+* Web crawling is bandwidth intensive, ensure there is enough bandwidth to sustain high throughput
+
+## Additional talking points
+
+> Additional topics to dive into, depending on the problem scope and time remaining.
+
+### SQL scaling patterns
+
+* [Read replicas](https://github.com/donnemartin/system-design-primer-interview#master-slave)
+* [Federation](https://github.com/donnemartin/system-design-primer-interview#federation)
+* [Sharding](https://github.com/donnemartin/system-design-primer-interview#sharding)
+* [Denormalization](https://github.com/donnemartin/system-design-primer-interview#denormalization)
+* [SQL Tuning](https://github.com/donnemartin/system-design-primer-interview#sql-tuning)
+
+#### NoSQL
+
+* [Key-value store](https://github.com/donnemartin/system-design-primer-interview#)
+* [Document store](https://github.com/donnemartin/system-design-primer-interview#)
+* [Wide column store](https://github.com/donnemartin/system-design-primer-interview#)
+* [Graph database](https://github.com/donnemartin/system-design-primer-interview#)
+* [SQL vs NoSQL](https://github.com/donnemartin/system-design-primer-interview#)
+
+### Caching
+
+* Where to cache
+    * [Client caching](https://github.com/donnemartin/system-design-primer-interview#client-caching)
+    * [CDN caching](https://github.com/donnemartin/system-design-primer-interview#cdn-caching)
+    * [Web server caching](https://github.com/donnemartin/system-design-primer-interview#web-server-caching)
+    * [Database caching](https://github.com/donnemartin/system-design-primer-interview#database-caching)
+    * [Application caching](https://github.com/donnemartin/system-design-primer-interview#application-caching)
+* What to cache
+    * [Caching at the database query level](https://github.com/donnemartin/system-design-primer-interview#caching-at-the-database-query-level)
+    * [Caching at the object level](https://github.com/donnemartin/system-design-primer-interview#caching-at-the-object-level)
+* When to update the cache
+    * [Cache-aside](https://github.com/donnemartin/system-design-primer-interview#cache-aside)
+    * [Write-through](https://github.com/donnemartin/system-design-primer-interview#write-through)
+    * [Write-behind (write-back)](https://github.com/donnemartin/system-design-primer-interview#write-behind-write-back)
+    * [Refresh ahead](https://github.com/donnemartin/system-design-primer-interview#refresh-ahead)
+
+### Asynchronism and microservices
+
+* [Message queues](https://github.com/donnemartin/system-design-primer-interview#)
+* [Task queues](https://github.com/donnemartin/system-design-primer-interview#)
+* [Back pressure](https://github.com/donnemartin/system-design-primer-interview#)
+* [Microservices](https://github.com/donnemartin/system-design-primer-interview#)
+
+### Communications
+
+* Discuss tradeoffs:
+    * External communication with clients - [HTTP APIs following REST](https://github.com/donnemartin/system-design-primer-interview#representational-state-transfer-rest)
+    * Internal communications - [RPC](https://github.com/donnemartin/system-design-primer-interview#remote-procedure-call-rpc)
+* [Service discovery](https://github.com/donnemartin/system-design-primer-interview#service-discovery)
+
+### Security
+
+Refer to the [security section](https://github.com/donnemartin/system-design-primer-interview#security).
+
+### Latency numbers
+
+See [Latency numbers every programmer should know](https://github.com/donnemartin/system-design-primer-interview#latency-numbers-every-programmer-should-know).
+
+### Ongoing
+
+* Continue benchmarking and monitoring your system to address bottlenecks as they come up
+* Scaling is an iterative process
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diff --git a/solutions/system_design/web_crawler/web_crawler_mapreduce.py b/solutions/system_design/web_crawler/web_crawler_mapreduce.py
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+# -*- coding: utf-8 -*-
+
+from mrjob.job import MRJob
+
+
+class RemoveDuplicateUrls(MRJob):
+
+    def mapper(self, _, line):
+        yield line, 1
+
+    def reducer(self, key, values):
+        total = sum(values)
+        if total == 1:
+            yield key, total
+
+    def steps(self):
+        """Run the map and reduce steps."""
+        return [
+            self.mr(mapper=self.mapper,
+                    reducer=self.reducer)
+        ]
+
+
+if __name__ == '__main__':
+    RemoveDuplicateUrls.run()
diff --git a/solutions/system_design/web_crawler/web_crawler_snippets.py b/solutions/system_design/web_crawler/web_crawler_snippets.py
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+++ b/solutions/system_design/web_crawler/web_crawler_snippets.py
@@ -0,0 +1,72 @@
+# -*- coding: utf-8 -*-
+
+class PagesDataStore(object):
+
+    def __init__(self, db);
+        self.db = db
+        ...
+
+    def add_link_to_crawl(self, url):
+        """Add the given link to `links_to_crawl`."""
+        ...
+
+    def remove_link_to_crawl(self, url):
+        """Remove the given link from `links_to_crawl`."""
+        ...
+
+    def reduce_priority_link_to_crawl(self, url)
+        """Reduce the priority of a link in `links_to_crawl` to avoid cycles."""
+        ...
+
+    def extract_max_priority_page(self):
+        """Return the highest priority link in `links_to_crawl`."""
+        ...
+
+    def insert_crawled_link(self, url, signature):
+        """Add the given link to `crawled_links`."""
+        ...
+
+    def crawled_similar(self, signature):
+        """Determine if we've already crawled a page matching the given signature"""
+        ...
+
+
+class Page(object):
+
+    def __init__(self, url, contents, child_urls):
+        self.url = url
+        self.contents = contents
+        self.child_urls = child_urls
+        self.signature = self.create_signature()
+
+    def create_signature(self):
+        # Create signature based on url and contents
+        ...
+
+
+class Crawler(object):
+
+    def __init__(self, pages, data_store, reverse_index_queue, doc_index_queue):
+        self.pages = pages
+        self.data_store = data_store
+        self.reverse_index_queue = reverse_index_queue
+        self.doc_index_queue = doc_index_queue
+
+    def crawl_page(self, page):
+        for url in page.child_urls:
+            self.data_store.add_link_to_crawl(url)
+        self.reverse_index_queue.generate(page)
+        self.doc_index_queue.generate(page)
+        self.data_store.remove_link_to_crawl(page.url)
+        self.data_store.insert_crawled_link(page.url, page.signature)
+
+    def crawl(self):
+        while True:
+            page = self.data_store.extract_max_priority_page()
+            if page is None:
+                break
+            if self.data_store.crawled_similar(page.signature):
+                self.data_store.reduce_priority_link_to_crawl(page.url)
+            else:
+                self.crawl_page(page)
+            page = self.data_store.extract_max_priority_page()