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<h1>Caching</h1>

<p>Caching can reduce the load on servers by storing the results of common

operations and serving the precomputed answers to clients. </p>

<p>For example, instead of retrieving data from database tables that rarely

change, you can store the values in-memory. Retrieving values from an

in-memory location is far faster than retrieving them from a database (which

stores them on a persistent disk like a hard drive.) When the cached values

change the system can invalidate the cache and re-retrieve the updated values

for future requests.</p>

<p>A cache can be created for multiple layers of the stack. </p>

<h2>Caching backends</h2>

<p><a href="http://memcached.org/">memcached</a> is a common in-memory caching system.</p>

<p><a href="http://redis.io/">Redis</a> is a key-value in-memory data store that can

easily be configured for caching with libraries such as

<a href="https://github.com/sebleier/django-redis-cache">django-redis-cache</a>.</p>

<h2>Caching resources</h2>

<p>"<a href="https://bjornjohansen.no/caching-varnish-or-nginx">Caching: Varnish or Nginx?</a>"

reviews some considerations such as SSL and SPDY support when choosing

reverse proxy Nginx or Varnish.</p>

<p><a href="http://bizcoder.com/caching-is-hard-draw-me-a-picture">Caching is Hard, Draw me a Picture</a>

has diagrams of how web request caching layers work. The post is relevant

reading even though the author is describing his Microsoft code as the

impetus for writing the content.</p>

<h2>Caching learning checklist</h2>

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Analyze your web application for the slowest parts. It's likely there are

complex database queries that can be precomputed and stored in an in-memory

data store.</p>

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Leverage your existing in-memory data store already used for session data

to cache the results of those complex database queries.

A <a href="/task-queues.html">task queue</a> can often be used to precompute the results

on a regular basis and save them in the data store.</p>

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Incorporate a cache invalidation scheme so the precomputed results remain

accurate when served up to the user.</p>

<h3>What do you want to learn now that your app is responding faster?</h3>

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