Advanced Caching in Rails

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Advanced Caching in Rails Presented with love in Helsinki for Frozen Rails 2012

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Tervetuloa! • Olen Adam Hawkins • @adman65 • gh://twinturbo • broadcastingadam.com • https://speakerdeck.com/u/twinturbo • Lead code money at Radium

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You May Have Read This

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Zero to Sixty

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HTTP Page Action Fragment Rails.cache Rack::Cache Rack::ConditionalGet Rack::ETag

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HTTP Page Action Fragment Rails.cache Rack::Cache Rack::ConditionalGet Rack::ETag 200 or 304 • Cache-Control • ETag • If-None-Match • If-Modiﬁed-Since • Protocol level caching -- take advantage!

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HTTP Page Action Fragment Rails.cache Rack::Cache Rack::ConditionalGet Rack::ETag Middleware to support HTTP Caching

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HTTP Page Action Fragment Rails.cache Rack::Cache Rack::ConditionalGet Rack::ETag • Follows HTTP caching rules • Respect freshness • Stores cacheable content in it’s own store • Forwards request down the stack

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HTTP Page Action Fragment Rails.cache Rack::Cache Rack::ConditionalGet Rack::ETag • Adds a 304 Not Modiﬁed header • Continues the request based on freshness

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HTTP Page Action Fragment Rails.cache Rack::Cache Rack::ConditionalGet Rack::ETag • Adds an ETag header if applicable • ETag is hash of the body

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HTTP Page Action Fragment Rails.cache Rack::Cache Rack::ConditionalGet Rack::ETag LOL, No

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HTTP Page Action Fragment Rails.cache Rack::Cache Rack::ConditionalGet Rack::ETag • Captures body and headers • Dump it in Rails.cache • Like HTTP caching, but worse

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HTTP Page Action Fragment Rails.cache Rack::Cache Rack::ConditionalGet Rack::ETag • Traditionally used to cache HTML • Only usable in views

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HTTP Page Action Fragment Rails.cache Rack::Cache Rack::ConditionalGet Rack::ETag • Writes content to the store • Usually Memcache • Actions and Fragments are built on this • Can cache whatever you like • Use it

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# HTTP Caching def show # 304 if fresh # else respond_with if stale? @post respond_with @post end end

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# Action Caching class PostsController < ApplicationController caches_action :index end

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# Fragment Caching <% cache 'posts' do %> <% end %>

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# Rails.cache Rails.cache.write("hash", {:this => :hash}) Rails.cache.write "foo", "bar" Rails.cache.read "foo" Rails.cache.read "hash" Rails.cache.read "foo"

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What do I use? • Rails.cache when you want to cache arbitrary computations (anywhere in your code!) • HTTP caching everywhere • Fragment caching for HTML • Action and page caching, not so much

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Hyvä, we can cache things

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Perkele! Now we to expire things

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“There are only two hard problems in Computer Science: GNU/Linux and cache invalidation” - RMS paraphrasing Phil Karlton

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Why So Hard? • Data is usually associated or dependent • Data is usually used to compose views • Output depend on different parts of data • Data can change in many places • Not all changes happen during HTTP requests (worker queues) • This is actually really hard

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Approaches • Dependency graphs • Auto-expiring cache keys • Manual expiration (puke) • Break cacheable content into smaller discrete chunks

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Dependency Graphs • The most complex thing that could possibly work • Track associations • Walk the graph to connected nodes when one node changes. Expiring nodes as you go along • gh://jcoglan/primer

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Manual Expiration • Rails.cache.delete “foo” • Sweepers -- seriously who use these? • Extremely complex because you must know every possible cached thing ever • Waste of time, don’t do it

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Auto-expiring Keys • The easiest thing that could possibly work • No need to manually expire • Cache key represents object state • Includes a hash or timestamp • PROTIP: reduce data to smallest unit where changes can be tracked.

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/posts/1/12348971239487 updated_at or last modiﬁed date

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Damn, that was easy

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Use Cases

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# Caching Collections class ActiveRecord::Relation def cache_key scoped.maximum(:updated_at).to_i end end # In your view for fragments cache @posts # for HTTP caching fresh? @posts

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• Cache one thing that caches another • Simple example: a cache list of posts. Each post is cached as well. List of posts uses a composite cache key. Expiring one post expires the list. Rerendering the list only needs the updated post. Everything else is still cached. • Magically coming to Rails 4* Russian Doll Caching

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<%# _posts.erb %> <%# the _posts partial gets linked to the _post partial %> <%# so a change there will expire this partial %> <%= render :partial => "posts" %> <%# _post.erb %> <%= post.title %> <%# you get some crazy cache key %> views/posts/065816632-20120810191209/d946707c67ab41d93cb2 Input hash. Input changes expire cache keys.

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JSON APIs w/AMS • AMS = ActiveModel::Serializers • HTTP caching everywhere • Auto expiring cache keys • Cache individual JSON fragments, responses composed of discrete JSON fragments. • belongs_to :touch => true • Russian doll caching for collections and records with associations

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# ActiveModel::Serializers Patch class PostSerializer < ActiveModel::Serializer # cache JSON string def to_json(*args) Rails.cache.fetch "/posts/#{object.cache_key}/to_json" do super end end # cache generated hash def serializable_hash(*args) Rails.cache.fetch "/posts/#{object.cache_key}/hash" do super end end end

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Date Handling • Write the UTC date into a data-timestamp attribute. • Use javascript to parse timestamps into local dates on the frontend

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Data Sharing • Eliminate permutations by using one generic copy. • Saves space in memcache • Example: the current user’s name is replaced with “You” on pages. The underlying view contains all the names. Use Javascript to replace the current user’s name with “You”.

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Wrap-Up • Use HTTP caching everywhere • Use auto expiring keys everywhere (belongs_to :touch => true) • Don’t use sweepers, page or action caching • Beware of things that happen outside the HTTP Request • So much to cover, not enough time.

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Questions? Read This • http://www.broadcastingadam.com/ 2012/07/advanced_caching_revised/