Advanced Caching in Rails

Presented at Frozen Rails in Helsinki 2012

Transcript

1. Advanced Caching in Rails Presented with love in Helsinki for Frozen Rails 2012
2. Tervetuloa! • Olen Adam Hawkins • @adman65 • gh://twinturbo • broadcastingadam.com • https://speakerdeck.com/u/twinturbo • Lead code money at Radium
3. You May Have Read This
4. Zero to Sixty
5. HTTP Page Action Fragment Rails.cache Rack::Cache Rack::ConditionalGet Rack::ETag
6. 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!
7. HTTP Page Action Fragment Rails.cache Rack::Cache Rack::ConditionalGet Rack::ETag Middleware to support HTTP Caching
8. 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
9. HTTP Page Action Fragment Rails.cache Rack::Cache Rack::ConditionalGet Rack::ETag • Adds a 304 Not Modified header • Continues the request based on freshness
10. HTTP Page Action Fragment Rails.cache Rack::Cache Rack::ConditionalGet Rack::ETag • Adds an ETag header if applicable • ETag is hash of the body
11. HTTP Page Action Fragment Rails.cache Rack::Cache Rack::ConditionalGet Rack::ETag LOL, No
12. 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
13. HTTP Page Action Fragment Rails.cache Rack::Cache Rack::ConditionalGet Rack::ETag • Traditionally used to cache HTML • Only usable in views
14. 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
15. # HTTP Caching def show # 304 if fresh # else respond_with if stale? @post respond_with @post end end
16. # Action Caching class PostsController < ApplicationController caches_action :index end
17. # Fragment Caching <% cache 'posts' do %> <!-- HTML HERE --> <% end %>
18. # 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"
19. 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
20. Hyvä, we can cache things
21. Perkele! Now we to expire things
24. “There are only two hard problems in Computer Science: GNU/Linux and cache invalidation” - RMS paraphrasing Phil Karlton
25. 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
26. Approaches • Dependency graphs • Auto-expiring cache keys • Manual expiration (puke) • Break cacheable content into smaller discrete chunks
27. 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
28. 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
29. 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.
30. /posts/1/12348971239487 updated_at or last modified date
31. Damn, that was easy
33. Use Cases
34. # 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
35. • 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
36. <%# _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.
37. 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
38. # 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
39. Date Handling • Write the UTC date into a data-timestamp attribute. • Use javascript to parse timestamps into local dates on the frontend
40. 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”.
41. 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.
42. Questions? Read This • http://www.broadcastingadam.com/ 2012/07/advanced_caching_revised/