Key-based caching in Rails uses cache keys that include the model id and a timestamp to automatically invalidate caches when the model is updated. Cache keys can also include extra arguments like view templates, and the "cache_digests" gem allows caching fragments based on a digest of the template and its dependencies so changes to templates invalidate caches. This provides a "Russian doll" caching strategy where caching nested objects invalidates parent caches.

1. Key-basedcacheexpirationin
Rails4
...or Rails 3 with a dependency
Wednesday, July 31, 13

2. BuildingBlocks(ActiveRecord::Base)
@product.cache_key
=>“products/1281-20130717091609”
“#{name}/#{id}-#{timestamp}”
Wednesday, July 31, 13

3. BuildingBlocks
(ActionView::Helpers::CacheHelper)
- cache @product do
%h1= @product.name
= image_tag @product.image.url
Wednesday, July 31, 13

4. Write fragment
views/products/1-20130728191246157664000
(“views/#{cache_key}”)
- cache @product do
%h1= @product.name
= image_tag @product.image.url
Wednesday, July 31, 13

5. Youcancomposefragmentkeys,too...
- cache [@product, “reviews”] do
%h1= @product.name
= image_tag @product.image.url
Write fragment
views/products/1-20130728191246157664000/reviews
“views/#{cache_key}/#{extra_args.join(‘/’)}”
Wednesday, July 31, 13

6. “ButwhatifI
change
atemplate?”
Wednesday, July 31, 13

7. Rails.cache.clear
Wednesday, July 31, 13

8. “ButwhatifI
deploy5times
aday”
Wednesday, July 31, 13

9. sucks
Wednesday, July 31, 13

10. Justkidding!™
Wednesday, July 31, 13

11. # Gemfile
gem ‘pg’
gem‘cache_digests’
gem ‘haml-rails’
gem ‘cancan’
# ...
(Rails 3 only – built into actionpack 4)
Wednesday, July 31, 13

12. Write fragment
views/products/1-20130728191246157664000/3b2b93d56d331aa7a1bf9267a656d3c9
(“views/#{cache_key}/#{template_digest}”)
- cache @product do
%h1= @product.name
= image_tag @product.image.url
Withcache_digests...
note: template_digest is a digest of the current template and all of it’s dependencies (more on that later)
Wednesday, July 31, 13

13. Cache digest for products/_product.html: 42eb891316f21e68798de5d5aa8021d5
Read fragment
views/products/1-20130728191246157664000/42eb891316f21e68798de5d5aa8021d5
Write fragment
views/products/1-20130728191246157664000/42eb891316f21e68798de5d5aa8021d5
- cache @product do
%h1= @product.name
- # image_tag @product.image.url
...changethetemplate
Wednesday, July 31, 13

14. templatechange
->newdigest
->newfragmentkey
->bustedcache!
Wednesday, July 31, 13

15. Andwedidn’t
havetodo
anything!
Wednesday, July 31, 13

16. “Russiandoll”cachingstrategy
thenotorious
Wednesday, July 31, 13

17. class Product < ActiveRecord::Base
has_many :reviews
end
class Review < ActiveRecord::Base
belongs_to :product, touch: true
end
Product’s cache key now dependent on its reviews
Wednesday, July 31, 13

18. @review.update!(rating: 3)
@review.product’s key gets bumped as well
Wednesday, July 31, 13

19. the “russian doll”...
# products/index.haml
- cache [@products.order(“updated_at ASC”).last, “index”] do
%ul.products
- @products.each do |product|
= render product
# products/_product.haml
- cache product do
%li.product
%h1= product.name
%h2 Reviews!
%ul.reviews
- product.reviews.each do |review|
= render review
# reviews/_review.haml
- cache review do
%li.review
%h4= “#{review.rating} stars!”
%p.content= review.content
Wednesday, July 31, 13

20. the “russian doll”...
index
product
reviews list
review
review
review
Wednesday, July 31, 13

21. @product.reviews.last.update!(rating: 5)
Last review will get re-rendered (and cached)
and container (review list) will get busted -
but when list re-renders all but this last
review can be fetched from cache.
Wednesday, July 31, 13

22. View the template dependency tree...
$ rake cache_digests:nested_dependencies TEMPLATE=your/template/name
ex:
and rake task for Rails 4: https://gist.github.com/nzaillian/6127613
Wednesday, July 31, 13

23. Explicit dependency
If Rails just can’t seem to figure out your dependency you can declare an explicit
dependency in a template by including a comment with a special form:
- # Template Dependency: reviews/summary
(...but you shouldn’t ever have to do this)
Wednesday, July 31, 13

24. OtherResources
https://github.com/rails/cache_digests
http://37signals.com/svn/posts/3113-how-key-based-cache-expiration-works
http://www.youtube.com/watch?v=yhseQP52yIY&t=0:39:39
mustwatch!(DHH’sRailsconf2013outlineofkey-basedcaching)
http://api.rubyonrails.org/classes/ActionView/Helpers/CacheHelper.html
Wednesday, July 31, 13