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 template digests, which change when the template code changes to bust caches on code deploys. The "Russian doll" caching strategy busts caches for a model when related models like reviews are updated. Declaring template dependencies can help Rails determine when to bust caches based on changes to templates or other files.

1. Key-basedcacheexpirationin
Rails4
...or Rails 3 with a dependency
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2. BuildingBlocks(ActiveRecord::Base)
@product.cache_key
=>“products/1281-20130717091609”
“#{name}/#{id}-#{timestamp}”
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3. BuildingBlocks
(ActionView::Helpers::CacheHelper)
- cache @product do
%h1= @product.name
= image_tag @product.image.url
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4. Write fragment
views/products/1-20130728191246157664000
(“views/#{cache_key}”)
- cache @product do
%h1= @product.name
= image_tag @product.image.url
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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(‘/’)}”
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6. “ButwhatifI
change
atemplate?”
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7. Rails.cache.clear
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8. “ButwhatifI
deploy5times
aday”
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9. sucks
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10. Justkidding!™
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11. # Gemfile
gem ‘pg’
gem‘cache_digests’
gem ‘haml-rails’
gem ‘cancan’
# ...
(Rails 3 only – built into actionpack 4)
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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)
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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
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14. templatechange
->newdigest
->newfragmentkey
->bustedcache!
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15. Andwedidn’t
havetodo
anything!
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16. “Russiandoll”cachingstrategy
thenotorious
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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
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18. @review.update!(rating: 3)
@review.product’s key gets bumped as well
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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
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20. the “russian doll”...
index
product
reviews list
review
review
review
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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.
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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
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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)
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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
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