Caching in RailsVysakh Sreenivasan

Only 2 hard Problems in CS

● Naming

● Cache invalidation

Caching in a web app● Prevent hitting databases.

● Cache the html responses.

● Help browser implement caching.

Before implementing caching we need a cache server

Memcached is the popular cache server

sudo apt-get install memcached

Before implementing caching in Rails we need to configure

config.action_controller.perform_caching = true

Using memcached with Rails 4.0

● gem ‘dalli’

● config.cache_store = :mem_cache_store

Lets get to the implementation

#1 Model level caching

Rails.cache.fetch

● First time, it can’t fetch a cache, so it will write a cache.

● Then it reads those caches.

Rails.cache.delete

● When passed an argument, it deletes the specific cache.

● You can clear caches in hooks like after_save

You can also use this to cache API results

#2 Fragment caching

<% cache @post do %>

<h3> <%= @post.title %> </h3>

<% end %>

cache_key: views/posts/1-201505056193031061005000/bea67108094918eeba42cd4a6e786901

based on @post.cache_key which is based on updated_at

md5 hash based on the view’s contents

Cache key changes when

● the record is updated -> a new cache_key is formed

● the view is changed -> a new md5 key is generated

The old keys will be there, which will be removed

automatically by memcached

#3 http caching

e-tag & 304 not modified

fresh_when(@post)

# or

fresh_when(etag: @post , last_modified: @post.updated_at)

based on @post.cache_key which is based on updated_at

if stale?(@post)

respond_to do |f|

…

end

end

Alternate syntax. Especially when using respond_to

def show

@post = Post.first

expires_in 5.minutes

end

Cache control: max_age

Recap1 Model level

● Rails.cache.fetch

● Rails.cache.delete

2 Fragment

<% cache @post do %>

<% end %>

3. etags

if stale?(@post)

Resources

● Docs: http://guides.rubyonrails.org/caching_with_rails.html

● Railscasts episodes on http caching, low level caching,

cache digests, dalli gem.