Part 5 Tag Based Caching

Tag Based Caching

Tag based caching is a way to solve the second hard problem in computer science: cache invalidation. I was working on a complex application that generated a ton of HTML. It was very repetitive in nature but highly associative. The same data would be displayed on many different pages. HTML fragments may need to reference many different objects to make it all work. At this scale I could no longer think of individual fragments. I could only think of the objects them selves. I simply wanted to express this statement: expire everything associated with this contact.

Here’s what I was dealing with:

  1. Maintain control over how long things are cached
  2. Large number of different associations. Actions or fragments no longer related to a specific resource.
  3. Content could be invalidated through HTTP requests or any number of background process.
  4. Hard to maintain specific keys. I thought of it as “resources”.

Enter Cashier

There is a ton of cached content in the system. Many different actions and fragments. There was also a cache hierarchy. Expiring a specific fragment would have to expire an action (so a cache miss would occur when a page was requested thus, causing the new fragment to be displayed) while other things on pages are still cached. One question to ask, is how can I expire groups of things based on certain events? Well, first you need a way to associate different keys. Once you can associate different keys, then you can expire them together. Since you’re tracking the keys being sent to Rails.cache, you can simply use Rails.cache to delete them. All of this is possible through one itty-bitty detail of the Rails caching system.

You may have noticed something in the Cache class in the previous section. There is a second argument for options. Anything in the option argument is passed to the cache store. This is where can tie in the grouping logic.

Through all of this trickery, you’ll be able to express this type of statement:

App.cache.expire_tag 'stats'
App.cache.expire_tag @account

The content could from anywhere, but all you know is that’s stale.

This is exactly where Cashier comes in. It (is my gem) that allows you associate actions and fragments with one or more tags, then expire based of tags. Of course you can expire the cache from anywhere in your code. Here are some examples:

caches_action :stats, :tag => proc {|c|

caches_action :show, :tag => 'account'
caches_cation :show, :tag => %w(account customer)

<%= cache @post, :tag => 'customer' do %>

Then you can expire like this:

Cashier.expire 'account' # wipe all keys tagged 'account'

I highly recommend you checkout Cashier. It may be useful in your application especially if you have complicated relationships and high performance requirements.