ACTIVE Network API Developer Blog

Russian Doll Caching with Elasticsearch

Here at Active, we eat our own dog food, which means our primary data store for information about our events on comes through the same API that you all use. We’re also religiously focused on improving the load times of our applications, especially of itself. For those of you who don’t know, is a ruby application, built with Rails. Like most frameworks, Rails can lose a lot of time to compiling it’s view templates with new information, and we’re no exception. On our event details pages, we spend an inordinate amount of time doing so, then cache it at Memcached for ~15 mins, which helps the overall response times (and during high traffic events). However when people request an event that isn’t already stored at this cache that full recalculation has to occur, which hurts our 98th percentile numbers, and more importantly hurts our users who just come to check out a new event.

This is all made more complicated by the fact that each of our event assets have child assets representing sub events, pricing information, etc. Rails 4 solved this problem by implementing Russian Doll Caching (a nested form of generational caching explained well here). Obviously this would be our first choice, however we would rather avoid linking directly to our database, as we prefer to continue eating our own dog food.

We did come up with a solution, a way to implement the Russian Doll cache on our site and expose it to you all too! The _version field in an ElasticSearch document provides a definitive version for a document. Thus we are now passing that through our api in a new field called “assetVersion” you’ll find in your api calls to Activity Search v2. All children of a given asset will now either be indexed with new guids or their versions will be incremented when they are changed. Roll the cache digests gem in, and you can now construct a functional Russian Doll Caching scheme backed by ElasticSearch.

class EventController < ApplicationController
  def show
    @event = ACTV.event params[:id]
<% cache("event/#{}/#{@event.assetVersion}") do %>
  <!-- Display Event Information -->
  <% @event.components.each do |component| %>
    <% cache("component/#{}/#{component.assetVersion}") do %>
      <!-- Display Individual Component Information -->
    <% end %>
  <% end %>
<% end %>

So now if the event itself is changed, it will bump the document version without changing the children. So just the section displaying that event information will have to be recompiled, while the others are drawn from Memcached. If the components (in this case the subevents and their pricing) are altered, that individual subevent will fall out due to either it’s guid or it’s document version changing, and the parent document will fall out due to us reindexing it (which bumps it’s assetVersion). Thus only that one subcomponent will be recompiled and the general information will be recompiled while everything else is drawn from Memcached. Additionally the inclusion of the cache digests gem will ensure that your cache keys have the digests of the views appended to the end of them.

Hopefully this will help you if you’re using our Activity Search v2 API or simply if you’re using ElasticSearch as a datastore in your application!