canvas-lms/gems/canvas_cache

CanvasCache

For all those things you don't want to go lookup again.

Usage

There are a collection of caching helpers in this library for different enhancements for the canvas system to help us scale effectively.

Redis Client Management

canvas_cache knows where the redis config info is and whether how the local environment is setup. You can ask from anywhere:

CanvasCache::Redis.enabled?

You can access the distrbitued redis client directly off the module:

CanvasCache::Redis.redis.set("key", "value")

If your config file has "servers: 'cache_store'", it will just give you a vanilla Rails.cache.redis instance, but if you give it a set of servers to work with in the config it will construct a patched version of the redis client.

It works pretty much like a standard redis client, but there are several enhancements that this library makes to the way redis caching works.

• uses the configurable HashRing described below for distributed configs
• includes some safety checks to prevent you from accidentally dropping your production cache with a "flushdb"
• performs logging with elapsed-time in the output
• uses a circuit breaker to tamp down on redis traffic if it gets connectivity issues.

Consistent Hashing on redis with Digest Injection

when spreading cache keys over a ring of nodes, you don't want the addition of a node to the ring to cause all keys to get re-positioned (that would be expensive). the redis-store library handles this internally with it's HashRing implementation, but it does not allow you to pass in your digest function of choice.

The HashRing implementation in this library provides a configurable digest and some statistics funcitonality for checking on the distribution of keys. You can use it by making sure the ":ring" option passed to your Redis::Distributed initialization is an instance of "CanvasCache::HashRing" if it isn't otherwise specified:

require 'redis/distributed'
module EnhancedDistributed
  def initialize(addresses, options = { })
    options[:ring] ||= CanvasCache::HashRing.new([], options[:replicas], options[:digest])
    super
  end
end

Redis::Distributed.prepend(EnhancedDistributed)

That way the "digest" option can be passed to any instantiation of the Redis::Distributed module (like in rails application initialization if you specify multiple redis urls).

Running Tests

This gem is tested with rspec. You can use test.sh to run it, or do it yourself with bundle exec rspec spec.