A good way to improve the performance of your application is caching. If you’re unsure whether your caching strategy is working it’s a good idea to start measuring cache hits and cache misses.

Assuming you have statsd set up, you can subscribe to ActiveSupport notifications and pass down the metrics. This is what you can do in an initialiser:

# config/initializers/instrumentation.rb
statsd = Statsd.new

ActiveSupport::Notifications.subscribe("cache_read.active_support") do |_name, _start, _finish, _id, payload|
  hit_or_miss = payload[:hit] ? "cache_hit" : "cache_miss"
  statsd.increment("cache.#{hit_or_miss}", 1)

You will be able to test this by listening to UDP port 8125 (statsd) in another terminal. If you excercise the code you’ll see something like this:

$ nc -u -l 8125

That means it’s working!