caching.md

Caching in Phoenix applications

Why?

Despite the speed of Phoenix applications, database queries are a bottleneck which can slow down your app. Oftentimes you may be loading the same or similar data anytime someone accesses a page. Caching previously loaded data can help speed up your app and avoid the query bottleneck!

What

There are a number of libraries available for caching data with Phoenix. We are going to be looking at con_cache specifically, which is an ETS based key/value storage.

How

Setup ConCache

• Add :con_cache to your deps

defp deps do
  [{:con_cache, "~> 0.12.0"}, ...]
end

• Add it to your application

def application do
  [applications: [:con_cache, ...]]
end

• Run mix deps.get
• Start the cache from a supervisor in lib/<my_app>.ex. You can add to the list of children as below.

    children = [
      supervisor(MyApp.Repo, []),
      supervisor(MyApp.Endpoint, []),
      # add ConCache here. :my_cache can be whatever you want to name your cache
      supervisor(ConCache, [[], [name: :my_cache]]),
      worker(Segment, [Application.get_env(:segment, :write_key)])
    ]

    # See http://elixir-lang.org/docs/stable/elixir/Supervisor.html
    # for other strategies and supported options
    opts = [strategy: :one_for_one, name: Healthlocker.Supervisor]
    Supervisor.start_link(children, opts)
  end

• You're ready to start caching!

Using ConCache

There are a number of functions you can use depending on how you would like to store your data. Below are some basics, but the full extent is covered in the docs.

ConnCache.get_or_store(cache_id, key, store_fun) is a particularly useful function which acts as a get from the cache if the item is already there, or stores the item if it is not.

Testing with ConnCache

Because ConnCache introduces state to your system, some tests may compromise the execution of other tests.

There are ways around this, as listed by the creator of ConnCache here.