Storage

AshCache keeps cached results in a store. This guide covers the stores that ship with the package, how to give a resource its own store, and how to write an adapter for other storage.

Built-in adapters

Choose one for the whole application in config:

config :ash_cache, adapter: AshCache.Store.Adapters.Noop

Or for the current process only:

Process.put(:ash_cache_adapter, AshCache.Store.Adapters.Noop)

Running on several servers (experimental)

The default Cachex store keeps each server's cache in that server's memory. On several servers, a write clears the cache only on the server that made it, and the others serve old results until ttl runs out.

The AshCache.Store.Adapters.Nebulex adapter is meant to share one cache between servers through a Nebulex cache that you set up yourself:

1. Add {:nebulex, "~> 3.1"} and a Nebulex adapter to your deps.
2. Define a Nebulex cache module, for example MyApp.Cache, and start it in your supervision tree instead of AshCache.Supervisor.
3. Set config :ash_cache, adapter: AshCache.Store.Adapters.Nebulex.
4. Set store MyApp.Cache in the cache block of every cached resource.

Give a resource its own store

All resources share the :ash_cache store by default. Set store to give a resource a store of its own:

cache do
  store :products_cache
  read :read, ttl: :timer.minutes(10)
end

With the default Cachex adapter, AshCache.Supervisor starts every store your resources name. To empty one store without touching the others:

AshCache.clear_store(:products_cache)

Write your own adapter

An adapter is a module that implements AshCache.Store.Adapter: get/2, put/4, delete/2 and clear/1. Start from the built-in Cachex adapter, which is about thirty lines.

Your adapter has to follow these rules:

• Keys are Elixir terms, not strings. They contain tuples, maps and structs. A store such as Redis needs them turned into strings first.
• A missing key returns {:ok, nil}, not an error.
• put/4 gets a :ttl in milliseconds, or :infinity.
• put/4 can get never_evict: true. Keep that entry when making room if you can. AshCache uses it for the version values that make clearing work. Losing one is safe but makes every cached read of that resource miss once.
• clear/1 removes only the given store's entries, not everything in the backend.

Set it as the adapter:

config :ash_cache, adapter: MyApp.MyCacheAdapter

Pass options to your adapter

Anything a read sets in store_options is passed to your put/4 along with :ttl:

cache do
  read :read, ttl: :timer.minutes(10), store_options: [prefix: "products"]
end

Test your adapter

Copy the package's contract test, test/support/store_contract.ex in the repository. It runs the same get, put, delete, clear and expiry checks against every built-in adapter.