Cache.MultiLayer (elixir_cache v0.4.6)

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Multi-layer caching strategy that cascades through multiple cache layers.

Keys are read from fastest to slowest, with automatic backfill on cache hits from slower layers. Writes go slowest-first to avoid polluting fast layers with data that failed to persist in slow ones.

Usage

Pass a list of layers as the strategy config. Each element can be:

  • A module that implements Cache (already running, not supervised by this adapter)
  • An adapter module (e.g. Cache.ETS) — will be auto-started and supervised
  • A tuple {AdapterModule, opts} — adapter with inline opts
defmodule MyApp.LayeredCache do
  use Cache,
    adapter: {Cache.MultiLayer, [Cache.ETS, MyApp.RedisCache]},
    name: :layered_cache,
    opts: []
end

__MODULE__ in Layers

You may include __MODULE__ in the layer list to position the current module's own underlying cache within the chain. If __MODULE__ is omitted, no local cache is created for the defining module—it acts as a pure facade.

defmodule MyApp.LayeredCache do
  use Cache,
    adapter: {Cache.MultiLayer, [Cache.ETS, __MODULE__, MyApp.RedisCache]},
    name: :layered_cache,
    opts: [uri: "redis://localhost"]
end

Read Behaviour

Layers are iterated fastest → slowest (list order). On a hit from layer N, the value is backfilled into layers 1..N-1.

Write Behaviour

Layers are written slowest → fastest (reverse list order). If a slow write fails, the write stops and an error is returned — preventing polluting faster layers with potentially-unsaved data.

Fetch Callback (Optional)

If all layers miss, an optional fetch callback can supply the value. The fetched value is then backfilled into all layers.

Define it as a module callback or pass it via opts:

defmodule MyApp.LayeredCache do
  use Cache,
    adapter: {Cache.MultiLayer, [Cache.ETS, MyApp.RedisCache]},
    name: :layered_cache,
    opts: [on_fetch: &__MODULE__.fetch/1]

  def fetch(key) do
    {:ok, "value_for_#{key}"}
  end
end

Options

  • :on_fetch - Optional fetch callback invoked on total cache miss. Receives the key, returns {:ok, value} or {:error, reason}.

  • :backfill_ttl - TTL in milliseconds to use when backfilling layers on a hit from a slower layer. Defaults to nil (no expiry).