The behaviour implemented by circuit breaker backends.

A service's breaker is chosen with the :backend circuit breaker option, and defaults to ExternalService.CircuitBreaker.Fuse — a node-local breaker built on the :fuse library.

Writing a backend

defmodule MyApp.CircuitBreaker do
  @behaviour ExternalService.CircuitBreaker

  @impl true
  def install(service, options) do
    {:ok, %{key: service, tolerate: options[:tolerate]}}
  end

  @impl true
  def ask(_service, config), do: if MyStore.open?(config.key), do: :blown, else: :ok

  @impl true
  def melt(_service, config), do: MyStore.record_failure(config.key)

  @impl true
  def reset(_service, config), do: MyStore.close(config.key)

  @impl true
  def remove(_service, config), do: MyStore.forget(config.key)
end

Backends are stateless modules. install/2 returns an opaque config term that is stored with the rest of the service state (in :persistent_term) and handed back to every other callback, so a backend needs no process, supervisor, or registry of its own.

This library never calls these functions on your behalf outside a guarded call. The user-facing view of a breaker stays at the level of ExternalService.available?/1, ExternalService.blown?/1, and ExternalService.reset/1 — the concept, not the individual operations.