Rexsilient.Classify behaviour (Rexsilient v1.0.0)

Decides whether the outcome of a call counts as a failure.

Every module that uses Rexsilient implements this behaviour, and the circuit breakers, retries and fallbacks it declares use it. The defaults count :error and {:error, _} returns, and every raise, throw or exit, as failures; any other return counts as a success.

Override classify/1 to change how returned values are classified:

defmodule MyApp.Resilience do
  use Rexsilient

  circuit_breaker :payments do
    # ...
  end

  @impl true
  def classify({:error, %{status: status}}) when status >= 500, do: :failure
  def classify(_), do: :success
end

Override classify/2 to change how raises, throws and exits are classified. The two are independent: overriding classify/1 keeps the default classify/2, so a catch-all clause on returns never turns an exception into a success.

@impl true
def classify(:error, %Mint.TransportError{}), do: :failure
def classify(_kind, _reason), do: :success

Circuit breakers and retries re-raise the exception unchanged either way; only how it is counted changes. A fallback replaces it when it counts as a failure, and re-raises it otherwise.

To classify one declaration differently from the others in the same module, name other functions with Rexsilient.classify_with/1.

Summary

Types

outcome()

What a call produced: {:value, value} for a value it returned, or {kind, reason} for a raise, throw or exit, as given to catch.

result()

Whether a call succeeded or failed.

Callbacks

classify(value)

Classifies a value returned by the call.

classify(kind, reason)

Classifies a call that raised, threw or exited.