Replaces the outcome of a failed call with the result of another function.

A fallback is declared in a module that uses Rexsilient, with the function to call instead:

defmodule MyApp.Resilience do
  use Rexsilient

  fallback :prices do
    function :cached_price
  end

  def cached_price(sku), do: Cache.last_price(sku)
end

and used with @fallback :prices from Rexsilient.Decorator, which calls the fallback function with the decorated function's arguments:

defmodule MyApp.Prices do
  use Rexsilient.Decorator

  @fallback :prices
  def fetch(sku) do
    PricesClient.fetch(sku)
  end
end

or, without a decorator, with run/3:

Rexsilient.Fallback.run(:prices, fn sku -> PricesClient.fetch(sku) end, sku)

Settings

• function - the function called on failure: a name, for a function in the declaring module, or {module, name} for one in another module. Required.

• classify_with - the name of the functions, in the declaring module, that decide which outcomes are failures and so are replaced. Defaults to classify; see Rexsilient.Classify.

Behaviour

The call is made. If its outcome is a success, it is returned unchanged. If it is a failure, the fallback function is called with the same arguments, and its result is returned instead; a raise, throw or exit counted as a failure is not re-raised.

{:error, :circuit_open} is always replaced, without asking the classifier: a circuit breaker rejected the call, so there is no result to keep. This lets a fallback declared around a breaker answer while the breaker is open.

An error raised by the fallback function itself propagates to the caller.