Examples of "fallback" tasks for error recovery?

One example that comes to my mind is with caching, where you try to keep a cache up to date, but if you fail you might just serve stale content you already have access to. In this case “updating an out of date cached value” is more complex than “using the out of date cached value”.

I’d also argue that circuit breakers are an implementation of that, even though the “simpler task” is telling the user that whatever isn’t executed is considered unavailable for the time being. Not that useful to the enduser, as functionality is not available in both the erroring case as well as in the being told things don’t work case, but the latter is saner for the system to handle and by preventing access it might allow downstream system to recover over time.

You might find a lot more of that in topics around distributed systems – where failure is much more obviously inevitable – than in pieces around coding for a single machine. Also commonly distributed setups involve some degree of not being able to control all pieces of software running, so actual faulty implementations are more of an consideration.

Thanks! Here are some references I found for caching and the “circuit breaker” pattern, along with some implementations in Elixir:

• How to write a caching server in Elixir

• Cachex

• CircuitBreaker (Martin Fowler)

  The basic idea behind the circuit breaker is very simple. You wrap a protected function call in a circuit breaker object, which monitors for failures. Once the failures reach a certain threshold, the circuit breaker trips, and all further calls to the circuit breaker return with an error, without the protected call being made at all. Usually you’ll also want some kind of monitor alert if the circuit breaker trips.

• Circuit Breaker Pattern in Elixir (Allan MacGregor)

• Hex package search