Fault tolerance

Real programs fail, and they fail in unpredictable ways. Akka, and the Scala community in general, favors planning explicitly for failure rather than trying to write infallible applications. A fault tolerant system is a system that can continue to operate when one or more of its components fails. The failure of an individual subsystem does not necessarily mean the failure of the application. How does this apply to Akka?

The actor model provides a natural unit to encapsulate failure: the actor. When an actor throws an exception while processing a message, the default behavior is for the actor to restart, but the exception does not leak out and affect the rest of the system. For instance, let's introduce an arbitrary failure in the response interpreter. We will modify the receive method to throw an exception when it is asked to interpret the response for misto, one of Martin Odersky's followers:

// ResponseInterpreter.scala
def receive = {
  case InterpretResponse("misto", r...