Halting failures

Scenario

C does not notice any activity from C' - Terminal failure?

• Distinguishing between a crash and a failure by omission/temporal can be impossible.

Asynchronous vs. Synchronous systems

• Asynchronous system: does not assume anything about the speed of execution of a process of delivery times of messages -> it is not possible to reliably detect crashing failures.

• Synchronous system: execution times and message deliveries are delimited -> it is possible to reliably identify temporal and omission failures.

In practice, we have partial synchronous systems: most of the time we can assume that the system behaves synchronously, despite the fact that there are no limits to the moments when it behaves asynchronously -> usually, failures by a crash can be detected.

Terminal failures

Type of stop	Description
Fail-stop	Failure by crash, but reliably detected.
Fail-noisy	Failure by crash, eventually not detected.
Fail-silent	Failure by omission or crash: client does not distinguish what happened.
Fail-safe	Arbitrary, but benign.
Fail-arbitrary	Arbitrary, with malicious failures.