Saturday, November 8, 2008

Thread-safe simple lifecycle (part 3)

In parts parts one and two of these mini-series, we started showing how going multi-thread could affect the simplest lifecycle of what we called and active object. In the end, what we have is a set of open questions, including (some of them were presented in previous posts):
  • What happens if you try to start the object and some other thread have previously done so?
  • What happens if several threads try to stop the object at the same time?
  • What happens if you try to stop the object when there are in-flight requests? Do we wait for them to finish? Do we stop and let them crash?
And the answer to all these questions is the same: it depends. It depends on the semantics of each particular object, so we can't provide a solutions suitable for everyone, every time. But this doesn't mean we can't do anything. I've checked in a new solution. Some of the highlights include:
  • The lifecycle interface is gone. As we have shown different problems need different semantics and thus, a different interface. Instead, we have a support class.
  • Though externally the same fixed-set of states is kept (implemented as an enumerated type), internally an infinite set of states is used, so that we can keep track of in-flight requests.
  • No synchronization is used, and state changes are performed using atomic references. Client-provided transition code is guaranteed to be run by just one thread without holding any locks.
  • No standard semantics are enforced. Instead, the support class provides methods so that you can build your own. Eg, if you call a method to start or stop the object while the transition is being performed in another thread an exception is thrown. However you can catch this exception and use the provided methods to wait for the object to reach a permanent exception to implement different semantics.

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