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OperationParker is an interface offered by SPI for the objects, such as Lock and Semaphore, to be used when a thread needs to wait for a lock to be released.

OperationParker keeps a list of waiters. For each notify operation:

  • it looks for a waiter

  • it asks the waiter whether it wants to keep waiting

  • if the waiter responds no, the service executes its registered operation (operation itself knows where to send a response)

  • it rinses and repeats until a waiter wants to keep waiting.

Each waiter can sit on a wait-notify queue for, at most, its operation’s call timeout. For example, by default, each waiter can wait here for at most 1 minute. A continuous task scans expired/timed-out waiters and invalidates them with CallTimeoutException. Each waiter on the remote side should retry and keep waiting if it still wants to wait. This is a liveness check for remote waiters.

This way, it is possible to distinguish an unresponsive member and a long (~infinite) wait. On the caller side, if the waiting thread does not get a response for either a call timeout or for more than 2 times the call-timeout, it will exit with OperationTimeoutException.

Note that this behavior breaks the fairness. Hazelcast does not support fairness for any of the data structures with blocking operations, such as Lock and Semaphore.