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Interface ISemaphore

A linearizable, distributed semaphore.

ISemaphore works on top of the Raft consensus algorithm. It offers linearizability during crash failures and network partitions. It is CP with respect to the CAP principle. If a network partition occurs, it remains available on at most one side of the partition.

ISemaphore is a cluster-wide counting semaphore. Conceptually, it maintains a set of permits. Each acquire() waits if necessary until a permit is available, and then takes it. Dually, each release() adds a permit, potentially releasing a waiting acquirer. However, no actual permit objects are used; the semaphore just keeps a count of the number available and acts accordingly.

This data structure also provides convenience methods to work with multiple permits at once. Beware of the increased risk of indefinite postponement when using the multiple-permit acquire. If permits are released one by one, a caller waiting for one permit will acquire it before a caller waiting for multiple permits regardless of the call order.

Correct usage of a semaphore is established by programming convention in the application.

ISemaphore has two variations:

  • The default implementation is session-aware. In this one, when a caller makes its very first `acquire()` call, it starts a new CP session with the underlying CP group. Then, liveliness of the caller is tracked via this CP session. When the caller fails, permits acquired by this caller are automatically and safely released. However, the session-aware version comes with a limitation, that is, a Hazelcast client cannot release permits before acquiring them first. In other words, a client can release only the permits it has acquired earlier.
  • The second implementation is sessionless. This one does not perform auto-cleanup of acquired permits on failures. Acquired permits are not bound to callers and permits can be released without acquiring first. However, you need to handle failed permit owners on your own. If a Hazelcast server or a client fails while holding some permits, they will not be automatically released. You can use the sessionless CP ISemaphore implementation by enabling JDK compatibility `jdk-compatible` server-side setting. Refer to Semaphore configuration documentation for more details.

Hierarchy

Index

Methods

acquire

  • acquire(permits?: number): Promise<void>
  • Acquires the given number of permits if they are available, and returns immediately, reducing the number of available permits by the given amount.

    If insufficient permits are available then the returned promise is not resolved until there are sufficient number of available permits or this ISemaphore is destroyed.

    Parameters

    • Optional permits: number

      optional number of permits to acquire; defaults to 1 when not specified

    Returns Promise<void>

availablePermits

  • availablePermits(): Promise<number>
  • Returns the current number of permits currently available in this semaphore.

    This method is typically used for debugging and testing purposes.

    Returns Promise<number>

    the number of permits available in this semaphore

destroy

  • destroy(): Promise<void>
  • Destroys this object cluster-wide. Clears all resources taken for this object.

    Returns Promise<void>

drainPermits

  • drainPermits(): Promise<number>
  • Acquires and returns all permits that are available at invocation time.

    Returns Promise<number>

    the number of permits drained

getName

  • getName(): string
  • Returns the unique name of this object.

    Returns string

getPartitionKey

  • getPartitionKey(): string
  • Returns the key of the partition that this DistributedObject is assigned to. For a partitioned data structure, the returned value will not be null, but otherwise undefined.

    Returns string

getServiceName

  • getServiceName(): string

increasePermits

  • increasePermits(increase: number): Promise<void>
  • Increases the number of available permits by the indicated amount. If there are some callers waiting for permits to become available, they will be notified. Moreover, if the caller has acquired some permits, they are not released with this call.

    Parameters

    • increase: number

      the number of permits to increase

    Returns Promise<void>

init

  • init(permits: number): Promise<boolean>
  • Tries to initialize this ISemaphore instance with the given permit count.

    Parameters

    • permits: number

      the given permit count

    Returns Promise<boolean>

    true if initialization success; false if already initialized

reducePermits

  • reducePermits(reduction: number): Promise<void>
  • Reduces the number of available permits by the indicated amount. This method differs from acquire() as it does not block until permits become available. Similarly, if the caller has acquired some permits, they are not released with this call.

    Parameters

    • reduction: number

      the number of permits to reduce

    Returns Promise<void>

release

  • release(permits?: number): Promise<void>
  • Releases the given number of permits and increases the number of available permits by that amount. If some callers in the cluster are waiting for acquiring permits, they will be notified.

    If the underlying ISemaphore implementation is non-JDK-compatible (configured via jdk-compatible server-side setting), then a client can only release a permit which it has acquired before. In other words, a client cannot release a permit without acquiring it first.

    Otherwise, which means the default implementation, there is no such requirement for clients. A client can freely release a permit without acquiring it first. In this case, correct usage of a semaphore is established by programming convention in the application.

    throws

    IllegalStateError if the Semaphore is non-JDK-compatible and the caller does not have a permit

    Parameters

    • Optional permits: number

      the number of permits to release

    Returns Promise<void>

tryAcquire

  • tryAcquire(permits?: number, timeout?: number): Promise<boolean>
  • Acquires the given number of permits and returns true, if they become available during the given timeout. If permits are acquired, the number of available permits in the ISemaphore instance is also reduced by the given amount.

    Parameters

    • Optional permits: number

      the number of permits to acquire; defaults to 1 when not specified

    • Optional timeout: number

      optional timeout in milliseconds to wait for the permits; when it's not specified the operation will return immediately after the acquire attempt

    Returns Promise<boolean>

    true if all permits were acquired, false if the waiting time elapsed before all permits could be acquired

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