Package com.hazelcast.cp

Interface ISemaphore

All Superinterfaces:
DistributedObject
public interface ISemaphore extends DistributedObject
ISemaphore is a fault-tolerant distributed alternative to the Semaphore. Semaphores are often used to restrict the number of threads than can access some physical or logical resource.

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

Hazelcast's distributed semaphore implementation guarantees that threads invoking any of the acquire methods are selected to obtain permits in the order of their invocations (first-in-first-out; FIFO). Note that FIFO ordering implies the order which the primary replica of an ISemaphore receives these acquire requests. Therefore, it is possible for one member to invoke acquire before another member, but its request hits the primary replica after the other member.

This class also provides convenience methods, such as acquire and release, 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 thread waiting for one permit will acquire it before a thread waiting for multiple permits regardless of the call order.

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

ISemaphore is accessed via CPSubsystem.getSemaphore(String). It 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.

It has 2 variations:

  • The default impl accessed via CPSubsystem 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 HazelcastInstance are automatically and safely released. However, the session-aware version comes with a limitation, that is, a HazelcastInstance cannot release permits before acquiring them first. In other words, a HazelcastInstance can release only the permits it has acquired earlier. It means, you can acquire a permit from one thread and release it from another thread using the same HazelcastInstance, but not different instances of HazelcastInstance. This behaviour is not compatible with Semaphore.release(). You can use the session-aware CP ISemaphore impl by disabling JDK compatibility via semaphore configuration. Although the session-aware impl has a minor difference to the JDK Semaphore, we think it is a better fit for distributed environments because of its safe auto-cleanup mechanism for acquired permits.
  • The second impl offered by CPSubsystem is sessionless. This impl does not perform auto-cleanup of acquired permits on failures. Acquired permits are not bound to HazelcastInstance and permits can be released without acquiring first. This one is compatible with Semaphore.release(). 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 impl by enabling JDK compatibility via semaphore configuration.

There is a subtle difference between the lock and semaphore abstractions. A lock can be assigned to at most one endpoint at a time, so we have a total order among its holders. However, permits of a semaphore can be assigned to multiple endpoints at a time, which implies that we may not have a total order among permit holders. In fact, permit holders are partially ordered. For this reason, the fencing token approach, which is explained in FencedLock, does not work for the semaphore abstraction. Moreover, each permit is an independent entity. Multiple permit acquires and reentrant lock acquires of a single endpoint are not equivalent. The only case where a semaphore behaves like a lock is the binary case, where the semaphore has only 1 permit. In this case, the semaphore works like a non-reentrant lock.

All of the API methods in the new CP ISemaphore impl offer the exactly-once execution semantics for the session-aware version. For instance, even if a release() call is internally retried because of a crashed Hazelcast member, the permit is released only once. However, this guarantee is not given for the sessionless, a.k.a., JDK-compatible CP ISemaphore.

  • Method Summary

    Modifier and Type
    Method
    Description
    void
    acquire()
    Acquires a permit if one is available, and returns immediately, reducing the number of available permits by one.
    void
    acquire(int permits)
    Acquires the given number of permits if they are available, and returns immediately, reducing the number of available permits by the given amount.
    int
    availablePermits()
    Returns the current number of permits currently available in this semaphore.
    int
    drainPermits()
    Acquires and returns all permits that are available at invocation time.
    String
    getName()
    Returns the name of this ISemaphore instance.
    void
    increasePermits(int increase)
    Increases the number of available permits by the indicated amount.
    boolean
    init(int permits)
    Tries to initialize this ISemaphore instance with the given permit count
    void
    reducePermits(int reduction)
    Reduces the number of available permits by the indicated amount.
    void
    release()
    Releases a permit and increases the number of available permits by one.
    void
    release(int permits)
    Releases the given number of permits and increases the number of available permits by that amount.
    boolean
    tryAcquire()
    Acquires a permit if one is available, and returns true immediately.
    boolean
    tryAcquire(int permits)
    Acquires the given number of permits if they are available, and returns true immediately.
    boolean
    tryAcquire(int permits, long timeout, TimeUnit unit)
    Acquires the given number of permits and returns true, if they become available during the given waiting time.
    boolean
    tryAcquire(long timeout, TimeUnit unit)
    Acquires a permit and returns true, if one becomes available during the given waiting time and the current thread has not been interrupted.

    Methods inherited from interface com.hazelcast.core.DistributedObject

    destroy, getPartitionKey, getServiceName

  • Method Details

    • getName

      String getName()
      Returns the name of this ISemaphore instance.
      Specified by:
      getName in interface DistributedObject
      Returns:
      name of this instance

    • init

      boolean init(int permits)
      Tries to initialize this ISemaphore instance with the given permit count
      Parameters:
      permits - the given permit count
      Returns:
      true if initialization success. false if already initialized
      Throws:
      IllegalArgumentException - if permits is negative

    • acquire

      void acquire() throws InterruptedException
      Acquires a permit if one is available, and returns immediately, reducing the number of available permits by one.

      If no permit is available, then the current thread becomes disabled for thread scheduling purposes and lies dormant until one of three things happens:

      • some other thread invokes one of the release() methods for this semaphore and the current thread is next to be assigned a permit,
      • this ISemaphore instance is destroyed, or
      • some other thread interrupts the current thread.

      If the current thread:

      • has its interrupted status set on entry to this method; or
      • is interrupted while waiting for a permit,
      then InterruptedException is thrown and the current thread's interrupted status is cleared.
      Throws:
      InterruptedException - if the current thread is interrupted
      IllegalStateException - if hazelcast instance is shutdown while waiting

    • acquire

      void acquire(int permits) throws InterruptedException
      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 current thread becomes disabled for thread scheduling purposes and lies dormant until one of three things happens:

      • some other thread invokes one of the release methods for this semaphore, the current thread is next to be assigned permits and the number of available permits satisfies this request,
      • or this ISemaphore instance is destroyed,
      • or some other thread interrupts the current thread.
      If the current thread:
      • has its interrupted status set on entry to this method, or
      • is interrupted while waiting for a permit,
      then InterruptedException is thrown and the current thread's interrupted status is cleared.
      Parameters:
      permits - the number of permits to acquire
      Throws:
      InterruptedException - if the current thread is interrupted
      IllegalArgumentException - if permits is negative or zero
      IllegalStateException - if hazelcast instance is shutdown while waiting

    • availablePermits

      int availablePermits()
      Returns the current number of permits currently available in this semaphore.

      This method is typically used for debugging and testing purposes.

      Returns:
      the number of permits available in this semaphore

    • drainPermits

      int drainPermits()
      Acquires and returns all permits that are available at invocation time.
      Returns:
      the number of permits drained

    • reducePermits

      void reducePermits(int reduction)
      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 - the number of permits to reduce
      Throws:
      IllegalArgumentException - if reduction is negative

    • increasePermits

      void increasePermits(int increase)
      Increases the number of available permits by the indicated amount. If there are some threads 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 - the number of permits to increase
      Throws:
      IllegalArgumentException - if increase is negative

    • release

      void release()
      Releases a permit and increases the number of available permits by one. If some threads in the cluster are blocked for acquiring a permit, one of them will unblock by acquiring the permit released by this call.

      If the underlying ISemaphore is configured as non-JDK compatible via server-side semaphore configuration then a HazelcastInstance can only release a permit which it has acquired before. In other words, a HazelcastInstance cannot release a permit without acquiring it first.

      Otherwise, which means the underlying impl is the JDK compatible Semaphore is configured via server-side semaphore configuration, there is no requirement that a HazelcastInstance that releases a permit must have acquired that permit by calling one of the acquire() methods. A HazelcastInstance 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:
      IllegalStateException - if the Semaphore is non-JDK-compatible and the caller does not have a permit

    • release

      void release(int permits)
      Releases the given number of permits and increases the number of available permits by that amount. If some threads in the cluster are blocked for acquiring permits, they will be notified.

      If the underlying ISemaphore impl is the non-JDK compatible CP impl that is configured via server-side semaphore configuration and fetched via CPSubsystem, then a HazelcastInstance can only release a permit which it has acquired before. In other words, a HazelcastInstance cannot release a permit without acquiring it first.

      Otherwise, which means the underlying impl is the JDK compatible Semaphore is configured via server-side semaphore configuration, there is no requirement that a HazelcastInstance that releases a permit must have acquired that permit by calling one of the acquire() methods. A HazelcastInstance 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.

      Parameters:
      permits - the number of permits to release
      Throws:
      IllegalArgumentException - if permits is negative or zero
      IllegalStateException - if the Semaphore is non-JDK-compatible and the caller does not have a permit

    • tryAcquire

      boolean tryAcquire()
      Acquires a permit if one is available, and returns true immediately. If no permit is available, returns false immediately.
      Returns:
      true if a permit was acquired, false otherwise

    • tryAcquire

      boolean tryAcquire(int permits)
      Acquires the given number of permits if they are available, and returns true immediately. If the requested number of permits are not available, returns false immediately.
      Parameters:
      permits - the number of permits to acquire
      Returns:
      true if the permits were acquired, false otherwise
      Throws:
      IllegalArgumentException - if permits is negative

    • tryAcquire

      boolean tryAcquire(long timeout, TimeUnit unit) throws InterruptedException
      Acquires a permit and returns true, if one becomes available during the given waiting time and the current thread has not been interrupted. If a permit is acquired, the number of available permits in the ISemaphore instance is also reduced by one.

      If no permit is available, then the current thread becomes disabled for thread scheduling purposes and lies dormant until one of three things happens:

      • some other thread releases a permit and the current thread is next to be assigned a permit,
      • or this ISemaphore instance is destroyed,
      • or some other thread interrupts the current thread,
      • or the specified waiting time elapses.

      Returns true if a permit is successfully acquired.

      Returns false if the specified waiting time elapses without acquiring a permit. If the time is less than or equal to zero, the method will not wait at all.

      If the current thread:

      • has its interrupted status set on entry to this method,
      • or is interrupted while waiting for a permit,
      then InterruptedException is thrown and the current thread's interrupted status is cleared.
      Parameters:
      timeout - the maximum time to wait for a permit
      unit - the time unit of the timeout argument
      Returns:
      true if a permit was acquired and false if the waiting time elapsed before a permit was acquired
      Throws:
      InterruptedException - if the current thread is interrupted
      IllegalStateException - if hazelcast instance is shutdown while waiting

    • tryAcquire

      boolean tryAcquire(int permits, long timeout, TimeUnit unit) throws InterruptedException
      Acquires the given number of permits and returns true, if they become available during the given waiting time. If permits are acquired, the number of available permits in the ISemaphore instance is also reduced by the given amount.

      If no sufficient permits are available, then the current thread becomes disabled for thread scheduling purposes and lies dormant until one of three things happens:

      • permits are released by other threads, the current thread is next to be assigned permits and the number of available permits satisfies this request,
      • or some other thread interrupts the current thread,
      • or the specified waiting time elapses.
      Returns true if requested permits are successfully acquired.

      Returns false if the specified waiting time elapses without acquiring permits. If the time is less than or equal to zero, the method will not wait at all.

      If the current thread:

      • has its interrupted status set on entry to this method,
      • or is interrupted while waiting for a permit,
      then InterruptedException is thrown and the current thread's interrupted status is cleared.
      Parameters:
      permits - the number of permits to acquire
      timeout - the maximum time to wait for the permits
      unit - the time unit of the timeout argument
      Returns:
      true if all permits were acquired, false if the waiting time elapsed before all permits could be acquired
      Throws:
      InterruptedException - if the current thread is interrupted
      IllegalArgumentException - if permits is negative or zero
      IllegalStateException - if hazelcast instance is shutdown while waiting