ISemaphore (Hazelcast Root 4.1.3 API)

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 SemaphoreConfig.setJDKCompatible(boolean). 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. Please see CPSession for the rationale behind the session mechanism.
- 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 SemaphoreConfig.setJDKCompatible(boolean).
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. For this version, you can tune execution semantics via CPSubsystemConfig.setFailOnIndeterminateOperationState(boolean).

Method Summary

All Methods Instance Methods Abstract Methods
Modifier and Type	Method and 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 Detail
  - 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 SemaphoreConfig 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 SemaphoreConfig, 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 SemaphoreConfig 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 SemaphoreConfig, 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

Interface ISemaphore

Method Summary

Methods inherited from interface com.hazelcast.core.DistributedObject

Method Detail

getName

init

acquire

acquire

availablePermits

drainPermits

reducePermits

increasePermits

release

release

tryAcquire

tryAcquire

tryAcquire

tryAcquire