Interface ISemaphore
- All Superinterfaces:
DistributedObject
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 firstacquire()
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 withSemaphore.release()
. You can use the session-aware CPISemaphore
impl by disabling JDK compatibility viaSemaphoreConfig.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 seeCPSession
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 withSemaphore.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 CPISemaphore
impl by enabling JDK compatibility viaSemaphoreConfig.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
Modifier and TypeMethodDescriptionvoid
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
Returns the current number of permits currently available in this semaphore.int
Acquires and returns all permits that are available at invocation time.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 countvoid
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
Acquires a permit if one is available, and returnstrue
immediately.boolean
tryAcquire
(int permits) Acquires the given number of permits if they are available, and returnstrue
immediately.boolean
tryAcquire
(int permits, long timeout, TimeUnit unit) Acquires the given number of permits and returnstrue
, if they become available during the given waiting time.boolean
tryAcquire
(long timeout, TimeUnit unit) Acquires a permit and returnstrue
, 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, getDestroyContextForTenant, getPartitionKey, getServiceName
-
Method Details
-
getName
String getName()Returns the name of this ISemaphore instance.- Specified by:
getName
in interfaceDistributedObject
- 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
- ifpermits
is negative
-
acquire
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,
InterruptedException
is thrown and the current thread's interrupted status is cleared.- Throws:
InterruptedException
- if the current thread is interruptedIllegalStateException
- if hazelcast instance is shutdown while waiting
- some other thread invokes one of the
-
acquire
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.
- has its interrupted status set on entry to this method, or
- is interrupted while waiting for a permit,
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 interruptedIllegalArgumentException
- ifpermits
is negative or zeroIllegalStateException
- if hazelcast instance is shutdown while waiting
- some other thread invokes one of the
-
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 fromacquire
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
- ifreduction
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
- ifincrease
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 viaSemaphoreConfig
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 theacquire()
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 viaSemaphoreConfig
and fetched viaCPSubsystem
, 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 theacquire()
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
- ifpermits
is negative or zeroIllegalStateException
- 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 returnstrue
immediately. If no permit is available, returnsfalse
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 returnstrue
immediately. If the requested number of permits are not available, returnsfalse
immediately.- Parameters:
permits
- the number of permits to acquire- Returns:
true
if the permits were acquired,false
otherwise- Throws:
IllegalArgumentException
- ifpermits
is negative
-
tryAcquire
Acquires a permit and returnstrue
, 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 theISemaphore
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,
InterruptedException
is thrown and the current thread's interrupted status is cleared.- Parameters:
timeout
- the maximum time to wait for a permitunit
- the time unit of thetimeout
argument- Returns:
true
if a permit was acquired andfalse
if the waiting time elapsed before a permit was acquired- Throws:
InterruptedException
- if the current thread is interruptedIllegalStateException
- if hazelcast instance is shutdown while waiting
-
tryAcquire
Acquires the given number of permits and returnstrue
, if they become available during the given waiting time. If permits are acquired, the number of available permits in theISemaphore
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.
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,
InterruptedException
is thrown and the current thread's interrupted status is cleared.- Parameters:
permits
- the number of permits to acquiretimeout
- the maximum time to wait for the permitsunit
- the time unit of thetimeout
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 interruptedIllegalArgumentException
- ifpermits
is negative or zeroIllegalStateException
- if hazelcast instance is shutdown while waiting
-