org.apache.ignite

Interface IgniteLock

All Superinterfaces:

AutoCloseable, Closeable, Lock

public interface IgniteLock
extends Lock, Closeable

This interface provides a rich API for working with distributed reentrant locks.

Functionality

Distributed reentrant lock provides functionality similar to java.util.concurrent.ReentrantLock.

Creating Distributed ReentrantLock

Instance of cache reentrant lock can be created by calling the following method: Ignite.reentrantLock(String, boolean, boolean, boolean).

Protection from failover

Ignite lock can automatically recover from node failure.

• If failoverSafe flag is set to true upon creation, in case a node owning the lock fails, lock will be automatically released and become available for threads on other nodes to acquire. No exception will be thrown.
• If failoverSafe flag is set to false upon creation, in case a node owning the lock fails, IgniteException will be thrown on every other node attempting to perform any operation on this lock. No automatic recovery will be attempted, and lock will be marked as broken (i.e. unusable), which can be checked using the method #isBroken(). Broken lock cannot be reused again.

Implementation issues

Ignite lock comes in two flavours: fair and non-fair. Non-fair lock assumes no ordering should be imposed on acquiring threads; in case of contention, threads from all nodes compete for the lock once the lock is released. In most cases this is the desired behaviour. However, in some cases, using the non-fair lock can lead to uneven load distribution among nodes. Fair lock solves this issue by imposing strict FIFO ordering policy at a cost of an additional transaction. This ordering does not guarantee fairness of thread scheduling (similar to java.util.concurrent.ReentrantLock). Thus, one of many threads on any node using a fair lock may obtain it multiple times in succession while other active threads are not progressing and not currently holding the lock. Also note that the untimed tryLock method does not honor the fairness setting. It will succeed if the lock is available even if other threads are waiting.

As a rule of thumb, whenever there is a reasonable time window between successive calls to release and acquire the lock, non-fair lock should be preferred:

 
      while(someCondition){
          // do anything
          lock.lock();
          try{
              // ...
          }
          finally {
              lock.unlock();
          }
      }
 

If successive calls to release/acquire are following immediately, e.g.

 
      while(someCondition){
          lock.lock();
          try {
              // do something
          }
          finally {
              lock.unlock();
          }
      }
 

using the fair lock is reasonable in order to allow even distribution of load among nodes (although overall throughput may be lower due to increased overhead).

Method Summary

Modifier and Type	Method and Description
void	close() Removes reentrant lock.
int	getHoldCount() Queries the number of holds on this lock by the current thread.
IgniteCondition	getOrCreateCondition(String name) Returns a Condition instance for use with this IgniteLock instance.
int	getWaitQueueLength(IgniteCondition condition) Returns an estimate of the number of threads on this node that are waiting on the given condition associated with this lock.
boolean	hasQueuedThread(Thread thread) Queries whether the given thread is waiting to acquire this lock.
boolean	hasQueuedThreads() Queries whether any threads on this node are waiting to acquire this lock.
boolean	hasWaiters(IgniteCondition condition) Queries whether any threads on this node are waiting on the given condition associated with this lock.
boolean	isBroken() Returns true if any node that owned the locked failed before releasing the lock.
boolean	isFailoverSafe() Returns true if this lock is safe to use after node failure.
boolean	isFair() Returns true if this lock is fair.
boolean	isHeldByCurrentThread() Queries if this lock is held by the current thread.
boolean	isLocked() Queries if this lock is held by any thread on any node.
void	lock() Acquires the distributed reentrant lock.
void	lockInterruptibly() Acquires the lock unless the current thread is interrupted.
String	name() Name of atomic reentrant lock.
Condition	newCondition() This method is not supported in IgniteLock, Any invocation of this method will result in UnsupportedOperationException.
boolean	removed() Gets status of reentrant lock.
boolean	tryLock() Acquires the lock only if it is free at the time of invocation.
boolean	tryLock(long timeout, TimeUnit unit) Acquires the lock if it is not held by another thread within the given waiting time and the current thread has not been interrupted.
void	unlock() Releases the lock.

Method Detail

name

String name()

Name of atomic reentrant lock.

Returns:

Name of atomic reentrant lock.

lock

void lock()
   throws IgniteException

Acquires the distributed reentrant lock.

Acquires the lock if it is not held by another thread and returns immediately, setting the lock hold count to one.

If the current thread already holds this lock then the hold count is incremented by one and the method returns immediately.

If the lock is held by another thread then the current thread becomes disabled for thread scheduling purposes and lies dormant until one of four things happens:

• The lock is acquired by the current thread; or
• Lock is broken (any node failed while owning this lock), and lock is created in non-failoverSafe mode.
• Local node is stopped.

Specified by:

lock in interface Lock

Throws:

IgniteException - if the node is stopped or broken in non-failoverSafe mode

lockInterruptibly

void lockInterruptibly()
                throws IgniteInterruptedException,
                       IgniteException

Acquires the lock unless the current thread is interrupted.

Acquires the lock if it is not held by another thread and returns immediately, setting the lock hold count to one.

If the current thread already holds this lock then the hold count is incremented by one and the method returns immediately.

If the lock is held by another thread then the current thread becomes disabled for thread scheduling purposes and lies dormant until one of four things happens:

• The lock is acquired by the current thread; or
• Some other thread interrupts the current thread.
• Lock is broken (any node failed while owning this lock), and lock is created in non-failoverSafe mode.
• Local node is stopped.

If the lock is acquired by the current thread then the lock hold count is set to one.

If the current thread:

• has its interrupted status set on entry to this method; or
• is interrupted while acquiring the lock; or then IgniteInterruptedException is thrown and the current thread's interrupted status is cleared.

IgniteException is thrown in case:

• the lock is broken before or during the attempt to acquire this lock; or
• local node is stopped,

In this implementation, as this method is an explicit interruption point, preference is given to responding to the interrupt over normal or reentrant acquisition of the lock.

Specified by:

lockInterruptibly in interface Lock

Throws:

IgniteInterruptedException - if the current thread is interrupted

IgniteException - if the lock is broken in non-failoverSafe mode (any node failed while owning this lock), or local node is stopped

tryLock

boolean tryLock()
         throws IgniteException

Acquires the lock only if it is free at the time of invocation.

Acquires the lock if it is available and returns immediately with the value true. If the lock is not available then this method will return immediately with the value false.

A typical usage idiom for this method would be:

 
 Lock lock = ...;
 if (lock.tryLock()) {
   try {
     // manipulate protected state
   } finally {
     lock.unlock();
   }
 } else {
   // perform alternative actions
 }

This usage ensures that the lock is unlocked if it was acquired, and doesn't try to unlock if the lock was not acquired. If node is stopped, or any node failed while owning the lock in non-failoverSafe mode, then IgniteException is thrown.

Specified by:

tryLock in interface Lock

Returns:

true if the lock was acquired and false otherwise

Throws:

IgniteException - if node is stopped, or lock is already broken in non-failover safe mode

tryLock

boolean tryLock(long timeout,
                TimeUnit unit)
         throws IgniteInterruptedException,
                IgniteException

Acquires the lock if it is not held by another thread within the given waiting time and the current thread has not been interrupted.

Acquires the lock if it is not held by another thread and returns immediately with the value true, setting the lock hold count to one.

If the current thread already holds this lock then the hold count is incremented by one and the method returns true.

If the lock is held by another thread then the current thread becomes disabled for thread scheduling purposes and lies dormant until one of five things happens:

• The lock is acquired by the current thread; or
• Some other thread interrupts the current thread; or
• Lock is broken (any node failed while owning this lock), and lock is created in non-failoverSafe mode.
• Local node is stopped.
• The specified waiting time elapses

If the lock is acquired then the value true is returned and the lock hold count is set to one.

If the current thread:

• has its interrupted status set on entry to this method; or
• is interrupted while acquiring the lock; or

then IgniteInterruptedException is thrown and the current thread's interrupted status is cleared.

IgniteException is thrown in case:

• the lock is broken before or during the attempt to acquire this lock; or
• local node is stopped,

If the specified waiting time elapses then the value false is returned. If the time is less than or equal to zero, the method will not wait at all.

In this implementation, as this method is an explicit interruption point, preference is given to responding to the interrupt over normal or reentrant acquisition of the lock, and over reporting the elapse of the waiting time.

Specified by:

tryLock in interface Lock

Parameters:

timeout - the time to wait for the lock

unit - the time unit of the timeout argument

Returns:

true if the lock was free and was acquired by the current thread, or the lock was already held by the current thread; and false if the waiting time elapsed before the lock could be acquired

Throws:

IgniteInterruptedException - if the current thread is interrupted

IgniteException - if node is stopped, or lock is already broken in non-failover safe mode

NullPointerException - if the time unit is null

unlock

void unlock()
     throws IgniteInterruptedException

Releases the lock. If lock is not owned by current thread, then an IllegalMonitorStateException is thrown. If lock is already broken prior to invocation of this method, and lock is created in non-failover safe mode, then IgniteException is thrown.

Specified by:

unlock in interface Lock

Throws:

IllegalMonitorStateException - if not owned by current thread

IgniteException - if node is stopped, or lock is already broken in non-failover safe mode

IgniteInterruptedException

getOrCreateCondition

IgniteCondition getOrCreateCondition(String name)
                              throws IgniteException

Returns a Condition instance for use with this IgniteLock instance.

• If this lock is not held when any of the Condition waiting or signalling methods are called, then an IllegalMonitorStateException is thrown.
• When the condition waiting methods are called the lock is released and, before they return, the lock is reacquired and the lock hold count restored to what it was when the method was called.
• If a thread is interrupted while waiting then the wait will terminate, an IgniteInterruptedException will be thrown, and the thread's interrupted status will be cleared.
• Waiting threads are signalled in FIFO order.

Parameters:

name - Name of the distributed condition object

Returns:

the Condition object

Throws:

IgniteException - if the lock is not initialized or already removed

newCondition

Condition newCondition()

This method is not supported in IgniteLock, Any invocation of this method will result in UnsupportedOperationException. Correct way to obtain Condition object is through method getOrCreateCondition(String)

Specified by:

newCondition in interface Lock

getHoldCount

int getHoldCount()
          throws IgniteException

Queries the number of holds on this lock by the current thread.

Returns:

the number of holds on this lock by the current thread, or zero if this lock is not held by the current thread

Throws:

IgniteException - if the lock is not initialized or already removed

isHeldByCurrentThread

boolean isHeldByCurrentThread()
                       throws IgniteException

Queries if this lock is held by the current thread.

Returns:

true if current thread holds this lock and false otherwise

Throws:

IgniteException - if the lock is not initialized or already removed

isLocked

boolean isLocked()
          throws IgniteException

Queries if this lock is held by any thread on any node. This method is designed for use in monitoring of the system state, not for synchronization control.

Returns:

true if any thread on this or any other node holds this lock and false otherwise

Throws:

IgniteException - if the lock is not initialized or already removed

hasQueuedThreads

boolean hasQueuedThreads()
                  throws IgniteException

Queries whether any threads on this node are waiting to acquire this lock. Note that because cancellations may occur at any time, a true return does not guarantee that any other thread will ever acquire this lock. This method is designed primarily for use in monitoring of the system state.

Returns:

true if there may be other threads on this node waiting to acquire the lock

Throws:

IgniteException - if the lock is not initialized or already removed

hasQueuedThread

boolean hasQueuedThread(Thread thread)
                 throws IgniteException

Queries whether the given thread is waiting to acquire this lock. Note that because cancellations may occur at any time, a true return does not guarantee that this thread will ever acquire this lock. This method is designed primarily for use in monitoring of the system state.

Parameters:

thread - the thread

Returns:

true if the given thread is queued waiting for this lock

Throws:

NullPointerException - if the thread is null

IgniteException - if the lock is not initialized or already removed

hasWaiters

boolean hasWaiters(IgniteCondition condition)
            throws IgniteException

Queries whether any threads on this node are waiting on the given condition associated with this lock. Note that because timeouts and interrupts may occur at any time, a true return does not guarantee that a future signal will awaken any threads. This method is designed primarily for use in monitoring of the system state.

Parameters:

condition - the condition

Returns:

true if there are any waiting threads on this node

Throws:

IllegalMonitorStateException - if this lock is not held

IllegalArgumentException - if the given condition is not associated with this lock

NullPointerException - if the condition is null

IgniteException - if the lock is not initialized or already removed

getWaitQueueLength

int getWaitQueueLength(IgniteCondition condition)
                throws IgniteException

Returns an estimate of the number of threads on this node that are waiting on the given condition associated with this lock. Note that because timeouts and interrupts may occur at any time, the estimate serves only as an upper bound on the actual number of waiters. This method is designed for use in monitoring of the system state, not for synchronization control.

Parameters:

condition - the condition

Returns:

the estimated number of waiting threads on this node

Throws:

IllegalMonitorStateException - if this lock is not held

IllegalArgumentException - if the given condition is not associated with this lock

NullPointerException - if the condition is null

IgniteException - if the lock is not initialized or already removed

isFailoverSafe

boolean isFailoverSafe()

Returns true if this lock is safe to use after node failure. If not, IgniteInterruptedException is thrown on every other node after node failure.

Returns:

true if this reentrant lock has failoverSafe set true

Throws:

IgniteException - if the lock is not initialized or already removed

isFair

boolean isFair()

Returns true if this lock is fair. Fairness flag can only be set on lock creation.

Returns:

true if this reentrant lock has fairness flag set true.

Throws:

IgniteException - if the lock is not initialized or already removed

isBroken

boolean isBroken()
          throws IgniteException

Returns true if any node that owned the locked failed before releasing the lock.

Returns:

true if any node failed while owning the lock since the lock on this node was initialized.

Throws:

IgniteException - if the lock is not initialized or already removed

removed

boolean removed()

Gets status of reentrant lock.

Returns:

true if reentrant lock was removed from cache, false in other case.

close

void close()

Removes reentrant lock.

Specified by:

close in interface AutoCloseable

Specified by:

close in interface Closeable

Throws:

IgniteException - If operation failed.