com.bigdata.zookeeper
Class ZNodeLockWatcher.ZLockImpl

java.lang.Object
  com.bigdata.zookeeper.ZNodeLockWatcher.ZLockImpl

All Implemented Interfaces:

ZLock

Enclosing class:

ZNodeLockWatcher

protected static class ZNodeLockWatcher.ZLockImpl
extends Object
implements ZLock

ZLock implementation class. The lock is realized as an EPHEMERAL SEQUENTIAL child of the lock node. If the child is in the first position of the lexically sorted children then it holds the lock.

Note: Assuming it has the correct ACL, any thread in any process MAY release the lock, NOT just the one that acquired it. This is done by deleting the ephemeral znode corresponding to the process holding the lock.

Version:

$Id$

Author:

Bryan Thompson

TODO:

The lock is not reentrant (you can not re-acquire it by requesting it while already held in the same or a different thread).

Field Summary

protected List<org.apache.zookeeper.data.ACL>	acl
protected org.apache.zookeeper.ZooKeeper	zookeeper
protected String	zpath The zpath of the lock node (the parent node whose ephemeral sequential children represent the queue of processes contending for the lock).

Constructor Summary

protected

ZNodeLockWatcher.ZLockImpl(org.apache.zookeeper.ZooKeeper zookeeper, String zpath, List<org.apache.zookeeper.data.ACL> acl)

Method Summary

void	destroyLock() Creates a marker node (a sibling of the lock node) to prevent new children from being added to the queue and then deletes all children in the queue in reverse lexical order so as to not trigger cascades of watchers and finally deletes the lock node itself and then the marker node.
String	getChild() The znode (not zpath) of the child and null iff the child does not hold the lock.
String	getZPath() The zpath of the lock node.
boolean	isLockHeld() Return true if the lock is currently held (this verifies that the zchild for the lock is in fact the leader in the queue of ephemeral znodes contending for the lock).
void	lock() Acquire the lock (blocking).
void	lock(long timeout, TimeUnit unit) Creates a new lock request (an EPHEMERAL SEQUENTIAL znode that is a child of the lock node) and awaits up to the timeout for the ZLock to be granted.
String	toString() Non-blocking representation of lock state.
void	unlock() Note: In the case where the lock was stolen by deleting the zchild node this method logs a warning but does not throw an exception.

Methods inherited from class java.lang.Object

clone, equals, finalize, getClass, hashCode, notify, notifyAll, wait, wait, wait

Field Detail

zookeeper

protected final org.apache.zookeeper.ZooKeeper zookeeper

zpath

protected final String zpath

The zpath of the lock node (the parent node whose ephemeral sequential children represent the queue of processes contending for the lock).

acl

protected final List<org.apache.zookeeper.data.ACL> acl

Constructor Detail

ZNodeLockWatcher.ZLockImpl

protected ZNodeLockWatcher.ZLockImpl(org.apache.zookeeper.ZooKeeper zookeeper,
                                     String zpath,
                                     List<org.apache.zookeeper.data.ACL> acl)

Parameters:

zookeeper -

zpath - The zpath of the lock node.

acl -

Method Detail

toString

public String toString()

Non-blocking representation of lock state.

Overrides:

toString in class Object

getZPath

public String getZPath()

The zpath of the lock node.

getChild

public String getChild()
                throws InterruptedException

The znode (not zpath) of the child and null iff the child does not hold the lock.

Throws:

InterruptedException

lock

public void lock()
          throws org.apache.zookeeper.KeeperException,
                 InterruptedException

Description copied from interface: ZLock

Acquire the lock (blocking).

Specified by:

lock in interface ZLock

Throws:

org.apache.zookeeper.KeeperException - if the lock node or the ephemeral child could not be created.

org.apache.zookeeper.KeeperException.SessionExpiredException - If the ZooKeeper client's session was expired. note that this exception is non-recoverable for a given ZooKeeper instance. However, if the application handles the exception according to its own semantics then it MAY obtain a new ZooKeeper instance associated with a new session.

InterruptedException - if the thread was interrupted while awaiting the lock.

isLockHeld

public boolean isLockHeld()
                   throws org.apache.zookeeper.KeeperException,
                          InterruptedException

Description copied from interface: ZLock

Return true if the lock is currently held (this verifies that the zchild for the lock is in fact the leader in the queue of ephemeral znodes contending for the lock).

Specified by:

isLockHeld in interface ZLock

Returns:

true iff the lock is held.

Throws:

org.apache.zookeeper.KeeperException

org.apache.zookeeper.KeeperException.SessionExpiredException - If the ZooKeeper client's session was expired. note that this exception is non-recoverable for a given ZooKeeper instance. However, if the application handles the exception according to its own semantics then it MAY obtain a new ZooKeeper instance associated with a new session.

InterruptedException

lock

public void lock(long timeout,
                 TimeUnit unit)
          throws org.apache.zookeeper.KeeperException,
                 InterruptedException,
                 TimeoutException

Description copied from interface: ZLock

Creates a new lock request (an EPHEMERAL SEQUENTIAL znode that is a child of the lock node) and awaits up to the timeout for the ZLock to be granted.

Specified by:

lock in interface ZLock

Parameters:

timeout - The timeout.

unit - The unit in which that timeout is expressed.

Throws:

org.apache.zookeeper.KeeperException

org.apache.zookeeper.KeeperException.SessionExpiredException - If the ZooKeeper client's session was expired. note that this exception is non-recoverable for a given ZooKeeper instance. However, if the application handles the exception according to its own semantics then it MAY obtain a new ZooKeeper instance associated with a new session.

InterruptedException - if the thread was interrupted while awaiting the lock.

TimeoutException - if the timeout noticeably expired before the lock was granted.

unlock

public void unlock()
            throws org.apache.zookeeper.KeeperException,
                   InterruptedException

Note: In the case where the lock was stolen by deleting the zchild node this method logs a warning but does not throw an exception.

Specified by:

unlock in interface ZLock

Throws:

org.apache.zookeeper.KeeperException

InterruptedException

destroyLock

public void destroyLock()
                 throws org.apache.zookeeper.KeeperException,
                        InterruptedException

Creates a marker node (a sibling of the lock node) to prevent new children from being added to the queue and then deletes all children in the queue in reverse lexical order so as to not trigger cascades of watchers and finally deletes the lock node itself and then the marker node.

Specified by:

destroyLock in interface ZLock

Throws:

org.apache.zookeeper.KeeperException

InterruptedException

TODO:

write unit tests for this., verify unlock() succeeds after destroyLock(), consider allowing even if you do not hold the lock., consider what happens if there is a transient disconnect.