I needed a set of distributed locks for
GaeVFS<http://code.google.com/p/gaevfs/> that
work across multiple JVM instances, and chose to implement them to the
Lock<http://java.sun.com/javase/6/docs/api/java/util/concurrent/locks/Lock.html>and
ReadWriteLock<http://java.sun.com/javase/6/docs/api/java/util/concurrent/locks/ReadWriteLock.html>interfaces
of the
java.util.concurrent.locks<http://java.sun.com/javase/6/docs/api/java/util/concurrent/locks/package-summary.html>package
in the hope that they might be generally useful for other
applications. The source
code<http://code.google.com/p/gaevfs/source/browse/trunk/src/com/newatlanta/appengine/locks/>for
the initial implementation of these locks is available from the GaeVFS
project web site in the
com.newatlanta.appengine.locks<http://code.google.com/p/gaevfs/source/browse/trunk/src/com/newatlanta/appengine/locks/>package
(junit tests are in the
com.newatlanta.appengine.junit.locks<http://code.google.com/p/gaevfs/source/browse/trunk/test/src/com/newatlanta/appengine/junit/locks/>package),
which includes the following classes:
ExclusiveLock
Implements a mutual exclusion lock with the following features:
- re-entrant locking by the thread that owns the lock (matching unlocks
are required to fully release the lock)
- only the thread that owns the lock can unlock it
- locks automatically expire after 30 seconds (the GAE request timeout
limit)
- locks can be acquired with a single call to memcache
ReadWriteLock
Implements a "many readers, one writer" scheme with the following features:
- the write lock is mutually exclusive and can only be acquired if no
other threads are holding the read lock
- the read lock is shared and can only be acquired if no other threads
are holding the write lock
- the write lock is re-entrant by the thread that owns the lock
(matching unlocks are required to fully release the lock)
- the thread that owns the write lock may also acquire the read lock
(it's not necessary to unlock the read lock--releasing the write
lock also
releases the read lock)
- write locks automatically expire after 30 seconds (the GAE request
timeout limit)
Using the ReadWriteLock can be fairly expensive; it takes a minimum of two
memcache calls to acquire the write lock, and a minimum of three memcache
calls to acquire a read lock. Therefore, it might be better to use an
ExclusiveLock--which can be acquired with a minimum of one memcache
call--depending on our application. For example, if your application's read
operations are short and fast, then an ExclusiveLock might perform better
than a ReadWriteLock.
SharedLock
Implements a counter-based shared lock that isn't particularly useful by
itself, but is used as a base class for the ReadWriteLock implementation.
AbstractLock
The base class for ExclusiveLock and SharedLock.
The implementations of these locks are based on the atomic features of the
MemcacheService<http://code.google.com/appengine/docs/java/javadoc/com/google/appengine/api/memcache/package-summary.html>,
and are therefore unreliable because memcache can evict them at any time.
This shouldn't be too much of a concern for the ExclusiveLock or the "write
lock" of the ReadWriteLock because the maximum lifetime of these locks is
only 30 seconds (the maximum lifetime of a request). However, this is a
concern for the counter-based SharedLock and the "read lock" of the
ReadWriteLock. I plan to address this by persisting the counters to the GAE
datastore via a write-behind mechanism as soon as Task Queues are available
for GAE/J.
I haven't done any performance testing of these locks (maybe when profiling
is available on GAE/J?). As with any synchronization mechanism, use these
sparingly, since misuse can badly harm the performance of your application.
Any comments or feedback is welcomed.
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