Atomic read OR write

[krisjenkins]

Hi,

I'm looking into migrating from OSCache to JBossCache, and I'm worried about a locking issue.

I'd like to be able to say, atomically, "Read this FQN/KEY, and if it comes back null, acquire a write lock and query the database". What I want to avoid is having two threads which read, find the key empty, first thread aquires a write lock, second thread queues for a write lock when it will only need a read lock.

Is it possible to do this with JBossCache?

Cheers,
Kris

[ben.wang]

This may be difficult to do without thinking it further. If the node does not exist, for example, there is no way to lock it. Therefore, atomicity can not be achieved.

-Ben

[belaban]

How about the following code:

// ReentrantLock lock;
lock.acquire();
Object val=cache.get(key);
if(val == null) {
val=fetchValueFromDB();
cache.put(node, key, val);
}
lock.release();

This simply uses external synchronization.

A more elegant solution would be to use a CacheLoader: a get() fetches the value from the DB and adds it to the cache with a write-lock. CacheLoader synchronizes access, so only 1 thread actually triggers the loading.

[krisjenkins]

Ah, that's a shame. Thanks for the rapid answer though, Ben.

Cheers,
Kris

[krisjenkins]

Bela,

Maybe I'm missing something, but it looks like that code fragment makes things worse. Now it will synchronize on every read, not just every read-that-requires-a-write. Is that right? And if so, would a cacheLoader suffer the same problem?

Kris

[belaban]

Use a read/write lock then. The problem you are trying to solve is exactly what the CacheLoader was designed to do.

With a CacheLoader, every get() on a non-existing node, or a node marked as not-yet-loaded will access the underlying store.

[krisjenkins]

I see. Thanks Bela. :-)