Deadlock issues with one thread?

Question

I've been trying to nail down a deadlock issue for quite a while now and I'm just at a loss. My application basically has one main thread of execution which does get(), put(), and delete() on a database, and another background thread that goes through the database periodically (e.g. every 5 minutes) and prunes the db. The pruning thread works by using a cursor to iterate over the db and evaluates each record and, if it's expired, deletes it, stopping when it reaches the end of the DB or when it's deleted 100,000 records. I had been getting DeadlockExceptions and figured it was a conflict between the main thread and the pruning thread competing for locks on the same objects, so I modified my code so that when the pruning thread wakes up and starts pruning, it makes the main thread wait(), so theoretically the two threads should never be doing db operations at the same time. However, this doesn't seem to have resolved the issue, as I still get DeadlockExceptions. I've raised the lock timeout to 1000 ms and set it to retry up to 20x to no avail. I've even tried it with the pruning threads completely disabled and I still got it, so it seems the deadlock is occurring even within the single thread. Any ideas? I'm not using transactions, if that matters.
Also, on this page - http://www.oracle.com/technology/documentation/berkeley-db/db/gsg_txn/JAVA/lockingsubsystem.html#deadlockresolve - there's reference to "EnvironmentConfig.setLockDetectMode()". I'm using JE 3.2.23 and there doesn't appear to be any such function. Is this out-of-date documentation or am I using and old version?

Answer 1

The first step is to take a look at the information in the DeadlockException -- it will tell you which threads are conflicting. Please post the stack traces and I'll be happy to help interpret them.
Also please take a look at this FAQ:
http://www.oracle.com/technology/products/berkeley-db/faq/je_faq.html#23
If you set je.txn.deadlockStackTrace and je.txn.dumpLocks as described, you'll get more information when the deadlock occurs.
Also, on this page -
http://www.oracle.com/technology/documentation/berkeley-db/db/gsg_txn/JAVA/lockingsubsystem.html#deadlockresolve
- there's reference to "EnvironmentConfig.setLockDetectMode()". I'm using JE 3.2.23 and
there doesn't appear to be any such function. Is this out-of-date documentation or am I
using and old version?The link above is for documentation in Berkeley DB the C Edition, not Berkeley DB Java Edition (JE) -- you're reading the doc for the wrong product. The JE doc is here:
http://www.oracle.com/technology/documentation/berkeley-db/je/TransactionGettingStarted/jelock.html
There is no lock detect mode in JE, it always uses a timeout of the oldest locker.
Mark