Last Resource Commit optimization for JDBC

There is one caveat with Last Resource Commit. There is a small chance that a transaction ends up with inconsistent results across participating resources if BTM crashes while a transaction is in-flight. The chance is small but it exists so be careful when using that feature. This is in no way a limitation of BTM but of the concept itself.

Please note that if you only intend to run transactions against a single database using Last Resource Commit this scenario is 100% safe, otherwise not.

In theory, only databases supporting XA and providing a javax.sql.XADataSource implementation can be used with transaction managers. In practice, there is a way around this limitation.

The Last Resource Commit optimization (sometimes referred to as Last Resource Gambit or Last Agent optimization) allows a single non-XA database to participate in a XA transaction by cleverly ordering the resources.

There can be at most one datasource emulating XA with Last Resource Commit participating in a transaction. If it happens that you're trying to use a second emulating datasource while one has already been used, BTM will throw an exception. Again, this is not a limitation of BTM but of the concept itself.

To enable it, you just have to create a PoolingDataSource using the as the XADataSource implementation.

Here's an example of code configuring a HSQLDB datasource:

PoolingDataSource myDataSource = new PoolingDataSource();
myDataSource.getDriverProperties().setProperty("driverClassName", "org.hsqldb.jdbcDriver");
myDataSource.getDriverProperties().setProperty("url", "jdbc:hsqldb:/the/db/path");
myDataSource.getDriverProperties().setProperty("user", "sa");
myDataSource.getDriverProperties().setProperty("password", "theSaPassword");

and the same example viewed as a Resource Loader configuration

The LRC implementation relies on the useTmJoin and deferConnectionRelease pool properties which must always be true (the default value).
Do not change these properties as this could force the transaction manager to reject the resource or worse, cause inconsistencies during commit phase.

BTM 1.3.1 and higher are immune to this problem as these settings are enforced when LrcXADataSource is detected.