13.4. Last Resource Commit Optimization (LRCO)

Although the XA transaction protocol is designed to provide ACID properties by using a two-phase commit protocol, model may not always be appropriate. Sometimes it is necessary to allow a non-XA-aware resource manager to participate in a transaction. This is often the case with data stores that do not support distributed transactions.

In this situation, you can use a technique known as Last Resource Commit Optimization (LRCO). This is sometimes called the Last Resource Gambit. The one-phase-aware resource is processed last in the prepare phase of the transaction, at which time an attempt is made to commit it. If the attempt is successful, the transaction log is written and the remaining resources go through the phase-two commit. If the last resource fails to commit, the transaction is rolled back. Although this protocol allows most transactions to complete normally, some errors can cause an inconsistent transaction outcome. For this reason, use LRCO as a last resort. When a single <local-tx-datasource> is used in a transaction, the LRCO is automatically applied to it. In other situations, you can designate a last resource by using a special marker interface. Refer to the JBoss Transactions Programmer's Guide for more details.

Using more than a single one-phase resource in the same transaction is not transactionally safe, and is not recommended. JBoss Transaction Service sees an attempt to enlist a second such resource as an error and terminates the transaction. This type of error is most often found when migrating from a legacy version of JBoss Application Server. Whenever possible the <local-tx-datasource> should be converted to an <xa-datasource> to resolve the difficulty.