All threads converge and wait at the start of a barrier transaction. The barrier transaction is suspended until the other threads reach its start position. If any threads were already processing part of the barrier transaction, those threads perform a rollback. Grouped transactions, such as those controlled by the BATCHSQL or GROUPTRANSOPS parameters, are also rolled back and then reapplied until they reach the start of the barrier transaction.

All of the threads converge and wait at the start of the next transaction after the barrier transaction as well. The two synchronization points, before and after the barrier transaction, ensure that metadata operations and EVENTACTIONS actions all occur in the proper order relevant to the data operations.

Once the threads are synchronized at the start of the barrier transaction, the barrier transaction is processed serially by the thread that has the lowest thread ID among all of the threads specified in the MAP statements, and then parallel processing across threads is resumed. You can force barrier transactions to be processed through a specific thread, which is always thread 0, by specifying the USEDEDICATEDCOORDINATIONTHREAD parameter in the Replicat parameter file.