67.4. Two-Phase Transactions
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## 67.4. Two-Phase Transactions #

PostgreSQL™ supports a two-phase commit (2PC) protocol that allows multiple distributed systems to work together in a transactional manner. The commands are PREPARE TRANSACTION, COMMIT PREPARED and ROLLBACK PREPARED. Two-phase transactions are intended for use by external transaction management systems. PostgreSQL™ follows the features and model proposed by the X/Open XA standard, but does not implement some less often used aspects.

When the user executes PREPARE TRANSACTION, the only possible next commands are COMMIT PREPARED or ROLLBACK PREPARED. In general, this prepared state is intended to be of very short duration, but external availability issues might mean transactions stay in this state for an extended interval. Short-lived prepared transactions are stored only in shared memory and WAL. Transactions that span checkpoints are recorded in the `pg_twophase` directory. Transactions that are currently prepared can be inspected using pg_prepared_xacts.

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