66.4. 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 arePREPARE TRANSACTION,COMMIT PREPARED andROLLBACK 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 executesPREPARE TRANSACTION, the only possible next commands areCOMMIT PREPARED orROLLBACK 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 thepg_twophase directory. Transactions that are currently prepared can be inspected usingpg_prepared_xacts.