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.