Explicit two-phase commit (2PC)

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Explicit two-phase commit (2PC)#

Note

Two-phase commit isn’t available with Group Commit or CAMO. See Commit scope limitations .

An application can explicitly opt to use two-phase commit with PGD. See Distributed Transaction Processing: The XA Specification .

The X/Open Distributed Transaction Processing (DTP) model envisions three software components:

  • An application program (AP) that defines transaction boundaries and specifies actions that constitute a transaction

  • Resource managers (RMs), such as databases or file-access systems, that provide access to shared resources

  • A separate component called a transaction manager (TM) that assigns identifiers to transactions, monitors their progress, and takes responsibility for transaction completion and for failure recovery

PGD supports explicit external 2PC using the PREPARE TRANSACTION and COMMIT PREPARED /ROLLBACK PREPARED commands. Externally, an EDB Postgres Distributed cluster appears to be a single resource manager to the transaction manager for a single session.

When bdr.commit_scope is local , the transaction is prepared only on the local node. Once committed, changes are replicated, and PGD then applies post-commit conflict resolution.

Using bdr.commit_scope set to local might not seem to make sense with explicit two-phase commit. However, the option is offered to allow you to control the tradeoff between transaction latency and robustness.

Explicit two-phase commit doesn’t work with either CAMO or the global commit scope. Future releases might enable this combination.

Use#

Two-phase commits with a local commit scope work exactly like standard PostgreSQL. Use the local commit scope:

BEGIN;
SET LOCAL bdr.commit_scope = local;

... other commands possible...

To start the first phase of the commit, the client must assign a global transaction id, which can be any unique string identifying the transaction:

PREPARE TRANSACTION some-global-id;

After a successful first phase, all nodes have applied the changes and are prepared for committing the transaction. The client must then invoke the second phase from the same node:

COMMIT PREPARED some-global-id;
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