Synchronous Commit v5.6

Commit scope kind: SYNCHRONOUS_COMMIT alias: SYNCHRONOUS COMMIT

Overview

PGD's SYNCHRONOUS_COMMIT is a commit scope kind that works in a way that's more like PostgreSQL's synchronous_commit option in its underlying operation. Unlike the PostgreSQL option, though, it's configured as a commit scope and is easier to configure and interact with in PGD.

Unlike other commit scope kinds, such as GROUP COMMIT and CAMO, the transactions in a SYNCHRONOUS_COMMIT operation aren't transformed into a two-phase commit (2PC) transaction. They work more like a Postgres synchronous_commit.

Example

In this example, when this commit scope is in use, any node in the left_dc group uses SYNCHRONOUS_COMMIT to replicate changes to the other nodes in the left_dc group. It looks for a majority of nodes in the left_dc group to confirm that they committed the transaction.

SELECT bdr.add_commit_scope(
    commit_scope_name := 'example_sc_scope',
    origin_node_group := 'left_dc',
    rule := 'MAJORITY (left_dc) SYNCHRONOUS_COMMIT',
    wait_for_ready := true
);

Configuration

SYNCHRONOUS_COMMIT supports the optional DEGRADE ON clause. See the SYNCHRONOUS_COMMIT commit scope reference for specific configuration parameters or see this section regarding Degrade on options.

Confirmation

Confirmation level	PGD Synchronous Commit handling
`received`	A remote PGD node confirms the transaction once it's been fully received and is in the in-memory write queue.
`replicated`	Same behavior as `received`.
`durable`	Confirms the transaction after all of its changes are flushed to disk. Analogous to `synchronous_commit = on` in legacy synchronous replication.
`visible` (default)	Confirms the transaction after all of its changes are flushed to disk and it's visible to concurrent transactions. Analogous to `synchronous_commit = remote_apply` in legacy synchronous replication.

Details

Currently SYNCHRONOUS_COMMIT doesn't use the confirmation levels of the commit scope rule syntax.

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Commit Scopes

Group Commit