Parameter · Replication / Primary Server

synchronous_standby_names

Number of synchronous standbys and list of names of potential synchronous ones.

Default: '' (empty)
Type: string
Applies: Reload

In plain words

Normally COMMIT returns as soon as the primary has saved the change to its own WAL; standbys get it a moment later (asynchronous). Listing standbys here makes replication synchronous: COMMIT waits until enough of them confirm they have the change too, so a primary failure loses no committed data.

FIRST 1 (s1, s2) waits for the highest-priority connected standby; ANY 2 (s1, s2, s3) waits for any two. If no listed standby is available, commits wait forever.

For DBAs and operators

Names match a standby's application_name (set in its primary_conninfo; default is cluster_name or walreceiver). * matches anything. Syntax:

FIRST n (a, b, c): priority. The first n connected names in list order are sync; others are potential and take over when one disappears.
ANY n (a, b, c): quorum. Any n confirmations suffice; all listed are quorum.
a, b (no keyword): legacy form, same as FIRST 1 (a, b).

What "confirm" means is set by synchronous_commit (remote_write, on = flushed, remote_apply). It can be turned off per transaction for unimportant writes.

LOG: standby "s1" is now a synchronous standby with priority 1
LOG: standby "s2" is now a candidate for quorum synchronous standby

SELECT application_name, state, sync_priority, sync_state, replay_lag
  FROM pg_stat_replication;

Support notes

Hanging commits with wait_event = 'SyncRep' in pg_stat_activity: not enough sync standbys connected. Fix the standby, or reduce the list and reload.
Cancelling a waiting commit gives WARNING: canceling wait for synchronous replication due to user request DETAIL: The transaction has already committed locally, but might not have been replicated to the standby.
Use at least one more candidate than n, otherwise one standby outage stops all writes.

How to change it

Change in postgresql.conf or ALTER SYSTEM, then run SELECT pg_reload_conf().

See it in context

Synchronous replication Streaming replication WAL sender

Related parameters

synchronous_commit primary_conninfo wal_sender_timeout max_wal_senders

All Replication parameters