synchronized_standby_slots

synchronized_standby_slots lists streaming replication standby server replication slot names that logical WAL sender processes will wait for. It is a sighup setting present in PG17–18; the latest recorded boot default is empty.

Note

Fact — official short description: “Lists streaming replication standby server replication slot names that logical WAL sender processes will wait for.”

Identity

Type , Valuestring: Upstream pg_settings type
Context , Valuesighup: Takes effect after configuration reload
Unit , Value—: Raw unit
Range , Value—: Raw limits in the last observed version
Enum values , Value—: — for non-enum types
Category , ValueReplication / Primary Server: Upstream classification
Latest boot value , Value"": empty string

Lifecycle

Fact	Value
First observed	PG17
Present in	PG17–19 Beta 3
Removed in	No
Introduction commit	`0f934b0739ad` — Rename standby_slot_names to synchronized_standby_slots.
Commit date	2024-07-01
Discussion	thread 1

Default history

Measured PG9.0–19 Beta 3 boot defaults
Versions	Raw `boot_val`	Unit	Human value
PG17–19 Beta 3	`""`	—	empty string

How it works

Lists streaming replication standby server replication slot names that logical WAL sender processes will wait for. A configuration reload applies a new value; existing work already in flight is not retroactively changed.

Logical senders on the primary wait until every listed physical standby slot has confirmed the relevant WAL before decoding sends changes. This protects failover-slot continuity, but a missing, invalid, or stalled listed slot can stop logical replication and slot-management functions.

Monitor and change synchronized_standby_slots together with sync_replication_slots, primary_slot_name, max_replication_slots. Validate on the relevant server role and real workload, then use its sighup context to choose session change, reload, or restart; a historical boot default is not the current effective value.

Tuning advice

Tip

Advice. These are workload-specific starting points and must be validated with measurements.

Workload	Guidance
OLTP	Size synchronized_standby_slots from topology, failover roles, slot/subscription count, and reconnect headroom. Test worst-case primary latency, standby replay, and disk retention before production.
OLAP	Read standbys and logical subscribers often see long queries or large transactions. Put explicit bounds on replay/apply and monitor lag, worker saturation, slot restart_lsn, and conflict cancellations.
Small nodes	Configure only replication capacity that is actually used. Even a small topology needs bounded timeouts and slot lifecycle; unlimited retention is not reliability.

Pigsty

Values use the fixed 8-vCPU, 32-GiB, 100-GiB SSD fixture and render the current Pigsty templates for PG19 Beta 3; this does not assert current Pigsty support for that historical or beta release.

Template	Effective value	Versus upstream boot	Source expression
OLTP	Unmodified	—	—
OLAP	Unmodified	—	—
CRIT	Unmodified	—	—
TINY	Unmodified	—	—

Caution

Advice — pending human review. Fact from the current Pigsty template projection: OLTP: PG17–19 Beta 3 unmodified; OLAP: PG17–19 Beta 3 unmodified; CRIT: PG17–19 Beta 3 unmodified; TINY: PG17–19 Beta 3 unmodified. No Pigsty-specific rationale is inferred from an absent override.

Common pitfalls

Listing a missing or invalid slot and blocking logical senders.
Confusing physical slot names with standby application_name values.
Failing to enable sync_replication_slots on the corresponding standbys.
Changing it on the wrong primary, standby, sender, or subscriber role.
Watching only configured bytes/time instead of actual lag, slot position, and worker state.

sync_replication_slots · primary_slot_name · max_replication_slots · max_slot_wal_keep_size · synchronous_standby_names · vacuum_defer_cleanup_age