max_parallel_apply_workers_per_subscription

max_parallel_apply_workers_per_subscription sets the maximum number of parallel apply workers per subscription. It is a sighup setting present in PG16–18; the latest recorded boot default is 2.

Note

Fact — official short description: “Maximum number of parallel apply workers per subscription.”

Identity

Type , Valueinteger: Upstream pg_settings type
Context , Valuesighup: Takes effect after configuration reload
Unit , Value—: Raw unit
Range , Value0 – 1024: Raw limits in the last observed version
Enum values , Value—: — for non-enum types
Category , ValueReplication / Subscribers: Upstream classification
Latest boot value , Value2: 2

Lifecycle

Fact	Value
First observed	PG16
Present in	PG16–19 Beta 3
Removed in	No
Introduction commit	`216a784829c2` — Perform apply of large transactions by parallel workers.
Commit date	2023-01-09
Discussion	thread 1

Default history

Measured PG9.0–19 Beta 3 boot defaults
Versions	Raw `boot_val`	Unit	Human value
PG16–19 Beta 3	`2`	—	2

How it works

Maximum number of parallel apply workers per subscription. A configuration reload applies a new value; existing work already in flight is not retroactively changed.

A subscription leader can use up to this many parallel workers to apply suitable in-progress transactions. More workers do not parallelize every transaction and consume the global logical-replication and background-worker pools.

Monitor and change max_parallel_apply_workers_per_subscription together with max_logical_replication_workers, max_sync_workers_per_subscription, max_worker_processes. 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 max_parallel_apply_workers_per_subscription 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: PG16–19 Beta 3 unmodified; OLAP: PG16–19 Beta 3 unmodified; CRIT: PG16–19 Beta 3 unmodified; TINY: PG16–19 Beta 3 unmodified. No Pigsty-specific rationale is inferred from an absent override.

Common pitfalls

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.
Failing over to a node that lacks the old primary’s capacity or prerequisites.
Using infinite waits or WAL retention to hide a failed consumer.

max_logical_replication_workers · max_sync_workers_per_subscription · max_worker_processes · max_replication_slots · max_active_replication_origins · hot_standby