# max_wal_senders

> max_wal_senders — Sets the maximum number of simultaneously running WAL sender processes. Observed in PG9.0–19 Beta 3; its last measured boot default is 10 in PG19 Beta 3, with postmaster context. This is a beta-snapshot fact and can change before PostgreSQL 19 GA.
---

> [!NOTE]
> **Fact — official short description:** “Sets the maximum number of simultaneously running WAL sender processes.”

## Identity {#identity}

| Field | Value | Meaning |
| --- | --- | --- |
| Type | `integer` | Upstream pg_settings type |
| Context | `postmaster` | Requires a server restart |
| Unit | — | Raw unit |
| Range | `0` – `262143` | Raw limits in the last observed version |
| Enum values | — | — for non-enum types |
| Category | Replication / Sending Servers | Upstream classification |
| Latest boot value | `10` | 10 |
{.fields meta="-"}

## Lifecycle {#lifecycle}

| Fact | Value |
| --- | --- |
| First observed | PG9.0 (research boundary) |
| Present in | PG9.0–19 Beta 3 |
| Removed in | No |
| Introduction commit | Not asserted: predates the PG9.0 research boundary |
| Commit date | — |
| Discussion | — |

## Default history {#default-history}

| Versions | Raw `boot_val` | Unit | Human value |
| --- | --- | --- | --- |
| PG9.0–9.6 | `0` | — | 0 |
| PG10–19 Beta 3 | `10` | — | 10 |
{.full-width caption="Measured PG9.0–19 Beta 3 boot defaults"}

## How it works {#mechanism}

Sets the maximum number of simultaneously running WAL sender processes. The value is fixed when the server starts, so changing it requires a restart.

Each streaming standby, WAL-streaming base backup, or logical replication connection can occupy a WAL sender. Zero disables sending, disconnected clients may retain a slot until timeout, and a standby must reserve at least as many senders as its primary to accept hot-standby queries safely.

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

## Tuning advice {#tuning-advice}

> [!TIP]
> **Advice.** These are workload-specific starting points and must be validated with measurements.

| Workload | Guidance |
| --- | --- |
| OLTP | Size max_wal_senders 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. |
{.full-width}

## Pigsty {#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 | `50` | different | `50` |
| OLAP | `50` | different | `50` |
| CRIT | `50` | different | `50` |
| TINY | `50` | different | `50` |
{.full-width}

> [!CAUTION]
> **Advice — pending human review.** Fact from the current Pigsty template projection: OLTP: PG9.0–19 Beta 3 = 50 (dcs); OLAP: PG9.0–19 Beta 3 = 50 (dcs); CRIT: PG9.0–19 Beta 3 = 50 (dcs); TINY: PG9.0–19 Beta 3 = 50 (dcs). Advice, pending human review — Editorial interpretation, pending human maintainer review: the override appears intended to reserve connection headroom for replicas, backups, and logical clients; confirm it against the current Pigsty templates, hardware fixture, and operational guarantees before publication.

## Common pitfalls {#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.

## Related parameters {#related-parameters}

[`wal_level`](/parameters/wal-level/) · [`max_replication_slots`](/parameters/max-replication-slots/) · [`archive_mode`](/parameters/archive-mode/) · [`wal_log_hints`](/parameters/wal-log-hints/) · [`output_plugin_libraries`](/parameters/output-plugin-libraries/) · [`wal_sender_timeout`](/parameters/wal-sender-timeout/)

## References {#references}

- [PostgreSQL 19 Beta 3 — max_wal_senders](https://www.postgresql.org/docs/19/runtime-config-replication.html#GUC-MAX-WAL-SENDERS)
- [PostgreSQL 19 release notes](https://www.postgresql.org/docs/19/release-19.html)
- [Machine-readable GUC export](/data/guc.jsonl)