IPC: WalReceiverWaitStart

Official description

Waiting for startup process to send initial data for streaming replication

Earlier names: Client/WalReceiverWaitStart (PG 13)

Trigger mechanism

WAIT_EVENT_WAL_RECEIVER_WAIT_START at src/backend/postmaster/pgstat.c:3738 is the grep-verified reporting path. The public identity/resource is mapped at src/backend/postmaster/pgstat.c:3739. The instrumented operation is: Waiting for startup process to send initial data for streaming replication. The WalReceiverWaitStart path has reached a process-coordination point and sleeps until a peer, worker, barrier, queue, or replication phase signals progress.

Normal or trouble?

• Normal: Brief rendezvous is normal when every participant continues to advance.
• Investigate: Investigate when the same phase persists across three samples, the expected peer is absent or blocked, or a queue and its dependants stop moving.

Diagnostic SQL

SELECT pid, backend_type, usename, datname, application_name,
       state, now() - query_start AS query_age,
       now() - xact_start AS xact_age,
       wait_event_type, wait_event,
       pg_blocking_pids(pid) AS blocking_pids,
       left(query, 160) AS query
FROM pg_stat_activity
WHERE wait_event_type = 'IPC'
  AND wait_event = 'WalReceiverWaitStart'
ORDER BY query_age DESC NULLS LAST;

SELECT wait_event, count(*) AS waiting_sessions,
       count(*) FILTER (WHERE state = 'active') AS active_waiters,
       max(now() - query_start) AS oldest_query
FROM pg_stat_activity
WHERE wait_event_type = 'IPC'
GROUP BY wait_event
ORDER BY waiting_sessions DESC, wait_event;

SELECT a.pid, l.locktype, l.mode, l.granted, l.fastpath,
       d.datname, n.nspname, c.relname,
       l.page, l.tuple, l.virtualxid, l.transactionid,
       l.classid, l.objid, l.objsubid
FROM pg_stat_activity AS a
LEFT JOIN pg_locks AS l ON l.pid = a.pid
LEFT JOIN pg_database AS d ON d.oid = l.database
LEFT JOIN pg_class AS c
  ON c.oid = l.relation
 AND l.database = (
       SELECT oid FROM pg_database WHERE datname = current_database()
     )
LEFT JOIN pg_namespace AS n ON n.oid = c.relnamespace
WHERE a.wait_event_type = 'IPC'
  AND a.wait_event = 'WalReceiverWaitStart'
ORDER BY a.pid, l.granted, l.locktype, l.mode;

Response

1. Identify the peer or phase named by the event.
2. Inspect that participant’s wait, error, and progress state.
3. Repair the stalled participant or upstream dependency instead of treating the waiter as the root cause.

Source evidence

• PostgreSQL 13.23 · src/backend/postmaster/pgstat.c:3738 — WAIT_EVENT_WAL_RECEIVER_WAIT_START
• PostgreSQL 13.23 · src/backend/postmaster/pgstat.c:3739 — WalReceiverWaitStart
• PostgreSQL 18.6 · src/backend/replication/walreceiver.c:784 — WAIT_EVENT_WAL_RECEIVER_WAIT_START
• PostgreSQL 18.6 · src/backend/utils/activity/wait_event_names.txt:160 — WAL_RECEIVER_WAIT_START

Related controls and signals

• GUCs: [guc:wal_buffers] · [guc:synchronous_commit]
• Metrics: [metric:waiting_sessions] · [metric:wait_event_share] · [metric:wal_bytes] · [metric:wal_write_time]