SyncRep

Mechanism

SyncRepLock protects synchronous-replication queues and shared sender state. Committers enqueue or inspect their wait position while WAL senders update which LSNs the configured synchronous standbys have acknowledged.

Expected behavior

Small bursts occur as synchronous commits queue and WAL senders publish acknowledgements.

When to investigate

Sustained SyncRep LWLock contention is different from waiting for a standby acknowledgement: it means queue/state coordination itself is hot, usually under extreme commit concurrency or sender churn.

Diagnostic actions

• Separate LWLock/SyncRep from IPC/SyncRep and replication-lag waits.
• Inspect synchronous standby health, sender churn, and commit rate.
• Stabilize replication and smooth commit bursts; change durability policy only with explicit incident authority.

Incident patterns

A standby flaps while a burst of synchronous committers repeatedly enters and leaves the wait queue, making queue coordination visible as LWLock/SyncRep.

Diagnostic queries

Sessions waiting on LWLock/SyncRep

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 = 'LWLock'
  AND wait_event = 'SyncRep'
ORDER BY query_age DESC NULLS LAST;

Current LWLock cohort

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 = 'LWLock'
GROUP BY wait_event
ORDER BY waiting_sessions DESC, wait_event;

Lock and relation context for these sessions

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 = 'LWLock'
  AND a.wait_event = 'SyncRep'
ORDER BY a.pid, l.granted, l.locktype, l.mode;