applytransaction

Mechanism

PG_WAIT_LOCK at src/backend/storage/lmgr/proc.c:1487 is the grep-verified reporting path. The public identity/resource is mapped at src/backend/utils/activity/wait_event_names.txt:429. The instrumented operation is: Waiting to acquire a lock on a remote transaction being applied by a logical replication subscriber. The lock manager could not grant the applytransaction heavyweight lock immediately. ProcSleep reports the lock wait and parks the backend on the lock's wait queue until owners release or the request is cancelled.

Expected behavior

Sub-second handoff during ordinary writes or planned DDL can be normal.

When to investigate

Investigate once a user-facing wait breaches its latency objective, a blocking chain grows, or the root holder is idle in transaction.

Diagnostic actions

• Build the pg_blocking_pids graph to its root.
• Inspect the root holder's state, transaction age, and business purpose.
• Choose cancellation, timeout, or workload sequencing only after identifying the safest root action.

Diagnostic queries

Sessions waiting on Lock/applytransaction

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 = 'Lock'
  AND wait_event = 'applytransaction'
ORDER BY query_age DESC NULLS LAST;

Current Lock 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 = 'Lock'
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 = 'Lock'
  AND a.wait_event = 'applytransaction'
ORDER BY a.pid, l.granted, l.locktype, l.mode;