WalSenderWriteData

Mechanism

WAIT_EVENT_WAL_SENDER_WRITE_DATA at src/backend/replication/walsender.c:1653 is the grep-verified reporting path. The public identity/resource is mapped at src/backend/utils/activity/wait_event_names.txt:93. The instrumented operation is: Waiting for any activity when processing replies from WAL receiver in WAL sender process. The frontend protocol path reports WalSenderWriteData immediately before a socket, TLS/GSS, or replication-client operation blocks. PostgreSQL is waiting for the remote side or network to progress.

Expected behavior

Idle ClientRead sessions are normal connection-pool inventory.

When to investigate

Investigate active writes, handshakes, old idle-in-transaction sessions, connection exhaustion, or replication-client stalls.

Diagnostic actions

• Check state and transaction age before counting the wait as load.
• Correlate with the owning application, pool, and network path.
• Fix consumer backpressure or pool policy rather than repeatedly terminating connections.

Diagnostic queries

Sessions waiting on Client/WalSenderWriteData

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 = 'Client'
  AND wait_event = 'WalSenderWriteData'
ORDER BY query_age DESC NULLS LAST;

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