postgres-errors-replication-lag

Quick Reference :

Three replication problems, three different views to inspect :

Problem	Where to look	Key signal
Streaming replica behind	`pg_stat_replication` (on PRIMARY)	`replay_lag`, byte diff vs `replay_lsn`
Slot retaining too much WAL	`pg_replication_slots` (on PRIMARY)	`active = false`, `wal_status` not `reserved`
Logical subscription behind / stuck	`pg_stat_subscription` (on SUBSCRIBER)	stale `latest_end_time`, errors in subscriber log

Two failures cause real outages :

Inactive slot fills the primary disk. A replication slot pins WAL from restart_lsn forward. If its consumer stops connecting, restart_lsn never advances and pg_wal grows without bound until the primary crashes on a full disk. ALWAYS set max_slot_wal_keep_size as a safety cap and ALWAYS alarm on inactive slots.

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Jun 17, 2026

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