The replication_slot collector monitors PostgreSQL replication slots, tracking WAL retention and slot activity.
Status
Default: Enabled
Metrics
pg_replication_slot_slot_current_wal_lsn
Type: Gauge
Description: Current WAL LSN (Log Sequence Number) as integer
Labels:
slot_name - Replication slot nameslot_type - Slot type (physical or logical)
pg_replication_slot_slot_confirmed_flush_lsn
Type: Gauge
Description: Last LSN confirmed flushed to the replication slot
Labels:
slot_name - Replication slot nameslot_type - Slot type (physical or logical)
Note: Only reported when the slot is active.
pg_replication_slot_slot_is_active
Type: Gauge
Description: Whether the replication slot is active
Labels:
slot_name - Replication slot nameslot_type - Slot type (physical or logical)
Values:
1 - Slot is active0 - Slot is inactive
pg_replication_slot_safe_wal_size_bytes
Type: Gauge
Description: Number of bytes that can be written to WAL such that this slot is not in danger of being lost
Labels:
slot_name - Replication slot nameslot_type - Slot type
Availability: PostgreSQL 13+
pg_replication_slot_wal_status
Type: Gauge
Description: Availability of WAL files claimed by this slot
Labels:
slot_name - Replication slot nameslot_type - Slot typewal_status - Status value (reserved, extended, lost)
Availability: PostgreSQL 13+
Values:
reserved - WAL files are reservedextended - WAL retention extended beyond max_wal_sizelost - Some WAL files have been removed (slot may be invalid)
SQL Query
PostgreSQL 13+:
SELECT
slot_name,
slot_type,
CASE WHEN pg_is_in_recovery() THEN
pg_last_wal_receive_lsn() - '0/0'
ELSE
pg_current_wal_lsn() - '0/0'
END AS current_wal_lsn,
COALESCE(confirmed_flush_lsn, '0/0') - '0/0' AS confirmed_flush_lsn,
active,
safe_wal_size,
wal_status
FROM pg_replication_slots;
SELECT
slot_name,
slot_type,
CASE WHEN pg_is_in_recovery() THEN
pg_last_wal_receive_lsn() - '0/0'
ELSE
pg_current_wal_lsn() - '0/0'
END AS current_wal_lsn,
COALESCE(confirmed_flush_lsn, '0/0') - '0/0' AS confirmed_flush_lsn,
active
FROM pg_replication_slots;
PostgreSQL Versions
Supported: PostgreSQL 9.4+ (when replication slots were introduced)
Version-specific features:
- PostgreSQL 13+:
safe_wal_size and wal_status columns
- PostgreSQL 10+: Function names changed from
pg_*_xlog_* to pg_*_wal_*
Required Permissions
The monitoring user needs:
- Access to
pg_replication_slots view
- Execute permission on WAL functions
Example Output
pg_replication_slot_slot_current_wal_lsn{slot_name="replica_1",slot_type="physical"} 123456789
pg_replication_slot_slot_confirmed_flush_lsn{slot_name="replica_1",slot_type="physical"} 123456000
pg_replication_slot_slot_is_active{slot_name="replica_1",slot_type="physical"} 1
pg_replication_slot_safe_wal_size_bytes{slot_name="replica_1",slot_type="physical"} 1073741824
pg_replication_slot_wal_status{slot_name="replica_1",slot_type="physical",wal_status="reserved"} 1
Use Cases
Monitoring Slot Lag
# Calculate lag between current and confirmed flush
pg_replication_slot_slot_current_wal_lsn - pg_replication_slot_slot_confirmed_flush_lsn
Detecting Inactive Slots
# Inactive slots that might be holding WAL
pg_replication_slot_slot_is_active == 0
WAL Retention Issues
Inactive replication slots can prevent WAL files from being removed, leading to disk space issues.
# Alert on low safe WAL size (PostgreSQL 13+)
pg_replication_slot_safe_wal_size_bytes < 1073741824 # < 1GB
# Alert on lost WAL status
pg_replication_slot_wal_status{wal_status="lost"} == 1
Troubleshooting
Check Slot Status
SELECT
slot_name,
slot_type,
active,
pg_wal_lsn_diff(pg_current_wal_lsn(), restart_lsn) AS lag_bytes,
safe_wal_size,
wal_status
FROM pg_replication_slots;
Remove Inactive Slots
SELECT pg_drop_replication_slot('slot_name');