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06. Replication and HA

Monitor streaming replication, replication lag, and high availability status.

Dashboard in development

This dashboard is currently under development. Replication metrics are collected as part of the health check system, and the full dashboard visualization is coming soon.

Purpose​

Ensure replication health for:

  • Disaster recovery readiness
  • Read replica performance
  • Failover preparedness

When to use​

  • Monitoring replica lag during high load
  • Investigating replication disconnections
  • Validating HA setup
  • Capacity planning for replicas

Key panels​

Replication lag (Bytes)​

What it shows:

  • Bytes of WAL not yet replayed on replica
  • Per-replica breakdown

Healthy range:

  • < 1 MB for synchronous replication
  • < 100 MB for async (depends on workload)

Warning signs:

  • Growing lag = replica can't keep up
  • Sudden spikes = network issues or replica overload

Replication lag (Time)​

What it shows:

  • Estimated time behind primary
  • More intuitive than bytes for SLA monitoring

Calculation: Based on WAL generation rate and byte lag.

Replication slot status​

What it shows:

  • Active slots and their consumers
  • Slot lag (retained WAL)

Warning signs:

  • Inactive slots with growing lag = WAL retention risk
  • Slots without active connections
WAL retention

Unused replication slots prevent WAL cleanup and can fill disk.

Sent vs replayed​

What it shows:

  • WAL sent to replica
  • WAL replayed (applied) on replica
  • Gap indicates apply lag

Replica connections​

What it shows:

  • Connected replicas
  • Connection state (streaming, catchup)

Variables​

VariablePurpose
cluster_nameCluster filter
node_namePrimary or replica

Replication modes​

Streaming replication​

Standard async or sync replication:

-- on primary
select * from pg_stat_replication;

Logical replication​

For selective table replication:

-- check subscriptions
select * from pg_stat_subscription;

Troubleshooting​

Replica not connecting​

  1. Check primary allows connections:

    show max_wal_senders;
    select * from pg_stat_replication;

  2. Verify pg_hba.conf allows replication

  3. Check network connectivity

Replication lag growing​

  1. Check replica resource usage (CPU, I/O)
  2. Review long-running queries on replica
  3. Consider hot_standby_feedback setting
  4. Check for replication conflicts: 
    select * from pg_stat_database_conflicts;

Replication slot bloat​

Remove unused slots:

-- List slots
select slot_name, active, pg_size_pretty(pg_wal_lsn_diff(pg_current_wal_lsn(), restart_lsn))
from pg_replication_slots;

-- Drop unused slot (CAUTION)
select pg_drop_replication_slot('unused_slot');