# Runbook — Postgres failover (`pg_auto_failover`)

> **Alerts** : `PostgresPrimaryUnreachable`, `PostgresReplicationLagHigh` · also reached from `api-availability-slo-burn.md` and `api-latency-slo-burn.md`.
> **Owner** : infra on-call.

## Topology recap

```
┌─────────────────┐
│  pgaf-monitor   │  ← state machine; assigns primary/standby roles
└────────┬────────┘
         │ pg_auto_failover protocol
         │
   ┌─────┴─────┐
   │           │
┌──▼───┐   ┌───▼────┐
│ pgaf-│   │ pgaf-  │
│primary│  │replica │
└───────┘  └────────┘
```

PgBouncer (`pgaf-pgbouncer`, port 6432) sits in front of whoever is currently primary. Backend reads `DATABASE_URL` from env that already points at the bouncer.

## What "failover" looks like

- Primary disappears (crash, host reboot, manual `incus stop`).
- Monitor notices within `pgaf_health_check_interval` (~10s).
- After `pgaf_failover_timeout` (60s), monitor promotes the replica to primary.
- PgBouncer is reconfigured by the monitor's notify hook ; new connections go to the new primary.

**Expected RTO is ~60 seconds.** RPO ≈ 0 if synchronous replication was caught up; up to one tx if async.

## Diagnose state

```bash
# From any node :
sudo -u postgres pg_autoctl show state

# Look for one node with state="primary" and one with state="secondary".
# If both are "wait_for_primary" the formation is wedged.

# Connection-level test (does the bouncer route to a live primary?) :
psql "$DATABASE_URL" -c "SELECT now(), pg_is_in_recovery();"
# pg_is_in_recovery = false ⇒ you're hitting the primary
```

## Common failure modes

### A. Monitor is up, primary is down, replica didn't get promoted

Either `pgaf_failover_timeout` hasn't elapsed yet (wait 60s) **or** the replica is too far behind to be safe.

```bash
# On the replica :
sudo -u postgres pg_autoctl show state
# Check the LSN distance — if it's > 1MB the monitor will refuse promote.
```

If monitor refused, manual promotion (only if you accept potential data loss) :

```bash
sudo -u postgres pg_autoctl perform failover --formation default --group 0
```

### B. Monitor itself is down

The data nodes keep serving their last-known role until the monitor returns. Reads keep working from the standby. **No automatic failover happens** without the monitor — start it before doing anything else.

```bash
sudo systemctl start pg_autoctl@monitor
sudo journalctl -u pg_autoctl@monitor -n 200 --no-pager
```

### C. Both data nodes are down (catastrophe)

Restore from pgBackRest. See the dr-drill runbook in `docs/archive/` (or the `pgbackrest` role README) for the manual procedure. **Estimated RTO ~30 min** with a full+diff already on MinIO.

## Connection routing

PgBouncer holds the routing decision, so during a failover :

```bash
# Confirm which Postgres backend is currently behind the bouncer :
psql -h pgaf-pgbouncer.lxd -p 6432 -U pgbouncer pgbouncer -c "SHOW SERVERS;"
```

If the bouncer is still pointing at the dead primary :

```bash
# Reload the bouncer config (the pg_auto_failover monitor's
# `host_change_hook.sh` should have done this automatically — if not,
# something is broken) :
sudo systemctl reload pgbouncer
```

## Backend behavior during failover

The backend's GORM connection pool drops dead connections lazily. Expect a few hundred 5xx during the 30-60s window — this trips `APIAvailabilitySLOFastBurn`. The alert clears once the pool refills.

## After recovery

1. Re-add the failed node as standby :
   ```bash
   sudo -u postgres pg_autoctl create postgres ...
   ```
2. Wait for `pg_autoctl show state` to show two healthy nodes.
3. Run the next dr-drill cycle to validate backups against the new primary.
4. Postmortem if downtime > 5 min.