Database Monitoring, Troubleshooting, and Diagnostics Questions

Situation & goal
I would quickly triage a cross-region replica lag to identify root cause, reduce customer impact, and keep stakeholders informed.

Step-by-step diagnostics

1. Inspect metrics:
   • Replica lag (seconds/messages), apply rate, replication backlog, redo/relay log sizes, CPU, memory, disk I/O, network throughput, packet loss.
   • Monitor master write QPS, transaction size, and commits/sec.
2. Check logs:
   • Replica DB error/replication logs (e.g., MySQL relaylog, Postgres wal_receiver), OS dmesg, NIC errors.
3. Network checks:
   • Run ping/latency, traceroute, mtr toward primary; measure throughput with iperf; check for MTU/misconfig and retransmits via netstat/sar.
4. Configuration & schema causes:
   • Long-running transactions, unindexed large scans, huge transactions or BLOBs, inappropriate max_wal_size, sync settings, compression differences, binlog_format issues.
5. Immediate mitigations:
   • Throttle heavy writes or pause noncritical batch jobs, promote compressed/incremental replication if available, increase replica apply workers, temporarily route reads to healthy replicas, increase network QoS, restart replication apply process if stuck (after safe checkpoint).
6. Communication:
   • I’d alert DBAs with timeline, root-candidate, and actions taken; notify product owners with impact, ETA, and mitigation steps; post updates every 15–30 minutes until resolved and provide a post-incident summary with root cause and remediation plan.

Follow-up

• Recommend long-term fixes: query tuning, indexing, change replication topology (multi-region primaries or async tuning), automated alerts and runbooks.