ha-topology-and-failover

Skill: HA topology & failover

Design availability from the targets down, and design the failover, not just the replica.

Step 0 — One opinion up front

A replica is not HA; a tested failover is. A standby you can't promote safely under load is decoration. The deliverable is the promotion path + split-brain guard, not the replica count.

Step 1 — State the targets

Write down RTO (how long can we be down?), RPO (how much data can we lose?), and the failure domains you must survive (node → AZ → region). No targets, no design.

Step 2 — Trace the topology tree

Traverse ../../knowledge/dbre-decision-trees.md §1 to a leaf. Record the path and the runner-up. Key forks: async vs sync (RPO vs write latency), single-AZ vs multi-AZ vs multi-region (which outage you survive), automatic vs manual failover (RTO vs complexity).

Step 3 — Design the failover mechanism

Specify explicitly:

• Promotion: what promotes a replica (orchestrator / managed control plane).
• Split-brain guard: fencing / quorum / STONITH so two primaries can't both accept writes.
• Trigger: who or what initiates, and the health signal it uses.
• App reconnection: how clients discover the new primary (DNS, proxy, service discovery).

Step 4 — Place across failure domains

Confirm primary, replicas, and backups live in different domains. A replica in the primary's AZ doesn't survive an AZ outage.

Step 5 — Prove it (game-day)

A topology is a hypothesis until failover is rehearsed. Hand the drill to database-operations-engineer (the ha-topology-and-failover design pairs with an operational failover drill).

Step 6 — Hand off

• The backup/PITR that complements HA → backup-and-restore-verification.
• Provisioning the instances/replicas → terraform-iac / cloud plugins.
• The schema the topology carries → database-engineering.

Output

An HA topology with the tree path + runner-up, RPO/RTO it meets, the failover mechanism (promotion + split-brain guard + trigger + app reconnection), the failure-domain placement, and a game-day plan to prove it.