Overview & Architecture

PostgreSQL is famously reliable, but out of the box it has no built-in automatic failover. If the primary server dies, nothing promotes a standby for you. High availability is therefore assembled from a few well-defined layers:

• Streaming replication — one primary and N hot-standby replicas kept in sync via the write-ahead log (WAL).
• Patroni — the orchestrator. It bootstraps PostgreSQL, runs leader election, promotes a standby when the primary fails, and exposes a REST API describing cluster state.
• etcd — the distributed configuration store (DCS). It holds the cluster state and provides the quorum that decides who the leader is, preventing split-brain.
• HAProxy — the client entry point. It sends writes to the current primary and reads to the replicas, following Patroni's health checks.

Architecture

What we will build

A 3-node PostgreSQL cluster (one primary + two replicas) with etcd co-located on the database nodes, fronted by a dedicated HAProxy node:

• pg-sv01, pg-sv02, pg-sv03 — PostgreSQL 18 + Patroni + etcd
• pg-haproxy — HAProxy entry point

Three nodes is the minimum for a real cluster: the quorum (more than half) survives the loss of one node, and an odd number avoids split-brain.

The end result

When the primary fails, Patroni elects a new leader within seconds, and HAProxy transparently repoints the write port to it — the application keeps working with no manual intervention and no failover logic of its own. We finish by proving exactly that with a hard-kill failover test.

Heads-up: every password in this guide is a placeholder. Replace them all with your own strong secrets before using this anywhere real.