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Why a failover test can stop the wrong node
“The wrong node” can mean different failures: an operator targets the wrong machine, an independent service manager restarts an old primary, or a standby is promoted while the original primary remains writable. The common risk is losing a trustworthy view of cluster identity and control. In a PostgreSQL cluster managed by Patroni, the leader lock coordinates leadership, and Patroni attempts to stop PostgreSQL if it cannot renew that lock. That safety model depends on Patroni retaining control of the database process; an outside restart path can undermine it. The Patroni FAQ says, “Only Patroni should be able to start, stop and promote Postgres instances in the cluster.” Patroni FAQ.
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Identify the failure you are testing
Do not treat every exercise as the same generic failover. A planned transfer, an unplanned primary loss, loss of access to the distributed configuration store (DCS), a network partition, and a multi-site disaster-recovery promotion have different safety conditions. Decide which case applies and define what success means before you act.
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- Primary loss: Check whether the surviving cluster can safely promote a candidate after the primary is unavailable.
- DCS or network failure: Test the consequences of losing coordination or communication, not just the database host.
- Multi-site recovery: Confirm the source site is isolated before promoting a separate standby cluster.
Set an acceptable recovery point objective (RPO): how much recent data, if any, the organization can tolerate losing. Also decide how the exercise will detect missing or divergent writes. In Patroni, asynchronous replication is the default, and a promoted replica may not contain every recent write; the README describes a configurable maximum-lag threshold. Patroni README.
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Verify identity and candidate health before the test
Use the cluster’s supported status interface to establish a baseline. Patroni’s REST API exposes member roles and states; record the names, roles, states, and leader identity. Check that the proposed candidate is the intended machine and is healthy enough to promote. A manual failover request names a candidate, and the API warns that manual failover can cause data loss. It can be used even when a leader exists, so do not mistake the endpoint for a planned switchover procedure. Patroni REST API.
- Capture cluster status and write down the current leader and each member’s role and state.
- Confirm the candidate’s identity against the machine or instance you intend to promote, not just an assumed ordering or hostname pattern.
- Check candidate health and replication state, then compare observed lag with the RPO you set.
- Choose the operation that matches the scenario; do not use an emergency/manual failover request as a substitute for a planned transfer.
Make sure one control plane owns PostgreSQL
Confirm that Patroni alone controls PostgreSQL start, stop, and promotion. Disable independent service-manager restart behavior for the database process in a Patroni deployment. Otherwise, a service manager may bring an old primary back after Patroni has relinquished leadership, creating two writable primaries.
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Before a live exercise, verify the fencing action and what happens if it fails. Patroni’s pre_promote hook runs after the candidate acquires the leader lock and before it promotes PostgreSQL. If the hook exits nonzero, Patroni blocks promotion and removes the leader key. This is a useful gate, but it is not proof that every old-primary path is fenced; validate the actual script and its effect in the environment. Patroni watchdog and fencing documentation.
Use layered protection against two writable primaries
Fencing is the action that prevents a node believed to be unavailable or demoted from continuing to serve writes. Patroni’s leader lock and attempt to stop PostgreSQL when lock renewal fails are important protections, but a process that Patroni cannot stop—or that another manager restarts—can still defeat the intended boundary.
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A watchdog provides another layer if Patroni crashes, is killed, runs too slowly, or cannot execute because a virtual machine is paused or heavily loaded. Patroni coordinates watchdog expiry with the DCS leader-lock time-to-live (TTL); timing margins matter. The documentation gives configuration examples, including a 30-second default TTL and five-second default safety margin, but these are defaults, not universal recommendations. Check the deployed values for loop_wait, retry_timeout, and ttl rather than copying example timings. Patroni watchdog documentation.
Run the exercise while watching the right signals
Observe the cluster using the same supported status and health interfaces used by operations and monitoring. Patroni’s REST API includes endpoints that help distinguish primary status from replica readiness. During the test, record whether the leader lock is valid, which node is writable, whether replicas catch up, and when application connections recover. Patroni REST API.
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- Check that the intended candidate, not another member, becomes leader.
- Verify that only the new leader accepts writes.
- Compare recovered data with the RPO and inspect for missing or divergent writes.
- Measure client recovery using the application’s actual connection path and health checks.
Apply stricter rules to a two-site standby
A standby site in a two-site asynchronous design cannot infer the state of the source site merely from a communication failure. In the documented Patroni arrangement, automatic promotion is not possible for that reason. Confirm that the old source is down or otherwise fenced—STONITH—before promoting the standby. Patroni’s multi-datacenter guide warns that promoting while the source cluster is still running creates split brain. After source recovery, reconcile the topology before returning it to service. Patroni multi-datacenter guide.
Close the test by proving safe rejoin
Promotion is not the end of the exercise. Verify that the old node cannot accept writes, that replicas follow the new leader, and that the former primary rejoins safely. Patroni’s README notes that redundancy is temporarily reduced until the failed member returns, so include restoration of redundancy in the test’s completion criteria. Patroni README.
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Keep instructions matched to the deployed release
These examples describe Patroni and PostgreSQL, not an identified incident behind the headline. The cited Patroni documentation is on the project’s mutable documentation and master pages; check the documentation for the release actually deployed before applying endpoint behavior, hook semantics, or configuration examples.
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