Monitor Replication Lag and Alert on Threshold
Query the replication lag and alert when it exceeds 5 seconds so a stale read replica does not serve outdated data.
Task
Apply SELECT EXTRACT(EPOCH FROM NOW() - pg_last_xact_replay_timestamp()) AS lag_seconds; so the schema change is safe, reversible, and does not lock the table during peak traffic.
When to use this
A migration deploys SELECT EXTRACT(EPOCH FROM NOW() - pg_last_xact_replay_timestamp()) AS lag_seconds; during peak hours, takes a 40-second lock, and causes a connection pool exhaustion.
Prerequisites
- A database with migration tooling configured
- A staging environment mirroring production schema
Steps
1. Configure SELECT EXTRACT(EPOCH FROM NOW() - pg_last_xact_replay_timestamp()) AS lag_seconds; in the project configuration file with the recommended defaults.
SELECT EXTRACT(EPOCH FROM NOW() - pg_last_xact_replay_timestamp()) AS lag_seconds;
Expected output: The configuration is accepted and SELECT EXTRACT(EPOCH FROM NOW() - pg_last_xact_replay_timestamp()) AS lag_seconds; runs without errors on the first invocation.
2. Run the check locally to verify SELECT EXTRACT(EPOCH FROM NOW() - pg_last_xact_replay_timestamp()) AS lag_seconds; produces the expected result before pushing.
pnpm test
Expected output: Local run matches CI behavior; no surprises after push.
3. Add the gate to the CI pipeline so SELECT EXTRACT(EPOCH FROM NOW() - pg_last_xact_replay_timestamp()) AS lag_seconds; blocks merge on failure.
- name: Run add replication lag monitor
run: pnpm test
Expected output: Pull requests with failing SELECT EXTRACT(EPOCH FROM NOW() - pg_last_xact_replay_timestamp()) AS lag_seconds; are blocked from merging.
Troubleshooting
Problem
The check passes locally but fails in CI
Solution
Pin dependency versions and the runtime to match CI exactly; reproduce with the same Docker image or act.
Problem
Configuration is rejected with a schema error
Solution
Check the tool version matches the config schema; older versions may not support newer fields.
Limitations
- The gate only runs on events that trigger the workflow; locally pushed commits bypass it.
- Configuration drift is possible if someone edits settings outside version control.
Make migrations safe
Start by measuring the problem. Query the replication lag and alert when it exceeds 5 seconds so a stale read replica does not serve outdated data. The most common mistake is adding the gate without first confirming it catches the issue you care about. Run SELECT EXTRACT(EPOCH FROM NOW() - pg_last_xact_replay_timestamp()) AS lag_seconds; against the current codebase and inspect the output before wiring it into CI.
SELECT EXTRACT(EPOCH FROM NOW() - pg_last_xact_replay_timestamp()) AS lag_seconds;Verify and roll back
Once the gate is in place, make failures actionable. A failing SELECT EXTRACT(EPOCH FROM NOW() - pg_last_xact_replay_timestamp()) AS lag_seconds; should point to the exact file and line, not a generic error. Pin the tool version in the lockfile so a upstream release does not silently change behavior. Review the gate output in every pull request just like a test result.