Section 3 of 3 · 7 minutes

Decide whether failover is safer than waiting

Your goal

What you will be able to do

Compare standby freshness, recovery objectives, application readiness, and split-brain controls before promotion.

Why this matters at work

The practical reason

Failover is a business and data-safety decision, not only a command that promotes a server.

Learn

The idea in plain English

Confirm the primary's state, the candidate standby's replay position, estimated data loss, timeline, write readiness, and application connection plan. Fence the old primary so both servers cannot accept independent writes.

Write the decision threshold in advance: for example, fail over when the primary is unavailable and the candidate is within the approved RPO. Record who authorises any known data loss.

Remember these points

Measure candidate freshness against the approved RPO.
Fence the old primary before accepting writes on the new one.
Test application reconnect and post-failover validation.

See it in SQL

Record standby replay freshness

Capture the latest replay time and WAL position for a failover decision.

SELECT
  clock_timestamp() AS measured_at,
  pg_is_in_recovery() AS is_standby,
  pg_last_wal_replay_lsn() AS replay_position,
  pg_last_xact_replay_timestamp() AS replayed_transaction_time,
  clock_timestamp() - pg_last_xact_replay_timestamp() AS replay_time_gap;

A replay-time gap must be interpreted with write activity; an idle primary may generate no recent transaction timestamp.

Combine this with primary WAL position, business transaction evidence, and the agreed RPO.

What you should see

A timestamped standby flag, replay position, latest replayed transaction time, and simple time gap.

Useful words

Important terms

Fencing: Preventing the old primary from accepting writes before a replacement primary is used.
Split brain: Two servers independently accept writes as primary, creating divergent data.

Quick check · Not graded

Check your understanding

What must happen before a promoted standby accepts application writes?

Choose one answerEvery standby should be promoted at the same time.Only the database name needs to be checked.The old primary must be fenced and the candidate must meet the approved recovery decision.

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