Index migration stalls read replicas
A migration to add an index on a large 'events' table (MySQL 8, primary + 2 read replicas) runs at 03:00 using a standard online DDL. The primary stays healthy and the migration completes there in 6 minutes. But starting at 03:00, replica lag on both read replicas climbs to 40 minutes, and the app — which reads recent events from replicas — starts showing users stale/missing data and the analytics dashboards go blank. After the index finishes replicating, lag drains and everything recovers. Triage and mitigate.
What a strong answer looks like
Stop the bleeding first (mitigate), then form hypotheses from real signals. Separate root cause from symptom, communicate status as you go, and close with what prevents a repeat.
0:00 of about 35 min
Which questions mattered is sealed until you submit. Telling you now would just be handing over the edge cases.
Run or narrate your approach, then ask the coach.