Writes disappear on replica read-after-write
MySQL primary + 3 read replicas behind a read-routing layer. Replication lag is healthy (<1 s on `Seconds_Behind_Master`) and there's no CPU/IO pressure. But after a deploy this morning, support is flooded with 'I updated my profile and it reverted' and 'I posted a comment and it's not there, then it is on refresh.' Writes go to the primary; the deploy changed the data layer to route more reads to replicas. Triage and fix, and say what monitoring would have caught it.
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 30 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.