Read-only transaction after failover
A managed Postgres failover (planned maintenance) promoted a standby and updated the cluster's writer DNS endpoint. Most app instances recovered, but a subset kept erroring with 'cannot execute INSERT in a read-only transaction' for ~25 minutes, and a few writes from that subset later appeared to be lost. The healthy instances use the writer endpoint and are fine. The affected subset is an older service. Triage, mitigate, and prevent.
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.