On-callMediumoc-g538

Subject Migration lockLevel Entry–Mid~20 minCommon in Databases & SQL · Concurrency interviewsIndustries Technology, Software development

Question

During a routine deploy, a database schema migration kicked off. Within a minute, requests that read or write one important table start timing out, and the dashboard shows a pile-up of queries waiting on that table. The DB CPU is low but query latency is huge. The migration is an ALTER on that table and it's still running. What's happening, and what do you do?

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.

Learn the concepts

Diagram & narrate the incident

Loading whiteboard…

Run or narrate your approach, then ask the coach.