Online schema migration
Design an online schema-migration system to alter a 3-billion-row, 2 TB MySQL table that serves 50k QPS, where a naive ALTER TABLE would lock writes for hours. You need to change a column type and add an index with zero downtime and the ability to abort safely if load spikes. How do you perform the change, keep the old and new schemas in sync, and cut over?
What a strong answer looks like
Clarify scale and constraints first. Propose a clean component breakdown, then go deep on the hard parts (data model, bottlenecks, consistency, failure modes) and name the trade-offs you are making.
Clarify4:30 left
Estimate4:30 planned
Design13:30 planned
Deep dive10:30 planned
Failure7:00 planned
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.