How do you migrate a hot table without long locks?
PICTURE THIS: HOW TO EXPLAIN IT
Simple meaning
Expand-contract: add column, dual-write, backfill in batches, switch reads, then drop old.
WHY — Data instead of guessing?
Why interviewers care about Data:
question about Data.
trade-offs, and what you would actually do on a Backend project - not buzzwords.
Name the idea, why it exists, then one short example.
End with when you use it and one common pitfall.
STEPS — What happens step by step?
Before you speak the answer, walk the interviewer through these steps:
- 1Expand-contract: add column, dual-write,
backfill in batches, switch reads, then drop old.
- 2Big bang ALTER on
huge tables locks traffic.
- 3I rehearse on a
staging copy first.
- 4Give an example
One tiny concrete case you can say aloud.
- 5Common mistake
What juniors usually get wrong.
- 6Close
When you pick this over the alternative.
EXAMPLE — See it in action
Here's a short line you can speak, broken into clear beats:
Note: Adapt this scaffold to your own project — keep it under 60–90 seconds.
Key takeaway
Expand-contract: add column, dual-write, backfill in batches, switch reads, then drop old. Big bang ALTER on huge tables locks traffic.