Backend Engineer interview questions
The 7 questions that actually come up, each with what a strong answer covers. These are not scripts to memorise — interviewers can tell — they are the shape of the answer that gets a yes.
7 questionsBackend Engineer career guide →
Design the schema for a marketplace order system.
What a strong answer coversNormalise first, denormalise with reasons. Cover money as integers, status as a constrained enum with history, and idempotency keys on writes.
An endpoint is slow. Walk through finding out why.
What a strong answer coversMeasure before guessing: APM/trace → query plan → N+1s → indexes → cache. Saying 'add Redis' first loses the question.
How do you make a payment webhook handler safe?
What a strong answer coversIdempotency (dedupe by event id), signature verification, fast-ack + async processing, and replay tolerance. This is a favourite trap.
Optimistic vs pessimistic locking — when each?
What a strong answer coversContention level decides. Bonus for version columns, SELECT … FOR UPDATE semantics, and what happens under retry.
What breaks when you go from one app server to ten?
What a strong answer coversLocal state (sessions, in-memory caches, cron double-runs), file storage, and connection-pool math against the database.
How do you version an API without breaking clients?
What a strong answer coversAdditive-first evolution, deprecation windows with telemetry, and why /v2 is a last resort rather than a habit.
A migration must run on a 200M-row table. Plan it.
What a strong answer coversOnline/batched migration, dual-write or backfill strategy, locks avoided, and a rollback story. 'Run ALTER TABLE at night' is the wrong answer.