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Backend Developer interview questions
Backend interviews look for people who think about data, failure and load. You'll be asked how a system behaves when a database slows down or a downstream service fails.
What interviewers look for
- Data modelling. Schemas, indexes and how queries behave at volume.
- Reliability. Timeouts, retries, idempotency and graceful failure.
- API design. Clear contracts that survive change.
How the interview usually runs
- Recruiter call.
- Coding screen.
- System design interview.
- Deep dive into past work, then a behavioural round.
Stages vary by company, so ask the recruiter what to expect before you start preparing.
10 Backend Developer interview questions, and how to answer them
1. How do you design a REST API for orders?
What they want to see: Clear contracts and edge cases.
How to answer: Cover resources, status codes, pagination, idempotency keys for payment retries and versioning.
2. What does a database index cost you?
What they want to see: Whether you see both sides of a trade-off.
How to answer: Faster reads for slower writes and more storage. Say how you'd confirm a query uses it with EXPLAIN.
3. What is an N+1 query and how do you fix it?
What they want to see: Real production awareness.
How to answer: Explain one query per row, how you spot it in logs, and fix with joins or batching.
4. How would you handle a downstream service that keeps timing out?
What they want to see: Failure thinking.
How to answer: Use timeouts, retries with backoff and jitter, circuit breakers and a sensible fallback.
5. When would you use a message queue?
What they want to see: Knowing when async helps.
How to answer: For slow or spiky work that doesn't need an instant reply. Mention at-least-once delivery and idempotent consumers.
6. SQL or NoSQL for this feature?
What they want to see: Reasoning from access patterns.
How to answer: Start with the data and the queries, then choose. Don't pick a side in general.
7. How do you keep a cache consistent?
What they want to see: Trade-offs in caching.
How to answer: Compare TTLs, write-through and invalidation on change, and say what stale data would cost here.
8. How do you deploy a schema change without downtime?
What they want to see: Practical operations.
How to answer: Expand, migrate, contract: add the new column, backfill, switch code, then remove the old one.
9. How do you secure an API?
What they want to see: Security basics.
How to answer: Cover authentication, authorisation on every object, input validation, rate limits and secrets handling.
10. Tell me about an outage you were part of.
What they want to see: Ownership and learning.
How to answer: Walk through detection, mitigation, root cause and what you changed afterwards, without blaming anyone.
A sample answer
How would you handle a downstream service that keeps timing out?
In a checkout service, our fraud provider began timing out at peak. I added a two-second timeout, one retry with jitter and a circuit breaker. When it opened, orders under a set value went through and larger ones were queued for review. Failed checkouts fell to almost zero, and we agreed the policy with the risk team beforehand.
A made-up example to show the structure. Use your own experience and real numbers.Common mistakes
- Designing for massive scale before asking about traffic.
- Forgetting timeouts and retries in any network call.
- Treating the database as a black box.
- Describing only the happy path of an API.
Practise this out loud
Run a mock interview for a Backend Developer role. Questions change every time, you answer by voice or typing, and each answer gets a score with feedback.
Frequently asked questions
What should a backend developer study first?
SQL, indexes and transactions come up constantly. After that, focus on API design, caching and how to reason about failure.
How is a backend interview different from a full-stack one?
It goes deeper on data, concurrency and reliability, with less on UI. Full-stack interviews tend to cover more ground at less depth.