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Home/Templates/Interviewing/Backend Engineer interview questions
Template & guide

Backend Engineer interview questions

Thirty vetted interview questions for Backend Engineer candidates covering API design, data modeling, performance, reliability, and collaboration, with strong-answer guidance.

  • Made forEmployers · Recruiters
  • Reading time~3 min
  • What's includedA complete, professionally written document you can adapt to your team.
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The template

Use these questions to interview Backend Engineer candidates at {{Company name}}. Group them by interviewer: API and data modeling suit a design-oriented session, performance and reliability suit a deep-dive with your most operationally experienced engineer. Everywhere, push past vocabulary to incidents and decisions the candidate personally owned.

API and service design

  • Walk me through an API you designed that other teams consume. What would you change in v2?
  • How do you evolve an API without breaking existing clients? Give a real migration you ran.
  • Design live: an endpoint for bulk-importing records with partial failures. What does the contract look like?
  • When do you choose synchronous calls versus queues or events between services? Give an example where you chose wrong.
  • How do you handle idempotency for payment-like operations?
  • Tell me about a service boundary that turned out to be wrong. What did it cost, and how was it fixed?

What good looks like: strong candidates design contracts from the consumer's seat — explicit error shapes, pagination and versioning decided upfront, idempotency keys for anything money-shaped — and they own a story where their boundary or contract choice aged badly. Candidates who have never regretted a design have not shipped enough of them.

Data modeling and databases

  • Walk me through the data model of the system you know best. Which table would you redesign?
  • How do you run a schema migration on a large, hot table with zero downtime?
  • When have you denormalized deliberately, and what consistency price did you pay?
  • Tell me about a slow query you diagnosed in production. Walk me through the actual steps.
  • How do you choose between a relational database and a document or key-value store? Give a decision you made.
  • What is your approach to transactions across service boundaries?

Performance and scalability

  • Tell me about the highest-load system you have operated. What were the numbers, and where was the ceiling?
  • Walk me through a latency problem you fixed: how did you measure, what did you find, what improved?
  • Where do you add caching, and how do you handle invalidation? Describe a caching bug you have caused or fixed.
  • How would you find a memory leak in a long-running service?
  • What load testing have you actually done before a launch? What did it change?

What good looks like: the best answers are measurement-first — profiles and percentiles before fixes, a named bottleneck, and numbers for before and after. Be wary of candidates who reach for horizontal scaling before understanding where the time goes, and of caching stories with no invalidation pain in them.

Reliability, testing, and operations

  • Tell me about the worst production incident you owned. Walk me through the timeline and your decisions.
  • What does your testing pyramid look like in practice — what do you actually cover at each level?
  • How do you make a deploy safe — flags, canaries, rollback? What is your real setup, not the ideal one?
  • What alerts have you configured that actually caught problems, and which just made noise?
  • How do you design a service to degrade gracefully when a dependency is down?

Collaboration and code quality

  • What do you look for first when reviewing a pull request?
  • Tell me about pushing back on a technical decision from someone more senior. How did it go?
  • How do you document a service so the on-call engineer at 3 a.m. can act?
  • Describe mentoring a junior engineer through their first production feature.
  • Tell me about a piece of tech debt you championed paying down. How did you sell it?

Not legal advice

This template is provided for general informational purposes only and is not legal advice. Laws differ by jurisdiction and change over time — have a qualified professional review any document before you rely on it.

For this role

  • Backend Engineer salary & market data
  • Backend Engineer skill assessment
  • Backend Engineer job description

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