18Software Engineering · Interview Prep · Free
Backend Developer interview questions — and how to answer them.
These are the questions Backend Developer candidates are most likely to face, from openers to the hard ones — each with a note on what a strong answer covers. Want more, tuned to your level? Use the free generator below.
What interviewers look for in a Backend Developer
- Concrete examples of systems you've built, with the trade-offs you weighed
- How you debug — the process, not just the fix
- Collaboration signals: code review, disagreements, mentoring
Likely Backend Developer interview questions
1. Tell us about a backend project you built from scratch. What technologies did you use and why?
Show decision-making process, tech choices justified by requirements, and full project lifecycle understanding.
2. How do you approach debugging a slow database query in production?
Mention query analysis tools, indexing strategies, execution plans, and avoiding impact on live users.
3. Describe your experience with REST APIs. How do you design endpoints that are scalable and maintainable?
Cover versioning, HTTP methods/status codes, pagination, filtering, rate limiting, and documentation practices.
4. Walk us through how you'd implement authentication and authorization in a new service.
Discuss JWT vs sessions, OAuth2 flows, password hashing, role-based access control, and security best practices.
5. Tell me about a time you had to refactor legacy code. What was your approach and what challenges did you face?
Emphasize planning, testing strategy, incremental changes, communication with team, and measurable improvements.
6. How would you design a caching strategy for a high-traffic application?
Address cache invalidation, cache layers (Redis/Memcached), TTL strategies, cache-aside vs write-through patterns.
7. Explain how you'd implement a robust retry mechanism for external API calls.
Cover exponential backoff, circuit breakers, idempotency keys, timeout handling, and failure logging.
8. Describe your experience with database design. How would you normalize vs denormalize data?
Discuss normal forms, trade-offs between query performance and storage, indexing strategies, and schema evolution.
9. How do you ensure your code is testable? Walk us through your testing pyramid strategy.
Explain unit/integration/e2e tests, mocking dependencies, test coverage goals, and continuous integration practices.
10. Tell us about a system you designed that had to scale horizontally. What bottlenecks did you encounter?
Discuss load balancing, stateless design, database sharding, message queues, and metrics/monitoring implementation.
11. How would you handle eventual consistency in a distributed system? Give a concrete example.
Address event sourcing, CQRS patterns, sagas for transactions, conflict resolution, and consistency guarantees.
12. Design a payment processing backend that must handle millions of transactions. Walk us through architecture decisions.
Cover idempotency, state machines, audit logging, reconciliation, PCI compliance, failure scenarios, and disaster recovery.
Want to practice answering live with scored feedback? Try the Mock Interview Coach. Applying too? See a Backend Developer cover letter example.
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