25 Backend Engineer Interview Questions

How do you design a RESTful API that's both intuitive and versioning-friendly?

Walk me through how you'd scale a service from 1,000 to 1,000,000 requests per day.

What's your approach to database schema design? How do you avoid painful migrations later?

How do you handle authentication and authorization at the API layer?

Describe a time you diagnosed and fixed a performance bottleneck in a backend service.

How would you implement idempotent API endpoints and why does it matter?

Explain the difference between optimistic and pessimistic locking — when do you use each?

How do you design a system to handle background jobs and retries reliably?

What's your approach to database indexing? How do you know when an index is hurting more than helping?

How would you implement pagination for a large dataset efficiently?

Describe how you'd build a multi-tenant SaaS application with strong data isolation.

How do you approach secrets management in a production environment?

Walk me through how you'd implement a webhook delivery system with guaranteed delivery.

How do you design services to be observable — what's your logging and metrics strategy?

What's your experience with message queues? When would you reach for one?

How do you handle database transactions in a microservices architecture?

Describe your approach to API documentation. What makes documentation actually useful?

How would you implement full-text search in an application?

What's your strategy for handling third-party API failures gracefully?

How do you approach data caching — what to cache, how long, and how to invalidate?

Describe a security vulnerability you discovered and how you fixed it.

How do you write backend code that non-backend engineers can understand and extend?

What's your approach to load testing before a major release?

How would you implement audit logging for compliance requirements?

What makes a good service boundary in a microservices architecture?