Backend Developer Interview Questions
10 curated questions with evaluation guidance for hiring managers.
How would you design an API that needs to handle 10,000 requests per second with sub-100ms response times?
Look for discussion of caching layers (Redis, CDN), database optimization (indexing, read replicas), horizontal scaling, load balancing, and async processing. Should mention monitoring and benchmarking.
Explain the CAP theorem and how it influences your database design decisions.
Should clearly explain Consistency, Availability, and Partition tolerance trade-offs. Look for practical examples: choosing PostgreSQL vs. DynamoDB for specific use cases.
How do you handle database migrations in a production environment with zero downtime?
Should mention expand-contract pattern, backward-compatible migrations, feature flags, blue-green deployments, and careful rollback strategies. Red flag if they suggest taking the system offline.
Describe your approach to API authentication and authorization. How do you decide between JWT, sessions, and OAuth?
Should explain the trade-offs of each approach: JWT for stateless microservices, sessions for traditional apps, OAuth for third-party integration. Look for understanding of refresh tokens and token invalidation.
How do you design for idempotency in distributed systems?
Should explain idempotency keys, deduplication strategies, and why they matter for payment systems and message queues. Look for awareness of at-least-once vs. exactly-once delivery.
Walk me through how you would implement a job queue system for processing background tasks.
Should discuss queue technologies (Redis, RabbitMQ, SQS), retry strategies, dead letter queues, concurrency control, and monitoring. Look for production experience with failure scenarios.
How do you approach logging and observability in microservices?
Should mention structured logging, correlation IDs, distributed tracing (Jaeger, OpenTelemetry), metrics dashboards, and alerting. Look for understanding of signal vs. noise in logs.
Explain how connection pooling works and why it matters for database performance.
Should describe pool lifecycle (create, reuse, destroy), sizing considerations, and the impact on throughput. Bonus if they mention tools like PgBouncer or HikariCP.
How do you handle data consistency across multiple services in a microservices architecture?
Should discuss saga pattern, eventual consistency, event sourcing, or two-phase commits. Look for understanding of compensating transactions and when strong consistency is truly needed.
What security practices do you follow when building APIs?
Should mention input validation, SQL injection prevention, rate limiting, CORS configuration, HTTPS enforcement, secrets management, and dependency scanning. Look for defense-in-depth thinking.
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