Backend Developer Interview Questions 2026
Questions that probe API design, database fundamentals, and distributed systems thinking.
10 questions3 categoriesWith answer hints
Technical
4 questions1What is the difference between a database index and a covering index, and when would you use each?
Hint: An index speeds up lookups; a covering index includes all columns the query needs, eliminating a table fetch. Use covering indexes on hot read paths with predictable column sets.
2Explain the CAP theorem and give an example of a real system that prioritises CP versus one that prioritises AP.
Hint: CP example: ZooKeeper/HBase (consistency during partition). AP example: Cassandra/DynamoDB (availability with eventual consistency). State that CA is only achievable without partitions (single-node).
3What is N+1 query problem and how do you detect and fix it in an ORM-based application?
Hint: Define: 1 query for list + N queries for each related record. Fix: eager loading (JOIN or IN clause). Detection: query logging, Bullet gem in Rails, Django Debug Toolbar.
4How does JWT authentication work, and what are its security risks compared to session-based auth?
Hint: JWTs are stateless and self-contained but cannot be invalidated before expiry. Risks: algorithm confusion attacks, storing sensitive data in payload, and insecure storage (localStorage). Sessions are revocable but require server-side storage.
Behavioral
3 questions5Tell me about a production incident you were involved in. How did you diagnose it, and what did you change afterward?
Hint: Show structured debugging (metrics → logs → traces), clear communication under pressure, and a concrete post-mortem action like adding an alert or circuit breaker.
6Describe a time you had to refactor a large, tightly coupled codebase. How did you plan it without breaking production?
Hint: Mention the strangler fig pattern, feature flags, incremental extraction, and how you maintained test coverage throughout the migration.
7Tell me about a technical decision you made that you later regretted. What would you do differently?
Hint: Interviewers value self-awareness and learning mindset over perfection. Be specific about the tradeoff you misjudged and the decision framework you now apply.
System Design
3 questions8Design a URL shortening service that handles 10,000 requests per second at peak.
Hint: Cover key generation (hash vs auto-increment), collision handling, cache layer (Redis for hot URLs), database sharding, and CDN for redirect latency. Estimate storage: ~100 bytes × 100M URLs = ~10 GB.
9How would you design a rate limiter for a public REST API?
Hint: Compare token bucket, sliding window, and fixed window algorithms. Discuss storage (Redis with atomic Lua scripts), key design (per-user vs per-IP), and returning 429 with Retry-After headers.
10Design a notification system that delivers emails, SMS, and push notifications to millions of users.
Hint: Use a message queue (Kafka/SQS) per channel, worker pools per channel type, retry with exponential backoff, deduplication keys, and a preference service to respect opt-outs.
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View Backend Developer RoadmapQuestions reflect commonly asked interview topics for Backend Developer roles across companies of various sizes. Hints summarize what strong answers typically cover — use them as a preparation guide, not a script.