Top 30 Backend Interview Questions 2026
Please explain the types of indexes and how to optimize queries when the database is slow.
Why is the query still slow even though there is an index? How do you debug?
What is the N+1 query problem and how to handle it?
OFFSET vs cursor pagination — when should you choose?
When should you use SQL vs NoSQL? Trade-off?
Normalization vs denormalization — what trade-off?
How to handle multiple requests to update the same resource? (for example, only 1 product left)
Optimistic vs pessimistic locking — how to choose?
What effect does transaction isolation levels have on data consistency?
What is deadlock and how to avoid it?
What is Idempotency? How to prevent double charges / double orders?
How will you scale the system from thousands to millions of users?
How to identify bottlenecks in a backend system?
Vertical vs horizontal scaling — when to use each type?
What does designing an effective caching strategy include? Where should the cache be placed?
What is cache invalidation and why is it difficult?
When does caching harm instead of helping performance?
Let's design a scalable URL shortener.
Design a mass email / notification system.
Design a rate limiter to prevent abuse.
How to ensure high availability and fault tolerance?
What is a circuit breaker? When to use it?
How to export or process millions of records without crashing the server?
Streaming vs buffering — what is the difference and when to use it?
What problem does Message queue solve? When should you use event-driven architecture?
Design a secure Authentication & Authorization system — How does JWT work, when not to use JWT, and how should passwords be hashed?
REST vs GraphQL — pros and cons and when to choose each?
Design API versioning How not to break the client?
What is the difference between PUT and PATCH? How should retry and timeout be designed?
An endpoint suddenly has high latency or 100% server CPU — in what order will you investigate and handle it?
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