Backend interviews go deeper than "can you write an endpoint." They test whether you understand the layers underneath the API — how data is stored and queried, what happens under load, how systems fail, and the trade-offs behind every design choice. This guide covers the backend developer interview questions that actually get asked, grouped by area.
APIs and HTTP
- What makes an API RESTful? (Statelessness, resources, proper HTTP verbs — see our REST API guide.)
- Difference between PUT and PATCH; idempotency and which methods are idempotent.
- How do you handle authentication and authorization? (JWT vs sessions, OAuth.)
- How do you version an API, and why?
- REST vs GraphQL vs gRPC — when would you choose each?
Databases
- SQL vs NoSQL — when do you reach for each?
- What is indexing, and how does it speed up queries (and cost you on writes)?
- Explain ACID and what each property guarantees.
- What is a transaction and an isolation level? What is a deadlock?
- How do you scale a database — replication vs sharding?
Caching, queues and concurrency
- Where and why would you add a cache (Redis), and what's the invalidation strategy?
- What is a message queue for, and when do you make work asynchronous?
- Explain a race condition and how you'd prevent one (locks, atomic operations).
- What is connection pooling and why does it matter under load?
- How do you handle rate limiting and graceful degradation?
System design
Mid and senior backend rounds include a design question — design a URL shortener, a rate limiter, a notification service, or an order system. They want requirements, data modeling, API design, scaling, and explicit trade-offs. Our system design guide gives you the framework.
How to prepare
Backend rounds are conversational — the interviewer keeps asking "why" and "what if traffic 10x's?" You can't prepare for that by reading; you prepare by defending your reasoning out loud against follow-ups. Greenroom runs spoken technical interviews that probe your design and trade-off thinking and give feedback on how clearly you reason. Pair it with our SQL and system design guides.
Frequently asked questions
What topics do backend developer interviews cover?
Backend interviews cover APIs and HTTP (REST principles, idempotency, auth, versioning), databases (SQL vs NoSQL, indexing, ACID, transactions, scaling via replication and sharding), caching and queues (Redis, cache invalidation, async processing), concurrency (race conditions, locks, connection pooling, rate limiting), and a system design round for mid and senior roles.
What database questions are asked in backend interviews?
Common database questions include SQL vs NoSQL trade-offs, what indexing does and its write cost, the ACID properties and what each guarantees, transactions, isolation levels and deadlocks, and how to scale a database through replication versus sharding. Interviewers want you to reason about when each choice is appropriate, not just recite definitions.
How do backend interviews test system design?
Mid and senior backend rounds include a design question like a URL shortener, rate limiter, notification service or order system. Interviewers look for clear requirements gathering, sensible data modeling, API design, a scaling plan, and an honest discussion of trade-offs. There's rarely one right answer — they score the reasoning you can defend under follow-up questions.
How should I prepare for a backend developer interview?
Study APIs, databases, caching, concurrency and system design, but the key is practising defending your reasoning out loud, because backend rounds are conversational and interviewers keep asking 'why' and 'what if traffic 10x's?' A voice-based mock interview that probes your trade-off thinking and gives feedback prepares you for that back-and-forth far better than silent reading.