Backend mock interviews
Backend rounds tend to start from something you have built and push until it breaks. The interesting part is not the design you present; it is what you say when asked what happens when a dependency stops answering.
Rounds with Sia from ₹49 · Live interviews with a working engineer from ₹500 · Pay per session
What a Backend round covers
API design
Resource shape, status codes that mean what they say, idempotency, and versioning you can live with. Expect to defend a choice rather than list REST conventions.
Data modelling and queries
Schema decisions and their cost later, indexing, and the N+1 that shows up under load. You may be asked to reason about a slow query without seeing the plan.
Concurrency and consistency
Race conditions, locking, transactions and what "at least once" costs you. Booking and payment flows are common ground here because the failure modes are concrete.
Failure and observability
Timeouts, retries and backoff, and what you would need in the logs at three in the morning. Answers that assume the happy path get followed up hard.
Backend interview questions you might be asked
Not a list to memorise. Each is the kind of opening a real round uses, with what a strong answer covers — the follow-ups depend on what you say.
A payment API is called twice because the client retried. How do you make sure the customer is charged once?
Idempotency keys, where they are stored and for how long, and what the second caller is told while the first request is still running.
An endpoint that was fast last month now takes four seconds. Where do you start?
Find where the time goes before guessing: the query, an N+1, a missing index or a slow dependency — and what you would log so it is quicker to find next time.
Two users book the last seat at the same moment. What stops both from getting it?
A transaction, a row lock, or a conditional update — with the trade-off of each, and what the losing user sees.
A service you call starts timing out. What does your code do?
Timeouts, retries with backoff, what is safe to retry, and when to stop calling the service altogether rather than piling on.
Design the tables for orders that can be partly refunded.
A model that keeps history instead of overwriting it, the queries it needs to answer, and the constraint that stops a refund exceeding what was paid.
Answer questions like these in a scored round and see exactly where your answer fell short.
What the interviewer is listening for
The signal is whether you reason about failure before you are prompted to. Candidates who name the failure mode themselves — and say which one they chose to accept — consistently score above those who only handle it when pushed.
Where candidates lose marks
- Designing only the happy path and waiting to be asked about errors.
- Reaching for a queue or a cache without saying what problem it solves.
- Describing a system you worked on without being able to say why it was built that way.
Two ways to interview
A round with Sia, on your own
One question at a time with real follow-ups on what you actually said. Choose three, five or eight questions. Scored the moment you finish, with a written breakdown across technical depth, problem solving and communication.
A live interview with an engineer
Forty minutes over Zoom with someone who runs backend interviews for a living. They score five criteria and write up what to fix, and you keep a numbered certificate for the session.
Pick your stack for a round with Sia: Node.js · Java · Python · PHP · Rails · Ruby.
Questions about backend interviews
Which languages can I use in a backend round?
Node.js, Java, Python, PHP, Rails or Ruby for a round with Sia. Most of the questions are about design and failure rather than syntax, so the follow-ups adapt to whichever you choose.
Is a backend round right for a fresher?
Yes. At Beginner difficulty the round stays on APIs, data modelling and the projects on your resume rather than large distributed systems.
Will I be asked about my own projects?
Usually, yes. Rounds often start from something you have built and ask why it was built that way — which is why being able to explain your own project matters as much as theory.
How is a backend round scored?
On technical depth, problem solving, communication, behavioural and code quality, with a written breakdown that names what was missing from each answer.
Read before your round
Backend interviews by technology
Other tracks
Not sure which to book? The FAQ covers how sessions are scored, and candidate stories show what people did with the feedback.