Backend Developer technical round: data, APIs and concurrency

Backend technical rounds combine algorithmic coding with practical server-side judgement. Most Java Developer Jobs at Indian product companies rely on exactly this mix of DSA, database and concurrency reasoning.

• Data structures and algorithms: hash maps, trees, and complexity analysis applied to real tasks.
• Databases: schema design, indexing, joins, transactions and isolation levels.
• API design: REST semantics, status codes, idempotency and pagination.
• Concurrency and reliability: race conditions, locking, retries and error handling.

The signal they want is someone who writes correct code and understands what happens under load and failure.

Practise these recurring shapes rather than random puzzles.

'Design the schema and a query for X, then make it fast.'

They watch whether you index the right columns and avoid N+1 queries.

'This endpoint is slow under load. How do you diagnose it?'

They want a layered approach: profiling, query plans, caching, and connection limits.

'How do you make a payment endpoint safe to retry?'

This probes idempotency keys and transactional correctness.

Question: 'Two requests try to book the last seat at the same time. How do you prevent a double booking?'

Strong: 'This is a race condition. I would enforce correctness at the database, using a transaction with row-level locking or a conditional update that only succeeds if the seat is still free, and rely on a unique constraint as a backstop. Optimistic locking with a version column is an alternative. I would not depend on application-level checks alone, since they race.' It names the failure and fixes it at the right layer.

Weak: 'I would check if the seat is available before booking.' That is exactly the check-then-act race the question is testing. It shows no awareness of concurrency, which is a core backend concern.

Balance coding practice with server-side reasoning.

• Keep data structures and complexity sharp, but always tie them to a realistic task.
• Practise schema design, indexing and reading a query plan.
• Be able to reason about transactions, race conditions and idempotency out loud.

Because interviewers push on trade-offs and failure modes, rehearse explaining your reasoning aloud. A free AI voice mock interview on InterviewPrep builds a backend mock from your CV and a real job description, then scores your reasoning delivery, pace and filler words, so you explain concurrency and design choices calmly under questioning.