Java and backend interview questions

Use JOIN FETCH in a dedicated query, an @EntityGraph on that use case, or a DTO projection that joins in SQL.

Error handlers have four arguments (err, req, res, next).

The cluster module or a process manager like PM2 forks workers that share the port.

Read the Authorization Bearer token, verify signature and expiry with the IdP public key or a shared secret, then attach req.user.

Key by IP plus username, count hits in Redis with an INCR and TTL window, and return 429 when the cap is exceeded.

If one team owns the whole domain and traffic is moderate, a modular monolith deploys faster and avoids distributed transactions.

It is a single entry point for auth, TLS, rate limits, and routing to many backend services.

Instances come and go with autoscaling, so callers cannot hardcode IPs.

Use synchronous REST when the user is waiting and you need an immediate consistent answer, like checking wallet balance.

After a dependency fails past a threshold, the breaker opens and fails fast instead of waiting on timeouts.

Checked exceptions must be declared or caught at compile time, like IOException.

Keys are hashed into buckets.

Repeating the same request leaves the same server state.

An index that includes all columns a query needs so the heap or table is not touched.

Spring creates objects and supplies their dependencies instead of new inside business code.

Join an existing transaction or create one.

Responses include links to related actions so clients discover flows.

Short TTL if profiles change often

SMTP is slow and flaky.

Session cookies suit browser apps with same-site protection.