Scenario Based Questions

spring-boot-starter-data-jpa is on the classpath, so Boot tries to create a DataSource, but no spring.datasource.url is set and no embedded DB is present. Fix: add the URL/credentials (check the active profile is the one holding them), add H2 for local runs, or exclude DataSourceAutoConfiguration if the service truly has no database.

• The class is outside the main class's package, so component scan misses it.
• It is missing @Component/@Service, or its @Profile/@Conditional does not match.
• You created the object with new, so Spring never manages it.
• You inject the implementation class but only a proxy of the interface exists (JDK proxy).

1. Client sends an Idempotency-Key (UUID) with POST /api/payments.
2. Server stores the key with the result in a table with a unique constraint; a repeated key returns the stored response instead of charging again.
3. Also verify payment gateway webhooks by signature and process each event id only once.

It is a race condition (lost update). Options: optimistic locking with @Version on Product (one transaction fails with ObjectOptimisticLockingFailureException and you retry or return 409), an atomic SQL update UPDATE product SET stock = stock - :qty WHERE id = :id AND stock >= :qty and check the updated row count, or a pessimistic lock (SELECT ... FOR UPDATE) for very hot items.

1. Check metrics (Actuator/Prometheus/Grafana): latency per endpoint, DB pool usage, GC, CPU.
2. Look at traces / logs with SQL logging: often an N+1 query or a missing index on a grown table.
3. Check HikariCP "connection is not available" timeouts (pool exhausted by slow queries or a leak).
4. Check slow downstream calls (payment API) and ensure timeouts and circuit breakers are set.
5. Take a thread dump (/actuator/threaddump) to see where threads are blocked.

Always set connect/read timeouts on RestClient/WebClient (the default may wait for a long time). Add retries with backoff only for idempotent calls, a circuit breaker with a fallback (Resilience4j), and move non-essential calls (SMS, email) to @Async or a queue so the user does not wait.

Accept MultipartFile, set spring.servlet.multipart.max-file-size, and stream it (file.getInputStream()) directly to object storage (S3/GCS) instead of loading bytes into memory. Validate content type and size, generate your own file name, and store only the URL/key in the database.

Package as a Docker image (layered JAR), run at least two instances behind a load balancer, and do a rolling or blue-green deployment. Use readiness probes (/actuator/health/readiness) so traffic arrives only when the app is ready, and server.shutdown=graceful so in-flight requests finish. Database migrations (Flyway) must be backward-compatible with the old version.

Add a correlation id: a filter reads X-Request-Id (or generates one), puts it in the SLF4J MDC, and the log pattern prints it; return it in the response header. With Micrometer Tracing the trace id/span id is added automatically and propagated to downstream services.