Top 20 REST API Interview Questions and Answers

Top 20 REST API Interview Questions and Answers Work through these 20 questions out loud. Open any question for the full model answer, the trap to avoid, and the follow-up a panel usually asks.

1. How do Spring Boot Actuator health probes work with Kubernetes?

   Enable probes, configure K8s readinessProbe on /actuator/health/readiness, liveness on /actuator/health/liveness. Custom HealthIndicator for critical deps on readiness group...

   Full answer

2. What is ProblemDetail (RFC 7807) in Spring 6 error handling?

   @ExceptionHandler(NotFoundException.class) ProblemDetail handle(NotFoundException ex) { return ProblemDetail.forStatusAndDetail(NOT_FOUND, ex.getMessage()); } — consistent 404...

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3. springdoc-openapi vs legacy springfox — what should new projects use?

   Add springdoc-openapi-starter-webmvc-ui dependency. Annotate with @Operation, @Schema on DTOs. Disable in prod via springdoc.api-docs.enabled=false or secure endpoint. Generate...

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4. How does Micrometer Tracing replace Spring Cloud Sleuth?

   Dependencies: micrometer-tracing-bridge-otel, opentelemetry-exporter-otlp. Traces propagate Authorization-safe headers across RestClient/WebClient calls. Correlate logs with...

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5. How configure Spring Security OAuth2 Resource Server for JWT?

   Configure issuer-uri pointing to IdP. SecurityFilterChain: authorizeHttpRequests(auth ->...

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6. SecurityFilterChain vs deprecated WebSecurityConfigurerAdapter?

   @Bean SecurityFilterChain filterChain(HttpSecurity http) throws Exception { http.csrf(csrf -> csrf.disable()).authorizeHttpRequests(auth ->...

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7. What are idempotency keys and how implement them in REST APIs?

   Client sends Idempotency-Key: uuid on POST /payments. Server Redis SETNX key, process, store response JSON. Retry with same key returns 201 + original body without double...

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8. Explain the transactional outbox pattern.

   OrderService @Transactional saves Order and OutboxEvent(type=ORDER_CREATED, payload=json). OutboxProcessor @Scheduled polls FOR UPDATE SKIP LOCKED, sends to Kafka, updates...

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9. What is the Backend-for-Frontend (BFF) pattern?

   Mobile BFF /mobile/home aggregates user profile + feed + notifications in one round trip. Web BFF different field selection. BFF handles OAuth BFF cookie pattern. Domain...

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10. Kubernetes liveness vs readiness vs startup probes?

    deployment.yaml: startupProbe HTTP /actuator/health with failureThreshold*period covering 60s startup. readinessProbe /actuator/health/readiness includes DB. livenessProbe...

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11. Kafka consumer groups, offsets, and delivery semantics?

    @KafkaListener with AckMode.MANUAL — ack after DB save. Consumer group order-service scales to partition count max. Idempotency key in message header dedup table. Monitor...

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12. A REST API is slow only in production, not locally. How do you debug it?

    Enable Micrometer timers and Hibernate statistics or datasource-proxy in staging with prod-like data. Check Hikari `connections-active`, JVM GC pauses, and Feign/RestClient...

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13. @RestController vs @Controller?

    @Controller is for traditional MVC — return "user/list" view name. @RestController adds @ResponseBody so return types become JSON/XML body — standard for REST APIs. Use...

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14. How do you handle exceptions globally in Spring?

    Create @RestControllerAdvice with @ExceptionHandler(MethodArgumentNotValidException.class) returning 400 + field map, NotFoundException → 404, generic Exception → 500 without...

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15. How would you design a rate-limited REST API?

    API gateway or filter applies token bucket per API key — Redis stores tokens with TTL. Exceeding limit returns 429 + Retry-After. Different tiers per plan. Log rejections for...

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16. What is the difference between JPA and Hibernate?

    JPA is the API contract; Hibernate is the engine under Spring Boot by default. You write @Entity per JPA spec; Hibernate handles SQL generation. Spring Data JpaRepository adds...

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17. How does Stream API parallel stream work?

    parallelStream() splits work across common pool threads. Good for CPU-heavy map on large in-memory collections. Bad for small lists, IO inside stream, or ordered stateful...

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18. How do you version REST APIs?

    We use /api/v1/users for clarity. Non-breaking: add optional JSON fields. Breaking: launch v2 path, maintain v1 for 6 months with Deprecation header. Spring maps...

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19. Explain sealed classes and their purpose in Java 17+.

    sealed interface Shape permits Circle, Rectangle limits who implements Shape. Compiler verifies switch (Shape s) covers all cases — no default needed. Use for domain variants...

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20. Explain structured concurrency and why it matters.

    In a request handler, open StructuredTaskScope, fork fetchUser and fetchOrders, join both — if orders fail, user fetch cancelled automatically. Safer than manual Future...

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