Microservices & REST APIs Interview Questions

Microservices Architecture structures an application as a collection of small, independent services that communicate over a network.

Monolithic applications are deployed as a single unit, whereas microservices are independently deployable services.

REST is an architectural style for building web services using standard HTTP methods and stateless communication.

REST principles include statelessness, client-server architecture, cacheability, layered systems, and uniform interfaces.

GET retrieves data and should not modify resources, while POST creates new resources or submits data.

PUT replaces an entire resource, while PATCH updates only specific fields.

DELETE removes a specified resource from the server.

HTTP status codes indicate request outcomes, such as 200 OK, 201 Created, 400 Bad Request, 404 Not Found, and 500 Internal Server Error.

An idempotent operation produces the same result regardless of how many times it is executed.

API versioning manages changes to APIs while maintaining backward compatibility for existing clients.

Swagger/OpenAPI is a specification and toolset used for documenting, testing, and designing REST APIs.

Service Discovery enables services to dynamically locate and communicate with each other.

An API Gateway acts as a single entry point for client requests and handles routing, authentication, and monitoring.

The Circuit Breaker Pattern prevents cascading failures by stopping requests to failing services and providing fallback mechanisms.

Microservices communicate using synchronous protocols such as REST and gRPC or asynchronous messaging systems such as Kafka and RabbitMQ.

Name concrete APIs (collections, concurrency, streams, annotations) tied to Microservices & REST APIs. Interviewers want syntax-level clarity plus when you would reach for each feature in production.

Prefer specific checked/unchecked exceptions, wrap lower-level failures with context, and avoid empty catch blocks. For Microservices & REST APIs, decide what is recoverable vs fatal to the request thread.

JUnit 5 with Arrange-Act-Assert, Mockito for collaborators, and parameterized tests for edge cases. Keep Microservices & REST APIs logic pure where possible so tests stay fast.

Incorrect equals/hashCode break HashMap/HashSet behavior. If Microservices & REST APIs stores objects in collections, explain the contract and mutability pitfalls.

Retain only needed references, prefer streaming for large datasets, and avoid accidental caches. Relate leaks or GC pressure to a concrete Microservices & REST APIs scenario.