Java Microservices Interview Questions and Answers

Microservices interviews test both Java/Spring Boot knowledge and understanding of distributed systems concepts. Here are commonly asked questions.

1. What are microservices, and how do they differ from a monolith?

Microservices break an application into small, independently deployable services organized around business capabilities, unlike a monolith which is a single deployable unit.

2. How do microservices communicate with each other?

Commonly through synchronous REST APIs, or asynchronously through message brokers like Kafka or RabbitMQ for event-driven communication.

3. What is an API Gateway, and why is it used?

An API Gateway is a single entry point that routes external requests to the appropriate microservice, and can also handle concerns like authentication, rate limiting, and load balancing.

4. What is service discovery?

Service discovery (e.g., using Eureka) allows microservices to find and communicate with each other dynamically without hardcoding network locations.

5. What is a Circuit Breaker pattern?

A Circuit Breaker prevents a failing service from being repeatedly called, "opening" the circuit after a threshold of failures and providing a fallback response instead.

@CircuitBreaker(name = "paymentService", fallbackMethod = "fallback")
public String processPayment() {
    return paymentClient.charge();
}

6. How do you handle data consistency across microservices?

Since each service owns its own database, patterns like the Saga pattern are used to maintain eventual consistency across services through a sequence of local transactions and compensating actions.

7. What is the difference between orchestration and choreography in microservices?

Orchestration uses a central coordinator to direct service interactions, while choreography lets services react to events independently without a central controller.

8. How do you secure communication between microservices?

Common approaches include using HTTPS/TLS, OAuth2/JWT tokens for authentication, and API gateways to enforce security policies centrally.

9. What is the role of a Config Server in a microservices architecture?

A Config Server centralizes configuration for all services, allowing configuration changes without redeploying individual services.

10. How do you monitor and debug issues across multiple microservices?

Through centralized logging, metrics collection (e.g., Prometheus), and distributed tracing tools that track a request as it flows across multiple services.