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Spring Cloud API Gateway Interview Questions and Answers

Master Spring Cloud API Gateway with interview questions covering routing, filters, predicates, authentication, rate limiting, load balancing, path rewriting, circuit breakers, and production best practices.

Spring Cloud API Gateway Interview Questions and Answers

Introduction

In a microservices architecture, clients should not communicate directly with individual services.

Instead, every request should pass through a centralized API Gateway.

The API Gateway acts as the single entry point for all client requests and provides:

  • Request Routing
  • Authentication & Authorization
  • SSL Termination
  • Load Balancing
  • Rate Limiting
  • Request Filtering
  • Logging
  • Distributed Tracing
  • Circuit Breaking

Spring Cloud Gateway is the recommended gateway solution in modern Spring Cloud applications and is built on Spring WebFlux for high performance.

Spring Cloud Gateway Architecture

flowchart LR

Client --> ApiGateway["API Gateway"]

ApiGateway["API Gateway"] --> CustomerService

ApiGateway["API Gateway"] --> PaymentService

ApiGateway["API Gateway"] --> LoanService

ApiGateway["API Gateway"] --> NotificationService

CustomerService --> PostgreSQL

PaymentService --> Kafka

LoanService --> MongoDB

Q1. What is Spring Cloud Gateway?

Answer

Spring Cloud Gateway is a lightweight, reactive API Gateway that sits between clients and backend microservices.

Responsibilities

  • Route requests
  • Authenticate users
  • Filter requests
  • Apply rate limits
  • Load balance traffic
  • Collect metrics
  • Perform request transformation

Instead of exposing every service, only the Gateway is publicly accessible.

Q2. Why do we need an API Gateway?

Without Gateway

flowchart LR

Client --> CustomerService

Client --> PaymentService

Client --> LoanService

Client --> NotificationService

Problems

  • Multiple endpoints
  • Repeated authentication
  • Tight coupling
  • Difficult monitoring

With Gateway

flowchart LR

Client --> ApiGateway["API Gateway"]

ApiGateway["API Gateway"] --> CustomerService

ApiGateway["API Gateway"] --> PaymentService

ApiGateway["API Gateway"] --> LoanService

Benefits

  • Single entry point
  • Centralized security
  • Simplified client communication

Q3. How does Spring Cloud Gateway work?

Request Flow

  1. Client sends request.
  2. Gateway matches a route.
  3. Predicates evaluate the request.
  4. Filters execute.
  5. Request is forwarded.
  6. Response is returned.

Gateway Request Flow

sequenceDiagram
Client->>Gateway: HTTP Request
Gateway->>Route Predicate: Match Route
Route Predicate-->>Gateway: Route Found
Gateway->>Filters: Apply Filters
Filters->>Payment Service: Forward Request
Payment Service-->>Gateway: Response
Gateway-->>Client: Response

Q4. What are Route Predicates?

Route Predicates determine whether a request matches a configured route.

Common predicates

  • Path
  • Method
  • Host
  • Header
  • Cookie
  • Query Parameter

Example

spring:

  cloud:

    gateway:

      routes:

      - id: payment

        uri: lb://payment-service

        predicates:

        - Path=/payments/**

The request is routed only if the predicate matches.

Q5. What are Gateway Filters?

Filters modify requests or responses.

Types

  • Global Filters
  • Route-specific Filters

Common filters

  • Authentication
  • Authorization
  • Logging
  • Header Modification
  • Rate Limiting
  • Path Rewriting
  • Circuit Breaker

Filter Flow

flowchart LR

Request --> Authentication

Authentication --> Logging

Logging --> RateLimiter

RateLimiter --> Routing

Routing --> Response

Q6. How does Gateway integrate with Eureka?

Instead of fixed URLs,

Gateway routes using service names.

Example

uri:

lb://payment-service

Discovery Flow

flowchart LR

Gateway --> Eureka

Eureka --> PaymentService1

Eureka --> PaymentService2

Eureka --> PaymentService3

Gateway automatically discovers available instances.

Q7. How is authentication implemented?

Authentication is usually performed once at the Gateway.

Typical mechanisms

  • JWT
  • OAuth2
  • OpenID Connect

Authentication Flow

flowchart LR

Client --> Gateway

Gateway --> JWTValidation

JWTValidation --> CustomerService

JWTValidation --> PaymentService

Backend services trust the authenticated request.

Q8. What is Rate Limiting?

Rate Limiting protects APIs from excessive traffic.

Example

100 Requests

Per Minute

Per User

Benefits

  • Prevent abuse
  • Protect backend systems
  • Improve availability

Rate Limiting

flowchart LR

Client --> Gateway

Gateway --> RateLimiter

RateLimiter --> Allowed

RateLimiter --> Rejected

Redis is commonly used to store rate-limiting counters.

Q9. What is Path Rewriting?

Gateway can modify request paths before forwarding.

Example

Client

/api/payments/100

Forwarded

/payments/100

Useful when exposing user-friendly APIs while backend services use different paths.

Q10. API Gateway Best Practices

Secure the Gateway

Authenticate all incoming requests.

Enable Rate Limiting

Prevent abuse.

Use HTTPS

Encrypt communication.

Integrate with Service Discovery

Avoid hardcoded URLs.

Enable Observability

Use

  • Micrometer
  • Prometheus
  • Grafana
  • OpenTelemetry

Banking Example

flowchart TD

MobileApp --> ApiGateway["API Gateway"]

ApiGateway["API Gateway"] --> Authentication

Authentication --> CustomerService

Authentication --> PaymentService

Authentication --> LoanService

Gateway --> Prometheus

Prometheus --> Grafana

All traffic flows through a single secured gateway.

Common Interview Questions

  • What is Spring Cloud Gateway?
  • Why use an API Gateway?
  • How does Gateway work?
  • What are Route Predicates?
  • What are Gateway Filters?
  • How does Gateway integrate with Eureka?
  • How is JWT authentication implemented?
  • What is Rate Limiting?
  • What is Path Rewriting?
  • API Gateway best practices?

Quick Revision

Topic Summary
API Gateway Single entry point
Route Predicate Route matching
Gateway Filter Request/response processing
Eureka Dynamic service discovery
JWT Authentication
OAuth2 Authorization
Rate Limiting Traffic control
Path Rewrite Modify request path
Load Balancing Route to healthy instance
Observability Metrics and tracing

API Gateway Request Lifecycle

sequenceDiagram
Client->>Gateway: HTTP Request
Gateway->>JWT Validator: Validate Token
JWT Validator-->>Gateway: Valid
Gateway->>Eureka: Discover Service
Eureka-->>Gateway: Payment Service
Gateway->>Gateway Filters: Apply Filters
Gateway Filters->>Payment Service: Forward Request
Payment Service-->>Gateway: Response
Gateway-->>Client: HTTP Response

Production Example – Banking Digital Platform

A banking platform exposes APIs to:

  • Mobile Banking
  • Internet Banking
  • ATM Systems
  • Third-party Partners

Gateway Responsibilities

  • Authenticate users using JWT.
  • Perform OAuth2 token validation.
  • Apply rate limits (100 requests/minute).
  • Discover backend services using Eureka.
  • Load balance requests across multiple service instances.
  • Rewrite external API paths to internal service paths.
  • Collect metrics using Micrometer.
  • Export metrics to Prometheus.
  • Send traces to OpenTelemetry.
flowchart LR

MobileApp --> ApiGateway["API Gateway"]

WebPortal --> ApiGateway["API Gateway"]

PartnerAPI --> ApiGateway["API Gateway"]

ApiGateway["API Gateway"] --> Eureka

ApiGateway["API Gateway"] --> CustomerService

ApiGateway["API Gateway"] --> PaymentService

ApiGateway["API Gateway"] --> LoanService

ApiGateway["API Gateway"] --> RedisRateLimiter

ApiGateway["API Gateway"] --> Micrometer

Micrometer --> Prometheus

Prometheus --> Grafana

This architecture centralizes security, routing, monitoring, and traffic management while keeping backend services simple and secure.

Key Takeaways

  • Spring Cloud Gateway is the recommended API Gateway solution for Spring Cloud and provides a centralized entry point for microservices.
  • Gateway responsibilities include routing, authentication, authorization, rate limiting, filtering, logging, and request transformation.
  • Route Predicates determine which requests match a route, while Gateway Filters process requests and responses.
  • Integration with Eureka enables dynamic service discovery and client-side load balancing without hardcoded URLs.
  • Authentication is typically implemented using JWT, OAuth2, or OpenID Connect at the Gateway layer.
  • Rate Limiting protects backend services from excessive or malicious traffic and is commonly implemented with Redis.
  • Gateway integrates with Micrometer, Prometheus, Grafana, and OpenTelemetry for complete observability.
  • A properly designed API Gateway improves security, scalability, maintainability, and operational efficiency in enterprise microservice architectures.