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Spring Cloud API Gateway

Introduction

API Gateway is the single entry point for all microservices. It receives client requests, routes them to the appropriate service, and returns the response. It acts as the primary contact for all incoming client requests.

Function Description
Routing Sends requests to the correct microservice
Load Balancing Distributes traffic across service instances
Security Authentication, Authorization, JWT
Rate Limiting Controls number of requests (prevents overload)
Logging & Monitoring Tracks requests and responses
Transformation Modify request/response (headers, body, URL)

API Gateway Implementation

1. Add Gateway Starter Dependency

Add the Spring Cloud Gateway (Reactive) dependency in pom.xml

    <dependency>
    <groupId>org.springframework.cloud</groupId>
    <artifactId>spring-cloud-starter-gateway-server-webflux</artifactId>
    </dependency>

2. Configure Routing for Microservices

Routing can be configured in two ways:

  • Programmatic : Define routes using a @Bean

  • Properties / Declarative : Define routes in application.properties or application.yml

    2.1 Routing Using application.properties

    Example route for User Microservice

    # user microservice
spring.cloud.gateway.server.webflux.routes[0].id=user-service
spring.cloud.gateway.server.webflux.routes[0].uri=http://localhost:8081
spring.cloud.gateway.server.webflux.routes[0].predicates[0]=Path=/user/**

Example

API Gateway Configuration 

spring.application.name=api-gateway
server.port=8080

User Service Endpoint : http://localhost:8081/user/create Call the user-service API via the gateway as : http://localhost:8080/user/create

Request via Gateway Routes to backend service URL
http://localhost:8080/user/create http://localhost:8081/user/create

Then,

1. Flow

  • The gateway will receive this request at /user/create, match the predicate /user/**
  • proxy route it to the user-service running at http://localhost:8081/user/create.
  • Gateway routes predicate's Path=/user/** decides which URLs get forwarded.

2. Gateway Logging

When a request hits the API Gateway, logs like:

GET "/user/read", parameters={}
Mapped to HandlerFilterFunction
Completed 200 OK
This means:

  • Request/user/readcame to gateway
  • Gateway forwarded it to User Service
  • Response returned successfully