Spring Cloud Gateway tutorial

When we implement API gateway pattern, Spring Cloud Gateway is one of the options provided by Spring cloud framework. Spring Cloud Gateway enables routing to APIs and solves cross-cutting concerns like security, monitoring, resiliency. It is based on Spring 5 and uses Netty web server & reactor framework underneath. If you compare Spring Cloud Gateway with Netflix's Zuul 1.0, Spring Cloud Gateway supports asynchronous & non blocking connections while Zuul 1.0 does not.

Spring Cloud Gateway is based on below three concepts

• Route: A route consists of a unique id, destination uri, list of predicates & a list of filters. All predicates need to be fullfilled to match a route
• Predicates: Contains logic to match a uri, for e.g if a request type if GET type, follows a particular path like /customers/
• Filters: Used to modify the incoming request, for e.g it can be used to add headers, or modify path.

In this Spring Cloud Gateway tutorial, we will create a Customer service which returns a customer details but the actual service call to Customer service will go through Spring Cloud Gateway. So a caller will send request to gateway which will route the call to the customer service. Similarly you can add more services like order service, account service.

<parent>
    <groupId>org.springframework.boot</groupId>
    <artifactId>spring-boot-starter-parent</artifactId>
    <version>2.6.3</version>
    <relativePath/> 
</parent>
    
<dependencies>
    <dependency>
        <groupId>org.springframework.cloud</groupId>
        <artifactId>spring-cloud-starter-gateway</artifactId>
    </dependency>
        <dependency>
        <groupId>org.springframework.cloud</groupId>
        <artifactId>spring-cloud-starter-circuitbreaker-reactor-resilience4j</artifactId>
    </dependency>
    <dependency>
        <groupId>org.springframework.cloud</groupId>
        <artifactId>spring-cloud-starter-contract-stub-runner</artifactId>
        <exclusions>
            <exclusion>
                <artifactId>spring-boot-starter-web</artifactId>
                <groupId>org.springframework.boot</groupId>
            </exclusion>
        </exclusions>
    </dependency>
</dependencies>

<dependencyManagement>
    <dependencies>
        <dependency>
            <groupId>org.springframework.cloud</groupId>
            <artifactId>spring-cloud-dependencies</artifactId>
            <version>2021.0.3</version>
            <type>pom</type>
            <scope>import</scope>
        </dependency>
    </dependencies>
</dependencyManagement>

package com.example.demo;

import org.springframework.boot.SpringApplication;
import org.springframework.boot.autoconfigure.SpringBootApplication;
import org.springframework.cloud.gateway.route.RouteLocator;
import org.springframework.cloud.gateway.route.builder.RouteLocatorBuilder;
import org.springframework.context.annotation.Bean;
import org.springframework.http.HttpMethod;
import org.springframework.web.bind.annotation.RestController;

@SpringBootApplication
@RestController
public class GatewayApplication {

    public static void main(String[] args) {
        SpringApplication.run(GatewayApplication.class, args);
    }

    @Bean
    public RouteLocator myRoutes(RouteLocatorBuilder builder) {
        return builder.routes()
            route(p -> p.method(HttpMethod.GET).and().path("/customers/{id}")
                .filters(f -> f.addRequestHeader("Hello", "World").prefixPath("/v1"))
                    .uri("http://localhost:8080")).build();
    }

}

server:
  port: 9999 

server:
  port: 9999
  
spring:
  cloud:
    gateway:
      routes:
      - id: customer-service
        uri: http://localhost:8080
        predicates:
        - Method=GET
        - Path=/customers/{id}
        filters:
        - AddRequestHeader=Hello, World
        - PrefixPath=/v1

spring:
  application:
    name: customer-service

server:
  port: 8080

spackage com.example.demo;

public class Customer {

	private String id;
	private String name;
    
	// removed getters, setters for brevity
}

package com.example.demo;

import java.util.ArrayList;
import java.util.List;

import org.slf4j.Logger;
import org.slf4j.LoggerFactory;
import org.springframework.web.bind.annotation.PathVariable;
import org.springframework.web.bind.annotation.RequestMapping;
import org.springframework.web.bind.annotation.RestController;

@RestController
public class CustomerRestController {

    private static final Logger LOG = LoggerFactory.getLogger(CustomerRestController.class);

    private List<Customer> customers = new ArrayList<>();

    public CustomerRestController() {
        customers.add(new Customer("superman", "Clark Kent"));
        customers.add(new Customer("batman", "Ben Affleck"));
        customers.add(new Customer("thor", "Chris Hemsworth"));
    }

    @RequestMapping(value = "/v1/customers/{id}")
    public Customer getCustomer(@PathVariable String id) {
        LOG.info("returning toy");
        return customers.stream().filter(customer -> 
            customer.getId().equalsIgnoreCase(id)).findFirst().get();
    }

}

package com.example.demo;

import org.springframework.boot.SpringApplication;
import org.springframework.boot.autoconfigure.SpringBootApplication;

@SpringBootApplication
public class CustomerServiceApplication {

    public static void main(String[] args) {
        SpringApplication.run(CustomerServiceApplication.class, args);
    }
}