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12 - Making Microservices Talk to Each Other

This guide is part of the Azure Spring Apps training

Creating a microservice that talks to other microservices.

In Section 6 we deployed a microservice that returns a list of cities. In Section 7, we deployed a microservice that, given a city, returns the weather for that city. And in Section 9, we created a front-end application that queries these two microservices.

There is a glaring inefficiency in this design: the browser first calls city-service, waits for it to respond, and upon getting that response, calls weather-service for each of the cities returned. All these remote calls are made over public internet, whose speed is never guaranteed.

To resolve this inefficiency, we will create a single microservice that implements the Transaction Script pattern: it will orchestrate the calls to individual microservices and return the weather for all cities. To do this, we will use Spring Cloud OpenFeign. OpenFeign will automatically obtain the URLs of invoked microservices from Spring Cloud Registry, allowing us to build our all-cities-weather-services microservice without needing to resolve the locations of the constituent microservices.

Note how the code we create in this section is endpoint-agnostic. All we specify is the name of the services we want to invoke in the @FeignClient annotation. OpenFeign and the Azure Spring Apps discovery service then work together behind the scenes to connect our new microservice to the services we've created previously.

Create a Spring Boot Microservice

To create our microservice, we will invoke the Spring Initializr service from the command line:

curl https://start.spring.io/starter.tgz -d type=maven-project -d dependencies=cloud-feign,web,cloud-eureka,cloud-config-client -d baseDir=all-cities-weather-service -d bootVersion=3.1.3 -d javaVersion=17 | tar -xzvf -

Add Spring code to call other microservices

Next to the DemoApplication class, create a Weather record:

package com.example.demo;

public record Weather (String city, String description, String icon ) {
}

Note: this corresponds to the Weather class that we created in Section 7 when we defined the original weather-service with two important differences: we no longer annotate the class as a JPA entity for data retrieval, and we define it as a Java record instead of a class. The differences between records and classes are many but in this particular case, they can be used interchangeably.

Next, in the same location create the City class. This is the same City class that we created in Section 6.

package com.example.demo;

public record City (String name) {
}

Then, in the same location, create an interface called CityServiceClient with the following contents. When we run our new service, OpenFeign will automatically provide an implementation for this interface.

package com.example.demo;

import java.util.List;

import org.springframework.cloud.openfeign.FeignClient;
import org.springframework.web.bind.annotation.GetMapping;

@FeignClient("city-service")
public interface CityServiceClient{

    @GetMapping("/cities")
    List<List<City>> getAllCities();
}

Create a similar OpenFeign client interface for the weather service, named WeatherServiceClient.

package com.example.demo;

import org.springframework.cloud.openfeign.FeignClient;
import org.springframework.web.bind.annotation.GetMapping;
import org.springframework.web.bind.annotation.RequestParam;


@FeignClient("weather-service")
public interface WeatherServiceClient {

    @GetMapping("/weather/city")
    Weather getWeatherForCity(@RequestParam("name") String cityName);
}

To enable Spring Cloud to discover the underlying services and to automatically generate OpenFeign clients, add the annotations @EnableDiscoveryClient and @EnableFeignClients to the DemoApplication class (as well as the corresponding import statements):

package com.example.demo;

import org.springframework.boot.SpringApplication;
import org.springframework.boot.autoconfigure.SpringBootApplication;
import org.springframework.cloud.client.discovery.EnableDiscoveryClient;
import org.springframework.cloud.openfeign.EnableFeignClients;

@SpringBootApplication
@EnableDiscoveryClient
@EnableFeignClients
public class DemoApplication {
    public static void main(String[] args) {
        SpringApplication.run(DemoApplication.class, args);
    }
}

Everything is now in place to implement the all-cities-weather-service. Create the class AllCitiesWeatherController as follows:

package com.example.demo;

import org.springframework.web.bind.annotation.GetMapping;
import org.springframework.web.bind.annotation.RestController;

import java.util.Collection;
import java.util.List;
import java.util.stream.Collectors;
import java.util.stream.Stream;

@RestController
public class AllCitiesWeatherController {

    private CityServiceClient cityServiceClient;
    private WeatherServiceClient weatherServiceClient;

    public AllCitiesWeatherController(CityServiceClient cityServiceClient, WeatherServiceClient weatherServiceClient) {
        this.cityServiceClient = cityServiceClient;
        this.weatherServiceClient = weatherServiceClient;
    }

    @GetMapping("/")
    public List<Weather> getAllCitiesWeather() {
        Stream<City> allCities = cityServiceClient.getAllCities().stream().flatMap(Collection::stream);

        //Obtain weather for all cities in parallel
        return allCities.parallel()
                .peek(city -> System.out.println("City: >>" + city.name() + "<<"))
                .map(city -> weatherServiceClient.getWeatherForCity(city.name()))
                .collect(Collectors.toList());

    }
}

Add time-out settings

In order to stop the Feign services timing out automatically, open up the src/main/resources/application.properties file and add:

spring.cloud.openfeign.client.config.default.connectTimeout=160000000
spring.cloud.openfeign.client.config.default.readTimeout=160000000

Create the application on Azure Spring Apps

As before, create a specific all-cities-weather-service application in your Azure Spring Apps instance:

az spring app create -n all-cities-weather-service --runtime-version Java_17

Deploy the application

You can now build your "all-cities-weather-service" project and send it to Azure Spring Apps:

cd all-cities-weather-service
./mvnw clean package -DskipTests
az spring app deploy -n all-cities-weather-service --artifact-path target/demo-0.0.1-SNAPSHOT.jar
cd ..

Test the project in the cloud

You can use the gateway created in Section 8 to access the all-cities-weather-service directly.

https://<Your gateway URL>/ALL-CITIES-WEATHER-SERVICE

You should get the JSON output with the weather for all the cities:

[{"city":"Paris, France","description":"It's always sunny on Azure Spring Apps","icon":"weather-sunny"},
{"city":"London, UK","description":"It's always sunny on Azure Spring Apps","icon":"weather-sunny"}]

⬅️ Previous guide: 11 - Configure CI/CD

➡️ Next guide: Conclusion