What is the simplest way to consume an external REST service in Lagom?

Question

According to Lagom documentation, we can define external service URI (like below) and can get it from ServiceLocator.

lagomUnmanagedServices in ThisBuild := Map("weather" -> "http://localhost:3333")

http://www.lagomframework.com/documentation/1.0.x/ServiceLocator.html#Communicating-with-external-services

What is the simplest way to call the external REST API in Lagom?

I considered using WsClient in Lagom, but I didn't choose it. Lagom includes only WsClient for Scala, therefore it provides result values as not java.util.concurrent.CompletionStage but scala.concurrent.Future. It makes the pain to combine with other Lagom APIs like CompletionStage#doWithService.

Nathan

That's a really good question. +1 – Teresaterese

Here's a sample java project (not a lagom project) that uses Lagom's libraries (not the the whole framework) to consume third party API's: github.com/ignasi35/lagom-client-demo – Thayne

Answer 1

A way to consume 3rd party REST services from lagom is by writing the 3rd party's REST spec using a Lagom Descriptor.

Imagine your code wanted to interact with Slack's API, you would create a slack-api project in your app and create the Slack descriptor there (you wouldn't need to create a slack-implof course).

Then, on your fancy-impl code you would depend on slack-api and in your FancyServiceImpl implementation you would inject SlackService in the constructor.

PS: The gist is scala code, but same idea applies to Lagom's Java DSL.

Thayne

Thank you for your answer! I'll try it. – Nathan

Answer 2

As none of the previous answers contain full information on how to implement it in Java, here's how I did it:

1. Create Lagom API module, e.g. external-service-api and put request/response DTOs and Lagom service in it, e.g.:

   public interface ExternalService extends Service {
   
       ServiceCall<ExternalServiceRequest, ExternalServicePostResponse> somePostMethod();
   
       ServiceCall<NotUsed, ExternalServiceGetResponse> someGetMethod();
   
       @Override
       default Descriptor descriptor(){
           return Service.named("external-service").withCalls(
               Service.pathCall("/post-endpoint", this::somePostMethod),
               Service.pathCall("/get-endpoint", this::someGetMethod)
           ).withAutoAcl(true);
       }
   }
   

2. Add dependency to your external-service-api in impl module where you want to use it.

3. Register your service in *Module.java. Use: bindClient(ExternalService.class);

4. Now the tricky part, I found some tutorials/repositories in which unmanaged service was defined in pom.xml in implementation module. It didn't work for me, and I don't know why (Lagom v. 1.4.5, Scala binaries 2.12). What I had to do, was to put unmanagedServices definitions in projects root pom.xml. Take notice that unmanagedService child element should be named same as you defined in descriptor.

       <plugin>
           <groupId>com.lightbend.lagom</groupId>
           <artifactId>lagom-maven-plugin</artifactId>
           <version>${lagom.version}</version>
           <configuration>
               <unmanagedServices>
                   <external-service>http://api.url.com/</external-service>
               </unmanagedServices>
           </configuration>
       </plugin>
   

5. Inject your service:

   @Inject
   public SomeConstructor(ExternalService externalService){
       ...
   }
   

6. To use it call e.g.: externalService.somePostMethod().invoke(new ExternalServiceRequest(...))

Dachau

Answer 3

In Lagom, any 3rd party application/service can be accessed as an unmanaged service. You would need to add that in your application.conf like this

#external-services lagom.services { 3rd-party-service-identifier = "http://3rd-party-service-url" ... } and would also need to add the 3rd party service url as an unmanagedService in the pom.xml like this

<plugin>
    <groupId>com.lightbend.lagom</groupId>
    <artifactId>lagom-maven-plugin</artifactId>
    <version>${lagom.version}</version>
    <configuration>
      <kafkaEnabled>false</kafkaEnabled>
      <cassandraEnabled>false</cassandraEnabled>
      <unmanagedServices>
        <3rd-party-service-identifier>${3rd-party-service-URL}</3rd-party-service-identifier>
      </unmanagedServices>
    </configuration>
  </plugin>

this is how you let Lagom know about the 3rd party service, but to make lagom to work with the service please go to the link below. https://www.lagomframework.com/documentation/1.4.x/java/IntegratingNonLagom.html

Coniine