§Getting started with Lagom in sbt

This page shows how to create and run your first Lagom project using sbt.

§Creating a new Lagom project

A Lagom system is typically made up of a set of sbt builds, each build providing multiple services. The easiest way to get started with a new Lagom system is to create a new project using the lagom Activator template:

$ activator new my-first-system lagom-java

This will create a new system with two services in it: helloworld and hellostream.

§Anatomy of a Lagom project

The created project contains the following elements:

my-first-system          → Project root
 └ hellostream-api       → hellostream api project
 └ hellostream-impl      → hellostream implementation project
 └ helloworld-api        → helloworld api project
 └ helloworld-impl       → helloworld implementation project
 └ project               → sbt configuration files
   └ build.properties    → Marker for sbt project
   └ plugins.sbt         → sbt plugins including the declaration for Lagom itself
 └ build.sbt             → Your project build file

• Notice how each service is broken up into two projects: api and implementation. The api project contains a service interface through which consumers may interact with the service. While the implementation project contains the actual service implementation.
• The project folder contains sbt specific files.
• The build.sbt file, which contains all information necessary to build, run, and deploy your services.

§Understanding services projects

• The service interface is always placed in the api project. For instance, the service interface for the helloworld service can be found in the helloworld-api project (look for the HelloService.java source file).

public interface HelloService extends Service {

  ServiceCall<NotUsed, String> hello(String id);

  @Override
  default Descriptor descriptor() {
    return named("helloservice").withCalls(
        restCall(Method.GET,  "/api/hello/:id", this::hello)
      ).withAutoAcl(true);
  }
}

• The service interface needs to inherit from Service and provide an implementation of Service#descriptor method.

• The implementation of Service#descriptor returns a Descriptor. A Descriptor defines the service name and the REST endpoints offered by a service. For each declared endpoint, an abstract method is declared in the service interface, e.g., see the HelloService#hello method.

• The implementation of the service abstract methods is provided by the related implementation project. For instance, the service implementation of the HelloService#hello method, for the helloworld service, can be found in the helloworld-impl project (look for the HelloServiceImpl.java source file).

public class HelloServiceImpl implements HelloService {

  private final PersistentEntityRegistry persistentEntityRegistry;

  @Inject
  public HelloServiceImpl(PersistentEntityRegistry persistentEntityRegistry) {
    this.persistentEntityRegistry = persistentEntityRegistry;
    persistentEntityRegistry.register(HelloWorld.class);
  }

  @Override
  public ServiceCall<NotUsed, String> hello(String id) {
    return request -> {
      // Look up the hello world entity for the given ID.
      PersistentEntityRef<HelloCommand> ref = persistentEntityRegistry.refFor(HelloWorld.class, id);
      // Ask the entity the Hello command.
      return ref.ask(new Hello(id, Optional.empty()));
    };
  }
}

• The PersistentEntityRegistry allows to persist data in the database using Event Sourcing and CQRS.

§Running Lagom services

Lagom includes a development environment that let you start all your services by simply typing runAll in the activator console. Open the terminal and cd to your Lagom project:

$ cd my-first-system
$ activator
... (booting up)
> runAll
[info] Starting embedded Cassandra server
..........
[info] Cassandra server running at 127.0.0.1:4000
[info] Service locator is running at http://localhost:8000
[info] Service gateway is running at http://localhost:9000
[info] Service helloworld-impl listening for HTTP on 0:0:0:0:0:0:0:0:24266
[info] Service hellostream-impl listening for HTTP on 0:0:0:0:0:0:0:0:26230
(Services started, use Ctrl+D to stop and go back to the console...)

You can verify that the services are indeed up and running by exercising one of its endpoints, e.g:

$ curl http://localhost:9000/api/hello/World

And you should get back the message Hello, World!.

If you are wondering why we have created two services in the seed template, instead of having just one, the reason is simply that (quoting Jonas Bonér):

One microservice is no microservice - they come in systems.

Said otherwise, we believe you will be creating several services, and we felt it was important to showcase intra-service communication.