Configuring Applications

Making your own application class

By making your own application class, you can perform several additional tasks as a part of your setup:

• Pass the configuration into the base class constructor
• Perform asynchronous startup functions before starting the application
• Perform graceful cleanup functions when the application stops

{% include code-caption.html content="src/widget.application.ts" %}

import {Application} from '@loopback/core';
import {RestComponent} from '@loopback/rest';
import {UserController, ShoppingCartController} from './controllers';

export class WidgetApplication extends Application {
  constructor() {
    // This is where you would pass configuration to the base constructor
    // (as well as handle your own!)
    super({
      rest: {
        port: 8080,
      },
    });

    const app = this; // For clarity.
    // You can bind to the Application-level context here.
    // app.bind('foo').to(bar);
    app.component(RestComponent);
    app.controller(UserController);
    app.controller(ShoppingCartController);
  }

  async stop() {
    // This is where you would do whatever is necessary before stopping your
    // app (graceful closing of connections, flushing buffers, etc)
    console.log('Widget application is shutting down...');
    // The superclass stop method will call stop on all servers that are
    // bound to the application.
    await super.stop();
  }
}

Configuring your application

Your application can be configured with constructor arguments, bindings, or a combination of both.

Let's see how these configurations work below.

Manual binding configuration

Binding is the most commonly-demonstrated form of application configuration throughout our examples. Binding is the recommended method for setting up your application.

In addition to the binding functions provided by Context, the Application class also provides some sugar functions for commonly used bindings, like component, server and controller:

export class MyApplication extends Application {
  constructor() {
    super();
    this.component(MagicSuite);
    this.server(RestServer, 'public');
    this.server(RestServer, 'private');

    this.controller(FooController);
    this.controller(BarController);
    this.controller(BazController);
  }
}

You can find a complete list of these functions on the Application API docs page.

Additionally, you can use more advanced forms of binding to fine-tune your application's configuration:

export class MyApplication extends Application {
  constructor() {
    super();
    this.server(RestServer);
    this.controller(FooController);
    this.bind('fooCorp.logger').toProvider(LogProvider);
    this.bind('repositories.widget')
      .toClass(WidgetRepository)
      .inScope(BindingScope.SINGLETON);
  }
}

In the above example:

• injection calls for fooCorp.logger will be handled by the LogProvider class.
• injection calls for repositories.widget will be handled by a singleton instance of the WidgetRepository class.

Components

app.component(MyComponent);
app.component(RestComponent);

The component function allows binding of component constructors within your Application instance's context.

For more information on how to make use of components, see Using Components.

Controllers

app.controller(FooController);
app.controller(BarController);

Much like the component function, the controller function allows binding of Controllers to the Application context.

Servers

app.server(RestServer);
app.servers([MyServer, GrpcServer]);

The server function is much like the previous functions, but bulk bindings are possible with Servers through the function servers.

const app = new Application();
app.server(RestServer, 'public'); // {'public': RestServer}
app.server(RestServer, 'private'); // {'private': RestServer}

In the above example, the two server instances would be bound to the Application context under the keys servers.public and servers.private, respectively.

The above examples of binding demonstrate manual binding where LoopBack does not automatically register the above artifacts on your behalf. Alternatively, LoopBack can automatically register artifacts for you. Let's see how this is done with DataSources and Repositories below.

DataSources

Automatic binding configuration

Alternative to manually binding artifacts in your application LoopBack 4 comes with an automatic approach to binding artifacts like Controllers, DataSources, Models and Repositories. Using a Booter class LoopBack automatically discovers the above artifacts, as per the folder structure illustrated in the table below:

{% include tip.html content=" Automatic binding configuration is by default supported when using the Command-line interface tools " %}

Constructor configuration

The Application class constructor also accepts an ApplicationConfig object which contains component-level configurations such as RestServerConfig. It will automatically create bindings for these configurations and later be injected through dependency injections. Visit Dependency Injection for more information.

{% include note.html content=" Binding configuration such as component binding, provider binding, or binding scopes are not possible with the constructor-based configuration approach. " %}

export class MyApplication extends RestApplication {
  constructor() {
    super({
      rest: {
        port: 4000,
        host: 'my-host',
      },
    });
  }
}

Further Reading

• Booting an Application