Service.md

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Overview

We use the service concept in LoopBack 4 to refer to an object with methods to perform local or remote operations. Such objects are resolved from bindings within a LoopBack 4 context.

We support three types of services:

• proxy: A service proxy for remote REST/SOAP/gRPC APIs
• class: A TypeScript class that can be bound to the application context
• provider: A TypeScript class that implements Provider interface and can be bound to the application context

For remote services (type=proxy), there should be at least one valid (REST, SOAP, or gRPC) data source created already in the src/datasources directory.

Generate services using CLI

The lb4 service command can generate code for local and remote services.

See Service Generator for more details.

Register services with the application

Services added to src/services are automatically discovered and registered when the application is booted.

To register a service by code, use application.service(). For example,

• Bind a service class or provider

  // For a class, the interface is default to the class itself
  const binding = app.service(MyService);

  // A custom name can be specified when the service is registered
  const binding = app.service(MyService, 'my-service');

• Bind a service class or provider with an interface identified by a string

  // Register a service and mark the interface as `MyService`
  const binding = app.service(MyService, {interface: 'MyService'});

  // Register a service provider and mark the interface as `MyService`
  const binding = app.service(MyServiceProvider, {interface: 'MyService'});

  NOTE: The app.service() inspects the class to determine if it's a provider by the existence of a value prototype method. A provider class is bound using app.toProvider(). Otherwise app.toClass() is used.

• Bind a service with an interface identified by a symbol

  // Define a symbol as the id for a service interface
  const MyServiceInterface = Symbol('MyService');
  const binding = app.service(MyService, {interface: MyServiceInterface});

Inject service instances

A @service decorator is provided to declare injection of a service instance by interface.

A class can be used as the interface to identify the service:

class MyController {
  constructor(@service(MyService) public myService: MyService) {}
}

The service class can be inferred from the design type of the target if present. In the example below, the interface is MyService.

class MyController {
  constructor(@service() public myService: MyService) {}
}

If the service is modeled as a TypeScript interface, we need to use a string or symbol to represent the interface as TypeScript interfaces cannot be reflected at runtime.

const MyServiceInterface = 'MyService';
class MyController {
  constructor(@service(MyServiceInterface) public myService: MyService) {}
}

const MyServiceInterface = Symbol('MyService');
class MyController {
  constructor(@service(MyServiceInterface) public myService: MyService) {}
}

See Service Decorator for more details.