@loopback/example-binding-resolution

binding-resolution

This example application is created to demonstrate LoopBack 4's context binding resolution and dependency injection within a context hierarchy. The application is verbosely instrumented to print out context/binding information to help developers understand how bindings are resolved and injected based on the current context and the binding scope.

The application scenario

The context chain

When GET http://127.0.0.1/ping is served, the following context hierarchy is leveraged by various artifacts.

• Application
  • RestServer
    • RequestContext (for HTTP requests)
      • InvocationContext (for interceptors)

Binding scopes for key artifacts

The artifacts below are bound to the context chain with certain scopes.

• Spy middleware (Singleton or transient)
• Spy interceptor (Singleton or transient)
• Ping controller (Transient or singleton)
• Logger service (Singleton)
• Request Logger service (Transient)

Binding resolution and injection

• new BindingDemoApplication() creates the application context

• The RestApplication registers RestComponent to the application. As a result, the RestServer is bound to the application context. The RestServer instance is the server context.

• app.boot() discovers and loads the following bindings into the application context:

  • PingController
  • SpyInterceptor
  • LoggerService
  • RequestLoggerService

• app.middleware(SpyMiddlewareProvider) binds middleware.Spy to the server context.

• When an HTTP request http://localhost:/3000/ping is sent, the rest server accepts the request, creates a RequestContext and dispatches it to the middleware sequence.

• The Spy middleware is invoked. It creates LOGGER_SERVICE in the request context as an alias to RequestLoggerService.

• The PingController.ping is mapped for the route. A new instance of PingController is created within the request context. Its dependency of LOGGER_SERVICE is resolved and injected into the controller with an instance of RequestLoggerService.

• The Spy global interceptor is invoked and it has access to the invocation context.

• The ping method of PingController is invoked. Method parameter injection is performed to supply the requestCtx parameter value.

• The ping method calls Logger service.

Use

DEBUG=loopback:example:binding-resolution npm start

To force binding scope to be Transient:

DEBUG=loopback:example:binding-resolution BINDING_SCOPE=transient npm start

Open http://localhost:3000/ping in your browser.

The console will print similar information as below:

  loopback:example:binding-resolution [middleware.Spy*] Owner context: RestServer-v2IUN1iNSM6NCqpa6D8vWw-1 +0ms
  loopback:example:binding-resolution [middleware.Spy*] Current context: RestServer-v2IUN1iNSM6NCqpa6D8vWw-1 +0ms
  loopback:example:binding-resolution [middleware.Spy*] Request context: RequestContext-v2IUN1iNSM6NCqpa6D8vWw-3 +0ms
  loopback:example:binding-resolution Request: GET /ping {...} +0ms
  loopback:example:binding-resolution [services.RequestLoggerService] Owner context: BindingDemoApplication-v2IUN1iNSM6NCqpa6D8vWw-0 +9ms
  loopback:example:binding-resolution [services.RequestLoggerService] Current context: RequestContext-v2IUN1iNSM6NCqpa6D8vWw-3 +0ms
  loopback:example:binding-resolution <<services.RequestLoggerService>> (GET /ping) [controllers.PingController] Owner context: BindingDemoApplication-v2IUN1iNSM6NCqpa6D8vWw-0 +0ms
  loopback:example:binding-resolution <<services.RequestLoggerService>> (GET /ping) [controllers.PingController] Current context: RequestContext-v2IUN1iNSM6NCqpa6D8vWw-3 +0ms
  loopback:example:binding-resolution [globalInterceptors.Spy*] Owner context: BindingDemoApplication-v2IUN1iNSM6NCqpa6D8vWw-0 +1ms
  loopback:example:binding-resolution [globalInterceptors.Spy*] Current context: BindingDemoApplication-v2IUN1iNSM6NCqpa6D8vWw-0 +0ms
  loopback:example:binding-resolution [globalInterceptors.Spy*] Invocation context: InterceptedInvocationContext-v2IUN1iNSM6NCqpa6D8vWw-4 +0ms
  loopback:example:binding-resolution <<services.RequestLoggerService>> (GET /ping) [controllers.PingController] Request context: RequestContext-v2IUN1iNSM6NCqpa6D8vWw-3 +1ms
  loopback:example:binding-resolution <<services.RequestLoggerService>> (GET /ping) Response {...} +1ms
  loopback:example:binding-resolution [middleware.Spy*] Request context: RequestContext-v2IUN1iNSM6NCqpa6D8vWw-5 +188ms
  loopback:example:binding-resolution Request: GET /favicon.ico {...} +0ms

Contributions

• Guidelines
• Join the team

Tests

Run npm test from the root folder.

Contributors

See all contributors.

License

MIT