@loopback/example-binding-resolution NPM

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

binding-resolution

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