For people coming from different programming language backgrounds, it might be unexpected to learn that in Danet, almost everything is shared across incoming requests. We have a connection pool to the database, singleton services with global state, etc. Remember that Deno doesn't follow the request/response Multi-Threaded Stateless Model in which every request is processed by a separate thread. Hence, using singleton instances is fully safe for our applications.

However, there are edge-cases when request-based lifetime may be the desired behavior, for instance per-request caching in GraphQL applications, request tracking, and multi-tenancy. Injection scopes provide a mechanism to obtain the desired injectable lifetime behavior.

Provider scope ​

An injectable can have any of the following scopes:

Usage ​

Specify injection scope by passing the scope property to the @Injectable() decorator options object:

import { Injectable, Scope } from 'https://deno.land/x/danet/mod.ts';

@Injectable({ scope: Scope.REQUEST })
export class TodoService {}

Singleton scope is used by default, and need not be declared. If you do want to declare an injectable as singleton scoped, use the Scope.GLOBAL value for the scope property.

Controller scope ​

Controllers can also have scope, which applies to all request method handlers declared in that controller. Like injectable scope, the scope of a controller declares its lifetime. For a request-scoped controller, a new instance is created for each inbound request, and garbage-collected when the request has completed processing.

Declare controller scope with the scope property of the ControllerOptions object:

@Controller({
  path: 'todo',
  scope: Scope.REQUEST,
})
export class TodoController {}

Scope hierarchy ​

The REQUEST scope bubbles up the injection chain. A controller that depends on a request-scoped injectable will, itself, be request-scoped.

Imagine the following dependency graph: TodoController <- TodoService <- TodoRepository. If TodoService is request-scoped (and the others are default singletons), the TodoController will become request-scoped as it is dependent on the injected service. The TodoRepository, which is not dependent, would remain singleton-scoped.

Transient-scoped dependencies don't follow that pattern. If a singleton-scoped ItemService injects a transient LoggerService injectable, it will receive a fresh instance of it. However, ItemService will stay singleton-scoped, so injecting it anywhere would not resolve to a new instance of ItemService. In case it's desired behavior, ItemService must be explicitly marked as TRANSIENT as well.

Access context ​

You may want to access a reference to the original request object when using request-scoped injectables. You can access it using the beforeControllerMethodIsCalled method as following. And yes, it can be async.

import { Injectable, Scope, Inject, HttpContext } from 'https://deno.land/x/danet/mod.ts';

@Injectable({ scope: Scope.REQUEST })
export class TodoService {
  constructor() {}
  
  async beforeControllerMethodIsCalled(ctx: HttpContext) {
    //do something with the context
  }
}

Performance ​

Using request-scoped injectables will have an impact on application performance. We have to create an instance of your class on each request. Hence, it will slow down your average response time and overall benchmarking result. Unless an injectable must be request-scoped, it is strongly recommended that you use the default singleton scope.