Module Resolution in nestjs

Question

I have module a and module b . Module a instantiates its service providers asynchronously whilst module b instantiates synchronously. Both contain multiple endpoints. Both modules are imported by the AppModule .

Now I'm new to thinking of things from the server side and I'm also coming from angular lazy loading, so bear with me here.

If an endpoint from module b is activated(?) does module a load as well? Do the service providers defined in module a , that use async factories to instantiate, get instantiated as well?

Answer 1

The concept of async providers is a bit different from Angular's; it has nothing to do with the routes or controllers.

If a provider is async, it means it needs to wait for an asynchronous resource (Promise) to be resolved before it can be instantiated. That can be waiting for a database connection to be established or a configuration to be loaded from the file system. When your application has started, all your async providers have been fully instantiated.

---

To try this out, you can add the HttpModule asynchronously as follows:

HttpModule.registerAsync({
  useFactory: async () => {
    console.log('Starting to instantiate');
    // wait 5 seconds to simulate asynchronous task
    await new Promise(resolve => setTimeout(resolve, 5000));
    console.log('Done after 5 seconds');
    return {};
  },
}),

],

When you start the application, you will see that the startup will wait the 5 seconds. The HttpModule is not instantiated dynamically as you might expect coming from Angular.

[Nest] 242   - 3/18/2019, 2:08:04 PM   [NestFactory] Starting Nest application...
Starting to instantiate
[Nest] 242   - 3/18/2019, 2:08:04 PM   [InstanceLoader] AppModule dependencies initialized +8ms
Done after 5 minutes
[Nest] 242   - 3/18/2019, 2:08:09 PM   [InstanceLoader] HttpModule dependencies initialized +5004ms