Advanced NestJS: Dynamic Providers

Livio is a member of the NestJS core team and creator of the @nestjs/terminus integration

Intro

Dependency Injection (short DI) is a powerful technique to build a loosely coupled architecture in a testable manner. In NestJS an item which is part of the DI context is called provider. A provider consists of two main parts, a value, and a unique token. In NestJS you can request the value of a provider by its token. This is most apparent when using the following snippet.

import { NestFactory } from '@nestjs/core';
import { Module } from '@nestjs/common';

@Module({
  providers: [
    {
      provide: 'PORT',
      useValue: 3000,
    },
  ],
})
export class AppModule {}

async function bootstrap() {
  const app = await NestFactory.createApplicationContext(AppModule);

  const port = app.get('PORT');
  console.log(port); // Prints: 3000
}
bootstrap();

The AppModule consists of one provider with the token PORT.

• We bootstrap our application by calling NestFactory.createApplicationContext (this method does the same as NestFactory.create but does not initiate an HTTP instance).
• Later on, we request the value of our provider with app.get('PORT'). This will return 3000 as specified in our provider.

Fair enough. But what if you do not know what you will provide to the user? What if you need to compute the providers during runtime?

This article goes into a technique which we use often for various NestJS integrations. This technique will allow you to build highly dynamic NestJS applications and still make use of the advantages of DI.

What are we trying to achieve

To see use cases of dynamic providers we will use a simple but useful example. We want to have a parameter decorator called Logger which takes an optional prefix as string. This decorator will inject a LoggerService, which prepends the given prefix to every log message.

So the final implementation will look like this:

@Injectable()
export class AppService {
  constructor(@Logger('AppService') private logger: LoggerService) {}

  getHello() {
    this.logger.log('Hello World'); // Prints: '[AppService] Hello World'
    return 'Hello World';
  }
}

Setup a NestJS application

We will make use of the NestJS CLI to get started quickly. If you have not installed it, use the following command:

npm i -g @nestjs/cli

Now run the following command in your terminal of choice to bootstrap your Nest application.

nest new logger-app && cd logger-app

Logger service

Let's start off with our LoggerService. This service will get injected later when we use our @Logger() decorator. Our basic requirements for this service are:

• A method which can log messages to stdout
• A method which can set the prefix of each instance

Once again we will use the NestJS CLI to bootstrap our module and service.

nest generate module Logger
nest generate service Logger

To satisfy our requirements we build this minimal LoggerService.

// src/logger/logger.service.ts

import { Injectable, Scope } from '@nestjs/common';

@Injectable({
  scope: Scope.TRANSIENT,
})
export class LoggerService {
  private prefix?: string;

  log(message: string) {
    let formattedMessage = message;

    if (this.prefix) {
      formattedMessage = `[${this.prefix}] ${message}`;
    }

    console.log(formattedMessage);
  }

  setPrefix(prefix: string) {
    this.prefix = prefix;
  }
}

First of all, you may have realized that the @Injectable() decorator uses the scope option with Scope.TRANSIENT. This basically means every time the LoggerService gets injected in our application, it will create a new instance of the class. This is mandatory due to the prefix attribute. We do not want to have a single instance of the LoggerService and constantly override the prefix option.

Other than that, the LoggerService should be self-explanatory.

Now we only have to export our service in the LoggerModule, so we can use it in AppModule.

// src/logger/logger.module.ts

import { Module } from '@nestjs/common';
import { LoggerService } from './logger.service';

@Module({
  providers: [LoggerService],
  exports: [LoggerService],
})
export class LoggerModule {}

Let's see if it works in our AppService.

// src/app.service.ts

import { Injectable } from '@nestjs/common';
import { LoggerService } from './logger/logger.service';

@Injectable()
export class AppService {
  constructor(private readonly logger: LoggerService) {
    this.logger.setPrefix('AppService');
  }
  getHello(): string {
    this.logger.log('Hello World');
    return 'Hello World!';
  }
}

Seems fine - let's start the application with npm run start and request the website with curl http://localhost:3000/ or open up http://localhost:3000 in your browser of choice.

If everything is set up correctly we will receive the following log output.

[AppService] Hello World

That is cool. Though, we are lazy, aren't we? We do not want to explicitly write this.logger.setPrefix('AppService') in the constructor of our services? Something like @Logger('AppService') before our logger-parameter would be way less verbose and we would not have to define a constructor every time we want to use our logger.

Logger Decorator

For our example, we do not need to exactly know how decorators work in TypeScript. All you need to know is that functions can be handled as a decorator.

Lets quickly create our decorator manually.

touch src/logger/logger.decorator.ts

We are just going to reuse the @Inject() decorator from @nestjs/common.

// src/logger/logger.decorator.ts

import { Inject } from '@nestjs/common';

export const prefixesForLoggers: string[] = new Array<string>();

export function Logger(prefix: string = '') {
  if (!prefixesForLoggers.includes(prefix)) {
    prefixesForLoggers.push(prefix);
  }
  return Inject(`LoggerService${prefix}`);
}

You can think of @Logger('AppService') as nothing more than an alias for @Inject('LoggerServiceAppService'). The only special thing we have added is the prefixesForLoggers array. We will make use of this array later. This array just stores all the prefixes we are going to need.

But wait, our Nest application does not know anything about a LoggerServiceAppService token. So let's create this token using dynamic providers and our newly created prefixesForLoggers array.

Dynamic providers

In this chapter, we want to have a look at dynamically generating providers.
We want to

• create a provider for each prefix
  • each of these providers must have a token like this 'LoggerService' + prefix
  • each provider must call LoggerService.setPrefix(prefix) upon its instantiation

To implement these requirements we create a new file.

touch src/logger/logger.providers.ts

Copy & paste the following code into your editor.

// src/logger/logger.provider.ts

import { prefixesForLoggers } from './logger.decorator';
import { Provider } from '@nestjs/common';
import { LoggerService } from './logger.service';

function loggerFactory(logger: LoggerService, prefix: string) {
  if (prefix) {
    logger.setPrefix(prefix);
  }
  return logger;
}

function createLoggerProvider(prefix: string): Provider<LoggerService> {
  return {
    provide: `LoggerService${prefix}`,
    useFactory: logger => loggerFactory(logger, prefix),
    inject: [LoggerService],
  };
}

export function createLoggerProviders(): Array<Provider<LoggerService>> {
  return prefixesForLoggers.map(prefix => createLoggerProvider(prefix));
}

The createLoggerProviders-function creates an array of providers for each prefix set by the @Logger() decorator. Thanks to the useFactory functionality of NestJS we can run a the LoggerService.setPrefix() method before the provider gets created.

All we need to do now is to add these logger providers to our LoggerModule.

// src/logger/logger.module.ts

import { Module } from '@nestjs/common';
import { LoggerService } from './logger.service';
import { createLoggerProviders } from './logger.providers';

const loggerProviders = createLoggerProviders();

@Module({
  providers: [LoggerService, ...loggerProviders],
  exports: [LoggerService, ...loggerProviders],
})
export class LoggerModule {}

As simple as that. Wait no, that does not work? Because of JavaScript, man. Let me explain: createLoggerProviders will get called immediately once the file is loaded, right? At that point in time, the prefixesForLoggers array will be empty inside logger.decorator.ts, because the @Logger() decorator was not called.

So how do we bypass that? The holy words are Dynamic Module. Dynamic modules allow us to create the module settings (which are usually given as parameter of the @Module-decorator) via a method. This method will get called after the @Logger decorator calls and therefore prefixForLoggers array will contain all the values.

If you want to learn more about why this works, you may wanna check out this video about the JavaScript event loop

Therefore we have to rewrite the LoggerModule to a Dynamic Module.

// src/logger/logger.module.ts

import { DynamicModule } from '@nestjs/common';
import { LoggerService } from './logger.service';
import { createLoggerProviders } from './logger.providers';

export class LoggerModule {
  static forRoot(): DynamicModule {
    const prefixedLoggerProviders = createLoggerProviders();
    return {
      module: LoggerModule,
      providers: [LoggerService, ...prefixedLoggerProviders],
      exports: [LoggerService, ...prefixedLoggerProviders],
    };
  }
}

Do not forget to update the import array in app.module.ts

// src/logger/app.module.ts

@Module({
  controllers: [AppController],
  providers: [AppService],
  imports: [LoggerModule.forRoot()],
})
export class AppModule {}

...and that's it! Let's see if it works when we update the app.service.ts

// src/app.service.ts

@Injectable()
export class AppService {
  constructor(@Logger('AppService') private logger: LoggerService) {}

  getHello() {
    this.logger.log('Hello World'); // Prints: '[AppService] Hello World'
    return 'Hello World';
  }
}

Calling http://localhost:3000 will give us the following log

[AppService] Hello World

Yey, we did it!

Conclusion

We have touched on numerous advanced parts of NestJS. We have seen how we can create simple decorators, dynamic modules and dynamic providers. You can do impressive stuff with it in a clean and testable way.

As mentioned we have used the exact same patterns for the internals of @nestjs/typeorm and @nestjs/mongoose. In the Mongoose integration, for example, we used a very similar approach for generating injectable providers for each model.

You can find the code in this Github repostiory. I have also refactored smaller functionalities and added unit tests, so you can use this code in production. Happy hacking :)

Comments

[Bilal Haidar]

Hi Livio,
Can you please illustrate more on this:
So how do we bypass that? The holy words are Dynamic Module

How would that overcome the problem? Are classes' constructors called before calling a Dynamic Module?

Thanks

[Lucas Silva]

Very nice article :)

[Ruslan Gonzalez]

Great article... keep sharing things like that, we appreciate it.

[Trias Bratayudhana]

Very nice article :)

[Daniel Huly]

Great article. I was looking for this kind of example.

I had only one issue with re-exporting the dynamic LoggerModule.
The prefixesForLoggers was not call in time (same issues as you explained in the article) due to other modules loading.

In my case I have a CoreModule that includes many base modules and services.

I was able to overcome the issue by making the LoggerModule an Asyc module (using Promise and setTimeout(..., 0)). This let all the files and @Logger(...) to be called and add all the prefixes to prefixesForLoggers array before resolving.

// logger.module.ts

export class LoggerModule {
  static forRoot(): Promise<DynamicModule> {

    return new Promise(resolve => {
      setTimeout(() => {
        const prefixedLoggerProviders = createLoggerProviders();
        resolve({
          module: LoggerModule,
          providers: [LoggerService, ...prefixedLoggerProviders],
          exports: [LoggerService, ...prefixedLoggerProviders],
        })
      }, 0);
    })

  }
}

Thanks :)

[Жека уже в Гамбурге]

Nice work! Thank you.

[Eric Haynes]

Or, you could just...

class AppService {
  private logger = LoggerService.withPrefix('AppService');
  // ...
}

[Francisco Aguilera]

underrated comment right here. This is how the nestjs logger works.

[Eric Haynes]

I just keep banging my head against the wall. It's like some kind of contest to see who can overcomplicate things the most. It's rooted in this archaic belief that initializing objects is expensive, then just snowballs with tactics designed to prevent duplicate initializations across use cases where there would NEVER be duplicates for objects in places where it wouldn't matter if there were.

[Livio Brunner]

That is why the article is called "Advanced NestJS: Dynamic Providers", not "How to create your own Logging Library".

Seems like I failed to convey that "Logger" was just an easy-to-understand vehicle to bring closer how you can dynamically generate providers using NestJS. This is quite powerful pattern which is used for instance in @nestjs/typeorm.

Your average user won't need this. This is mainly targeted for advanced NestJS users (hence the title) + library authors

In heavens sake I don't wanna say "You should build your own logger like this". Off course not, that is why we already provide the logger you mentioned .

I am part of the NestJS core team -- off course I wouldn't try to promote a different logger. That wasn't my point of the article. Sorry for the misunderstanding will try to make it clearer next time.

[Eric Haynes]

Respectfully, that it was a logger wasn't relevant to my comment either. I would feel the same about it as a queue name, or a repository type. Rather, I was pointing out that this is quite a lot of complexity to solve the "problem" of passing a string to a function.

I don't get this obsession with decorators. They're not a particulary powerful pattern. You shed the runtime context that's necessary to instantiate things, and it takes a tremendous amount of effort to avoid falling back on static contexts, paradoxically creating the problem that DI frameworks claim to solve. As Daniel Huly points out, there are are numerous ways that consumers of this can be loaded later than the module, and this will mysteriously not work without leaving the user any way to trace it short of hacking around in the library's compiled code.

Further, while decorators can enforce types on their target, the NestJS ones don't, so anything relying on @Inject completely disposes of type safety:

@Logger('AppService') private logger: Potato;

"We do not want to explicitly write this.logger.setPrefix('AppService') in the constructor of our services?" The mutation is a poor pattern, yes. After all, your service has no real way to know if it's actually TRANSIENT. But otherwise, when something is used in exactly one place, the place that's using it is a perfectly fine place to instantiate it. The DI container is already managing the singleton service.

If you really need a bunch of other injected stuff to do so, expose the factory:

@Injectable()
class SomeService {
  private logger: LoggerService;

  constructor(loggerFactory: LoggerFactory) {
    this.logger = loggerFactory.withPrefix('AppService');
  }
}

It's actually dynamic, it's type-safe, it doesn't make assumptions about whether the consumer actually wants a transient instance or not, you can debug it, and most importantly, it doesn't rely on load order to work. All things a library author should consider.

[Livio Brunner]

Yes I actually agree. That is one of the reasons why I am personally pushing to always offer decorators alternative.

For me decorators are nice-to-have-syntax-sugars. It's alright for certain things (e.g. defining a Controller), but you always want to have the option to for instance create a Controller dynamically. This can't be done nicely with decorators (except you create class mixins). It's much more convenient to use a service or similar where you can call some functions.

Nonetheless, I believe decorators can be nice. Dynmically generating providers can be useful. I agree - my example wasn't the best - but a lot of the things you've mention (e.g. type-security) can be fixed depending on the context even with Decorators. Libraries authors should strive for a programmatic service-based API and as a nice-to-have maybe an applicable decorator.

In the end its up to the Library Author what they want to offer and how they do it. This article was meant to look into one strategy which might help in your specific use-case. If you -- as a library author -- believe its not the right fit then so be it.