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In LoopBack 3, predefined boot scripts are organized in the /server/boot directory, and are executed right before the server starts to perform some custom application initialization. The same functionality can also be achieved in LoopBack 4 application by adding observers.

The LoopBack 3 boot script is mapped to LoopBack 4 observer’s start function. And moreover, besides participating in an application’s start phase, an observer also allows you to perform logic in the stop phase.

LoopBack 3 Boot Script

LoopBack 3 supports three styles of boot scripts:

  • synchronous boot script
  • asynchronous boot script that takes in a callback
  • asynchronous boot script that returns a promise

They all have the first parameter as server (some people are used to name it as app). The callback based asynchronous script has a 2nd parameter done as the callback function.

A typical synchronous boot script that prints out a datasource’s name before application runs:

server/boot/print-info.js

'use strict';

module.exports = function printInfo(server) {
  // SUPPOSE your application has a datasource called `db`, which is usually the default
  // memory datasource in a sample LoopBack application created by CLI, regardless if it's
  // an LB3 app or LB4 app
  const db = server.datasources.db;
  console.log('This is a synchronous script.');
  console.log('Your app has a datasource called: ', db.name);
};

A typical asynchronous boot script that takes in a callback:

server/boot/create-sample-cb.js

// in file `server/boot/create-sample-cb.js`
'use strict';

module.exports = function createSample(server, done) {
  // SUPPOSE your application has a Todo model
  const Todo = server.models.Todo;
  const sample = {
    title: 'a todo sample - cb',
    desc: 'created by callback based script',
  };
  Todo.create(sample, (err, result) => {
    if (err) return done(err);
    console.log('Sample created as: ', result);
    done();
  });
};

A typical asynchronous boot script that returns a promise:

server/boot/create-sample-promise.js

'use strict';

module.exports = function createSample(server) {
  // SUPPOSE your application has a Todo model
  const Todo = server.models.Note;
  const sample = {
    title: 'a todo sample - promise',
    desc: 'created by promise based script',
  };
  return Todo.create(sample).then(result => {
    console.log('Sample created as: ', result);
  });
};

Since a LoopBack 4 application is created in TypeScript, which supports the latest ECMAScript features including async/await, there is no such signature difference any more, the start function in the observer is always an async function.

Migrating to LoopBack 4 Observer

To perform the same logic defined in a LoopBack 3 boot script, you should create a corresponding observer in your LoopBack 4 application.

LoopBack 4 has its own life cycles at runtime and has boot, start, stop as its transition states. Observer allows you to participate in the start state, which is equivalent to the LoopBack 3 custom boot script.

Taking the two scripts(server/boot/print-info.js and server/boot/create-sample-promise.js) above as examples, let’s first migrate server/boot/print-info.js to your LoopBack 4 application.

Creating Observer

Run command lb4 observer to create an observer called printInfo:

lb4 observer
? Observer name: printInfo
? Observer group:
   create src/observers/print-info.observer.ts
   update src/observers/index.ts

Observer PrintInfo was created in src/observers/

You will find a file called PrintInfo.observer.ts created in folder src/observers/. Open that file, there is a class called PrintInfoObserver, which implements LifeCycleObserver and has start and stop functions. The start function is where you should copy over the content of LB3’s printInfo function.

Injecting Application

The LoopBack 3 boot script takes in server as its first parameter, which is essentially the application injected using key CoreBindings.APPLICATION_INSTANCE:

// ... imports
export class PrintInfoObserver implements LifeCycleObserver {
  constructor(
    @inject(CoreBindings.APPLICATION_INSTANCE) private app: Application,
  ) {
    // ... other class members
  }
}

Now within the class PrintInfoObserver you have access to the application.

Migrating Artifacts

A boot script usually accesses or manipulates an application’s artifacts like models, datasources, etc… So when moving its content to the start function, make sure you edit the code to retrieve those artifacts in LoopBack 4’s way.

For the print info script, since it retrieves a datasource’s info, you need to MIGRATE the LoopBack datasource db to your LoopBack 4 application. The steps are well documented on page migration/datasources.md. And in the start function, EDIT CODE to inject the datasource using @inject('datasources.db') private ds: juggler.DataSource:

The code snippet is the complete observer file, the LB3 boot script logic is only in the start function

import {
  /* inject, Application, CoreBindings, */
  lifeCycleObserver, // The decorator
  LifeCycleObserver,
  inject,
  CoreBindings,
  Application, // The interface
} from '@loopback/core';
import {juggler} from '@loopback/repository';

/**
 * This class will be bound to the application as a `LifeCycleObserver` during
 * `boot`
 */
@lifeCycleObserver()
export class PrintInfoObserver implements LifeCycleObserver {
  constructor(
    // inject `app` if you need access to other artifacts by `await this.app.get()`
    @inject(CoreBindings.APPLICATION_INSTANCE) private app: Application,
    // inject a datasource with key `datasources.${dsName}`
    @inject('datasources.db') private ds: juggler.DataSource,
  ) {}

  /**
   * This method will be invoked when the application starts
   */
  async start(): Promise<void> {
    // Add your logic for start
    console.log('This is a migrated synchronous boot script.');
    console.log(`Your app has a datasource called: ${this.ds.name}`);
  }

  /**
   * This method will be invoked when the application stops
   */
  async stop(): Promise<void> {
    // Add your logic for stop
    console.log('print message observer has stopped.');
  }
}

For the other script server/boot/create-sample-promise.js, the creation of its corresponding observer is exactly the same as server/boot/print-info.js. To avoid duplicate content, the steps are omitted here.

The script retrieves LoopBack 3 model Todo and creates a sample through Todo.create(). While in LoopBack 4, model only describes the shape of data, and repository processes the persistence-related behavior. Therefore you need to call TodoRepository.create(sample) instead to create the sample. And to migrate the Todo model, you can follow the steps in Migrating model definitions and built-in APIs.

The migrated boot script as CreateSampleObserver’s start function:

The code snippet is the complete observer file, the LB3 boot script logic is only in the start function

import {
  /* inject, Application, CoreBindings, */
  lifeCycleObserver, // The decorator
  LifeCycleObserver,
  inject,
  CoreBindings,
  Application, // The interface
} from '@loopback/core';
import {TodoRepository} from '../repositories';

/**
 * This class will be bound to the application as a `LifeCycleObserver` during
 * `boot`
 */
@lifeCycleObserver()
export class CreateSampleObserver implements LifeCycleObserver {
  constructor(
    // inject `app` if you need access to other artifacts by `await this.app.get()`
    @inject(CoreBindings.APPLICATION_INSTANCE) private app: Application,
    // inject a repository with key `repositories.${repoName}`
    // or with the shortcut injector:
    // `@repository(TodoRepository) private todoRepo: TodoRepository`
    @inject('repositories.TodoRepository') private todoRepo: TodoRepository,
  ) {}

  /**
   * This method will be invoked when the application starts
   */
  async start(): Promise<void> {
    // Add your logic for start
    console.log('This is a migrated asynchronous boot script');

    const sample = {title: 'a todo sample', desc: 'Something to do.'};
    // Create the sample by calling TodoRepo.create()
    const result = await TodoRepo.create(sample);
    console.log('Sample created as ', result);
  }

  /**
   * This method will be invoked when the application stops
   */
  async stop(): Promise<void> {
    // Add your logic for stop
    console.log('create sample observer has stopped.');
  }
}

Setting Order for Migrated Boot Scripts

In LoopBack 3, boot scripts are loaded according to the script file’s name (alphabetically). LoopBack 4 observer provides an advanced way to control the order of scripts by specifying and setting group names. The tutorial is well documented in the section Notify life cycle observers of start/stop related events by order.

Predefined LoopBack 3 Boot Scripts

LoopBack 3 application has two predefined boot scripts: /server/boot/root.js and /server/boot/authentication.js. Please do not create corresponding observers for them.

In LoopBack 4, the router is automatically registered in the rest server and the authentication system is enabled by applying the authentication component.