Page Contents

Differences between LoopBack 3 and LoopBack 4 booting process

Because of the architectural differences, the booting process is very different in LoopBack 3 and LoopBack 4.

In LoopBack 3

The booting process in LoopBack 3 is handled by loopback-boot, which performs the following tasks:

1. Configuration of application settings

It determines the execution environment, the application host, the application port, the application root directory and other artifact directories.

Then it configures the LoopBack Application object from the config.json file in the application root directory.

2. Configuration of datasources

Datasources are determined from application options or datasource files. They are then attached to the LoopBack Application object.

3. Definition of models

Models are determined from application options or model files, mixins are applied, and they are attached to their respective datasources. They are then attached to the LoopBack Application object.

4. Setting up middleware

Middleware from the middleware configuration file are resolved and added to the LoopBack 3 middleware chain.

5. Setting up components

Components from the component configuration file are resolved and are added to the LoopBack 3 middleware chain.

6. Running boot scripts

Boot scripts are resolved from the server/boot directory and run.

In LoopBack 4

In LoopBack 4, the booting tasks are shared between the @loopback/core’s Application class and the @loopback/boot package. Since your application will extend BootMixin and Application, their separation will not be apparent to you, and understanding of how they work together as one is not mandatory for developing LoopBack 4 projects.

The following are the list of tasks that are performed in the LoopBack 3 booting process, and their equivalent in LoopBack 4.

1. Configuration of application settings

There is no config.json in LoopBack 4. The application options are passed in the constructor of the Application class. Read more about application configuration here.

2. Configuration of datasources

Datasources in LoopBack 3 are defined in a single datasources.json file. In LoopBack 4, datasources are defined as TypeScript classes. They all reside in the src/datasources directory.

BootMixin resolves the datasources, and attaches them to the application. Read more about datasource booter here.

Read more about LoopBack 4 datasources here. Read more about migrating datasources here.

3. Definition of models

Although REST APIs are built around Models, they are not a part of the booting process in LoopBack 4; Controllers and Repositories are.

Controllers and Repositories import models as regular TypeScript classes since they are required at compile time.

Read more about migrating models here.

4. Setting up middleware

Middleware are not supported at LoopBack level yet. However, you can mount a LoopBack 4 application on an Express application, which would allow you to use the familiar Express routing methods.

If you want to mount an Express router in a LoopBack 4 application, you can use the RestApplication.mountExpressRouter() API.

5. Setting up components

In LoopBack 3, a component is a simple Node.js module which exports a function with the signature function(app, options). In LoopBack 4, a component is a TypeScript class which can add servers, observers, providers, and controllers to the application using dependency injection.

LoopBack 3 components adding routes can be migrated to LoopBack 4 by moving the functionality to the controller of a LoopBack 4 component.

6. Running boot scripts

If you used LoopBack 3 boot scripts for adding routes to the application, it should now be moved to a standalone controller, a component, or implemented using app.mountExpressRouter().

The functionality of non-routing boot scripts can be implemented as life cycle observers. Read more about life cycle events and observers here.

For more information about migrating boot scripts in LoopBack 4, refer to the boot scripts migration guide.