@loopback/docs

--- lang: en title: 'Differences in LoopBack 3 and LoopBack 4 booting process' keywords: LoopBack 4.0, LoopBack 4, Node.js, TypeScript, OpenAPI, LoopBack 3.0, LoopBack 3 sidebar: lb4_sidebar permalink: /doc/en/lb4/LB3-vs-LB4-booting.html --- ## 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](https://github.com/loopbackio/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](https://loopback.io/doc/en/lb3/Defining-mixins.html) 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](../apidocs/apidocs.core.application.md) class and the [@loopback/boot](../apidocs/apidocs.boot.html) 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. {% include tip.html content="For all the details about booting in LoopBack 4, refer to the [booting documentation](../Booting-an-Application.md)." %} #### 1. Configuration of application settings There is no `config.json` in LoopBack 4. The application options are passed in the constructor of the [Application](../apidocs/apidocs.core.application.md) class. Read more about application configuration [here](../Application.md#configuring-your-application). #### 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. {% include note.html content="Before [loopback-next#5000](https://github.com/loopbackio/loopback-next/pull/5000) landed, datasource classes required a configuration JSON file, it is not required anymore." %} `BootMixin` resolves the datasources, and attaches them to the application. Read more about datasource booter [here](../Booting-an-Application.md#controller-booter). Read more about LoopBack 4 datasources [here](../DataSource.md). Read more about migrating datasources [here](./datasources.md). #### 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. {% include note.html content="Booters are LoopBack 4 artifacts that are used for modularly adding functionality to the framework. Controllers, Repositories, Datasources, Interceptors, etc., are added using booters, custom booters can be written for adding new functionality to the framework." %} Read more about migrating models [here](./models/overview.md). #### 4. Setting up middleware Middleware are not supported at LoopBack level yet. However, you can [mount a LoopBack 4 application](../express-with-lb4-rest-tutorial.md) on an Express application, which would allow you to use the familiar Express routing methods. {% include tip.html content="Follow our GitHub issues [#1293](https://github.com/loopbackio/loopback-next/issues/1293) and [#2035](https://github.com/loopbackio/loopback-next/issues/2035) to track the progress on supporting Express middlweware in LoopBack 4." %} If you want to mount an Express router in a LoopBack 4 application, you can use the [RestApplication.mountExpressRouter()](../Routes.html#mounting-an-express-router) API. #### 5. Setting up components In LoopBack 3, a [component](https://loopback.io/doc/en/lb3/LoopBack-components.html) is a simple Node.js module which exports a function with the signature `function(app, options)`. In LoopBack 4, a [component](../Creating-components.md) is a TypeScript class which can add [servers](../Server.md), [observers](../Life-cycle.md), [providers](../Creating-components.md#providers), and [controllers](../Controller.md) 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](../Life-cycle.md). For more information about migrating boot scripts in LoopBack 4, refer to the [boot scripts migration guide](./boot-scripts.md).