docs: booting in LB3 vs LB4

docs/site/migration/LB3-vs-LB4-booting.md

@@ -0,0 +1,153 @@
+---
+lang: en
+title: 'Differences in LoopBack 3 and LoopBack 4 booting process'
+keywords: LoopBack 4.0, LoopBack 4, 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/strongloop/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/strongloop/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](../DataSources.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/strongloop/loopback-next/issues/1293)
+and
+[#2035](https://github.com/strongloop/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](../Controllers.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).

docs/site/sidebars/lb4_sidebar.yml

@@ -797,6 +797,10 @@ children:
     url: Understanding-the-differences.html
     output: 'web, pdf'
 
+  - title: 'Booting process'
+    url: LB3-vs-LB4-booting.html
+    output: 'web, pdf'
+
   - title: 'Request/response cycle'
     url: LB3-vs-LB4-request-response-cycle.html
     output: 'web, pdf'