docs: booting in LB3 vs LB4 by hacksparrow · Pull Request #5178 · loopbackio/loopback-next

hacksparrow · 2020-04-21T16:36:40Z

Differences in LB3 and LB4 booting. Addresses #4838.

raymondfeng · 2020-04-22T16:40:59Z

+### In LoopBack 4
+
+In LoopBack 4, the booting tasks are shared between the
+[RestApplication](./apidocs/apidocs.rest.restapplication.html) class


What part of booting does RestApplication handle?

We should mention it's possible to plug in new types of booters.

Can we have link to https://loopback.io/doc/en/lb4/Booting-an-Application.html?

What part of booting does RestApplication handle?

The config part.

I don't think RestApplication is handling the config part, isn't this handled by Application from @loopback/core? AFAIK, RestApplication is a thin wrapper around Application that's adding a single REST server instance and convenience methods for accessing that server.

Let's build a better understanding of what you are trying to describe, @hacksparrow, and how is that actually implemented.

So refer to Application of @loopback/core instead? We need to tell where the app configuration is applied as part of the booting process.

@raymondfeng I have updated accordingly.

bajtos

Great start!

bajtos · 2020-04-23T11:26:39Z

+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.


FYI: In the first step, we define all models. In the second step, we attach the models to datasources and the application.

The list of models defined may be longer than the list of models attached:

loopback-boot looks at server/model-config to see which files should be added to the app.

Then it examines inheritance and mixin configurations, to determine dependencies (which models to add to the list)

Models are sorted topologically based on inheritance, so that we define parent models before children.

All models from the list above are defined via model registry.

Only models specified in server/model-config are added to the app and attached to a datasource (unless dataSource flag is set to null).

loopback-boot also supports universal mode, where the entire loopback application is bundled for browser. In that case, the boot process is split in two parts:

The first step finds out which models (and their parents, etc.) are needed by the app. It creates "instructions" for booting an app, these instructions are compiled into the browser bundle together with model source files (common/models/{model-name}.json).

The second step is performed in the browser, it processes the instructions from the first step.

Maybe not worth getting into all those details for something that we are moving away from. Especially since the corresponding step in LB4 is an import.

bajtos · 2020-04-23T11:28:15Z

+### In LoopBack 4
+
+In LoopBack 4, the booting tasks are shared between the
+[RestApplication](./apidocs/apidocs.rest.restapplication.html) class


I don't think RestApplication is handling the config part, isn't this handled by Application from @loopback/core? AFAIK, RestApplication is a thin wrapper around Application that's adding a single REST server instance and convenience methods for accessing that server.

Let's build a better understanding of what you are trying to describe, @hacksparrow, and how is that actually implemented.

bajtos · 2020-04-23T11:29:18Z

+
+There is no `config.json` in LoopBack 4. The application options are passed in
+the constructor of the
+[RestApplication](./apidocs/apidocs.rest.restapplication.html) class.


Using RestApplication is not mandatory, people can use the base Application class too. This will become more and more important as we add support for different API flavors (gRPC, event-stream consumers, etc.)

bajtos · 2020-04-23T11:29:43Z

+Datasources in LoopBack 3 are defined in a single `datasources.json` file. In
+LoopBack 4, datasources are defined as TypeScript classes that come with an
+associated JSON configuration file. They all reside in the `src/datasources`
+directory.


Please note that we are moving away from JSON config files for datasources, see #5000

bajtos · 2020-04-23T11:30:50Z

+associated JSON configuration file. They all reside in the `src/datasources`
+directory.
+
+`BootMixin` resolves the datasources, and attaches them to the application.


Do we have any documentation on how is the datasource booter working under the hood? For example, what files it looks for, how it decided which exported member of a datasource TS file it will add to the app, etc. If yes, then please add a link to it.

We don't have anything in-depth, AFAIK. The closest is https://loopback.io/doc/en/lb4/Booting-an-Application.html#controller-booter, added the link.

bajtos · 2020-04-23T11:31:45Z

+### 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.


Can you please briefly explain why we are not booting models?

The main reason is that we need Models at compile time, therefore they must be imported directly.

bajtos · 2020-04-23T11:33:07Z

+process in LoopBack 4; Controllers and Repositories are.
+
+Controllers and Repositories import models as regular TypeScript classes.
+


Can you please briefly explain why we are not booting models?

The main reason is that we need Models at compile time, because as you have correctly pointed out, controllers and repositories are importing them directly.

jannyHou · 2020-04-24T14:46:30Z

+[loopback-boot](https://github.com/strongloop/loopback-boot),
+which performs the following tasks:
+
+### In LoopBack 3


I feel this paragraph

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

should be under ### In LoopBack 3, instead of before it.

jannyHou

Good content 👍 lgtm, a few nitpicks

jannyHou · 2020-04-24T14:59:01Z

+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 as not


Suggested change

apparent to you, and understanding of how they work together as one as not

apparent to you, and understanding of how they work together as one is not

jannyHou · 2020-04-24T15:02:24Z

+
+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.


it'd be better to include a sample code, or the link https://loopback.io/doc/en/lb4/Application.html#configuring-your-application

bajtos

Thank you @hacksparrow for the updates.

Please make sure all relative links are still working after you moved the file to docs/site/migration.

Also #5000 has been landed earlier today, please update the text accordingly.

bajtos · 2020-04-24T15:48:39Z

+{% include note.html content="We are moving away from JSON files for datasource
+configuration. Refer to
+[loopback-next#5000](https://github.com/strongloop/loopback-next/pull/5000)
+for the progress." %}


The PR has been already landed, please update this section to reflect the current master.

bajtos · 2020-04-24T15:49:35Z

+
+`BootMixin` resolves the datasources, and attaches them to the application.
+Read more about datasource booter
+[here](./Booting-an-Application.md#controller-booter).


Did you update all links after moving the file to docs/site/migration? I don't think this link is valid now.

bajtos · 2020-04-24T15:49:57Z

+[here](./Booting-an-Application.md#controller-booter).
+
+Read more about LoopBack 4 datasource [here](./DataSources.md).
+


Should we add a link to https://loopback.io/doc/en/lb4/migration-datasources.html too?

bajtos · 2020-04-24T15:50:16Z

+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." %}
+


Should we add a link to https://loopback.io/doc/en/lb4/migration-models-overview.html?

nabdelgadir · 2020-04-26T04:25:53Z

+refer to the
+[booting documentation](./Booting-an-Application.md)." %}
+
+### 1. Configuration of application settings


Should this and the two below be fourth-level headings?

Indeed. Thanks.

nabdelgadir · 2020-04-26T04:26:42Z

+Read more about datasource booter
+[here](./Booting-an-Application.md#controller-booter).
+
+Read more about LoopBack 4 datasource [here](./DataSources.md).


Nitpick: datasource -> datasources

nabdelgadir · 2020-04-26T04:29:23Z

+
+If you want to mount an Express router in a LoopBack 4 application, you can use
+the
+[RestApplication.mountExpressRouter()](./apidocs/apidocs.rest.restapplication.mountexpressrouter.md)


Can we link to https://loopback.io/doc/en/lb4/Routes.html#mounting-an-express-router instead?

bajtos

I am not convinced that the new links work (e.g. I don't see any file called migration-datasources.md in our repository). But as long as you have run the preview of the doc site and verified that all links work, the I am fine.

hacksparrow · 2020-04-27T13:34:28Z

Thanks @bajtos, fixed them.

hacksparrow · 2020-04-27T13:39:36Z

@nabdelgadir I have applied your feedback. Can you PTAL?

Differences in LB3 and LB4 booting.

hacksparrow force-pushed the docs/lb3-vs-lb4-boot branch 2 times, most recently from 9bc3e7e to 3d518e2 Compare April 22, 2020 16:30

hacksparrow marked this pull request as ready for review April 22, 2020 16:32

hacksparrow requested review from agnes512, bajtos, deepakrkris, emonddr, jannyHou, nabdelgadir and raymondfeng as code owners April 22, 2020 16:32

raymondfeng reviewed Apr 22, 2020

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Comment thread docs/site/LB3-vs-LB4-booting.md Outdated

bajtos suggested changes Apr 23, 2020

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hacksparrow force-pushed the docs/lb3-vs-lb4-boot branch 3 times, most recently from aa01187 to bde8541 Compare April 24, 2020 14:48

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docs: booting in LB3 vs LB4

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Differences in LB3 and LB4 booting.

hacksparrow force-pushed the docs/lb3-vs-lb4-boot branch from b35d76b to 92c7347 Compare April 27, 2020 15:24

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hacksparrow merged commit f3bebe8 into master Apr 27, 2020

hacksparrow deleted the docs/lb3-vs-lb4-boot branch April 27, 2020 16:52

hacksparrow mentioned this pull request Apr 27, 2020

Migration Guide: Boot up #4838

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		process in LoopBack 4; Controllers and Repositories are.

		Controllers and Repositories import models as regular TypeScript classes.

	apparent to you, and understanding of how they work together as one as not
	apparent to you, and understanding of how they work together as one is not

		[here](./Booting-an-Application.md#controller-booter).

		Read more about LoopBack 4 datasource [here](./DataSources.md).

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hacksparrow commented Apr 21, 2020

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