How to build a new API transport extension (or migrate a LB3 one)

This story is extracted from #4099, where we researched existing LoopBack 3 components and various techniques they use.

Now we need to write documentation for extension authors to show how to solve the following use cases in LoopBack 4:

1. A new server type to handle requests coming from a non-REST source, e.g. Primus/WebSockets and RabbitMQ.

2. WebSockets: mount WebSocket-handler on the underlying http server used by @loopback/rest.

3. Message queues: allow consumers to also produce (publish) new messages, e.g. as a response to the incoming message.

Prior work:

• https://loopback.io/doc/en/lb4/Creating-servers.html

I see the following major areas to cover (feel free to add more items to this list):

• How can a transport implementation discover endpoint handlers (controllers, routes, message consumers, etc.) registered in the application context? (Related: How can extensions introspect application artifacts #5426)
  • Also how to handle handlers added dynamically after application has started
• How should be a transport extension structured like?
  • Ensure that app.start() will start the server/subscriber, app.stop() stop it
  • How to create a per-request context and populate it with request-specific metadata

In most cases, the new content will be useful to authors building new LB4 components too, therefore we should structure the content in two parts:

• A guide explaining how to build a component contributing a service, this should go to Extending LoopBack
• A guide explaining how to migrate LB3 components, this guide should go to Migrating components and extensions and focus on aspects unique to extensions migrating from LB3 and refer to content in Extending LoopBack wherever possible & appropriate.

まず既存の Creating servers ドキュメントを読み、その後 Extending LoopBack 4 と Migrating components and extensions のガイドを確認します。コンポーネントサービス、トランスポートハンドラーの検出、動的ハンドラー、アプリケーションの起動と停止、WebSockets とメッセージキューのリクエストごとのコンテキストを扱います。両方のガイドで、一覧にあるユースケースと移行固有の考慮事項が明確に説明されていれば完了です。