Documentation: Creating and communicating with microservices.

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User Story
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There are multiple ways to set up microservices:
- Have a public facing API Gateway responsible for authentication, that then acts as a proxy for internal microservices, or which can have orchestration logic on top of their responses.
- Have many public facing services- one responsible for the process of authentication and the frontends are responsible for knowing.

There are also multiple ways to perform communication:
- Loopback 4 supports creating an OpenAPI JSON based datasource when using `lb4 service`.
- HTTP REST
- rpc, gRPC, redis/socket notifications.

There are different ways to organize the source code:
- A monorepository, using something like lerna to manage versioning and publishing.
- Separate repositories managed by different teams.

It would be great to have documentation and examples that could cover these cases. Example:
- Creating a loopback4 microservice as a REST Server that can expose an `openapi.json` file, and a loopback4 api gateway responsible for authentication and consuming those microservices.
- Creating a loopback4 microservice as a gRPC server that can publish proto files (to a private repository, or just on github for consumption), and a loopback4 api gateway responsible for authenticating and consuming those microservices.
- Workflows for maintaining multiple services with minimal manual code intervention.

Beginnen Sie mit der Durchsicht der vorhandenen LoopBack-4-Dokumentation zu `lb4 service` und OpenAPI-JSON-Datenquellen. Legen Sie den Umfang für Beispiele zu REST- und gRPC-Microservices, API-Gateways, Authentifizierung, der Organisation von Services und Workflows mit mehreren Services fest. Als abgeschlossen gilt die Aufgabe, wenn die ausgewählten Fälle vollständig dokumentiert sind und ausführbare Beispiele enthalten; das Issue nennt jedoch keine zu aktualisierenden Dateien oder Tests.

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