Document the LoopBack 4 request-response cycle

[hacksparrow]

This will be a low-level description of all the components involved in the request-response handling process. It will help consumers and developers interested in knowing what's happening under the hood.

[marioestradarosa]

@hacksparrow , I can't wait to see this documentation. I am hungry to learn.

[b-admike]

based on @raymondfeng's feedback, here are some details to consider in this narrative (opt for visualization for clarity):

• node.js http layer
• express listening on given port (restServer)
• middleware processing
• sequence actions
• default sequence
• routing (find the route, parse parameters, request body (checking OpenAPI spec), invoking controller method/handler with parameters)
• serialize response, reject if necessary
• send action
• how each request creates a controller instance

Try to group it into two phases: 1st phase interaction between API client and LoopBack, 2nd phase API business logic interact with existing backend services using repositories/datasources/models.

[dhmlau]

Closing as duplicate of #2390

[bajtos]

@dhmlau I slightly disagree. The goal of #2390 is to describe request-handling flow at high level, especially in regards to different Express middleware types. I am concerned that by including low-level details listed in #1833 (comment), we will greatly inflate the scope of #2390. Was this change taken into account when #2390 was estimated?

[dhmlau]

When discussing with @raymondfeng , he mentioned that this is a duplicate of #2390. Since #2390 has more descriptions so we're closing this issue instead.
If two are different, is there any sequence we should work on the task?

@raymondfeng , what's your thoughts?

[dhmlau]

Let's reopen it for now so that we won't forget.

[bajtos]

If two are different, is there any sequence we should work on the task?

I would start with #2390, I think it's a smaller task.

[dhmlau]

FYI - There's a diagram at the top of https://loopback.io/doc/en/lb4/Concepts.html, it would be a good starting point.

https://loopback.io/pages/en/lb4/imgs/key-concepts-overview-diagram.png

[dhmlau]

PR in progress: #4914

[dhmlau]

Closing this issue as #4914 lands