Deprecate current-context API [2.x]

[bajtos]

Deprecate all current-context APIs in favour of loopback-context-cls.

This is a follow-up for strongloop/loopback-context#1

Based on our discussions, I think we should remove getCurrentContext from LoopBack 3.0. In order to simplify upgrade path and allow community to take over maintenance of CLS-based current context, I want to follow the same process we did with loopback-boot and that is to extract the functionality into a standalone module that can be versioned independently.

@raymondfeng @ritch please review
cc @strongloop/loopback-dev FYI

[richardpringle]

Would this somehow make it any easier to solve the following issue without a hack?

#1495

[bajtos]

Probably not. My intention here is to allow us to back out of getCurrentContext and stop trying to fix it, while not breaking all apps that may be depending on this feature. I think getCurrentContext may work in certain cases, so why to break them if there is a reasonably easy workaround (proposed here).

[raymondfeng]

This message is a bit confusing. Maybe we should say something like:

The current implementation is based on loopback-context-cls and it has some limitations.

[bajtos]

I am fine to improve the message, as long as it remains actionable. I.e. it should be clear to the user how to get rid of that deprecation warning.

IMO, your proposal @raymondfeng, describes the state but does not provide any clue to the user about how to proceed.

Which part of the message do you find confusing?

[ritch]

Can we link to a github issue or wiki (or other doc) so that we can keep the info up to date?

A deprecation warning will be sufficient to direct the person to a doc with some concrete info. We can also keep this up to date as we look into this further.

[raymondfeng]

The confusing part of As a short-term fix, use loopback-context-cls module. is that it seems to instruct a user to use loopback-context-cls, which is already being used.

[bajtos]

I have reworded deprecation warnings to point to our documentation page: https://docs.strongloop.com/display/APIC/Using%20current%20context

[bajtos]

@raymondfeng @ritch PTAL again, LGTY now?

[raymondfeng]

LGTM

[bajtos]

I am waiting for https://github.com/strongloop/loopback-context/pull/3/files before landing this PR.