[arch] Deprecate custom extension points for use in Bndtools

[kriegfrj]

For discussion:

Since experimenting with the "facade" pattern in the core Bndtools implementation, I have come across a few custom extension point definitions that are part of Bndtools. The most recent one that I discovered was for the IValidator interface. (Note: I'm not talking about custom implementations of existing extension points, but where we have both defined and used our own extension points.)

I think I have come to the conclusion that there are not any strong use cases for us to use custom extension points within Bndtools. Declarative Services can do the same thing, but they are easier to implement, maintain and deploy (particularly when Bnd is your build tool) and enable dynamic re-deployment (which isn't much help for the end user but it super-helpful for the developer). ServiceTrackers (and now even the ExtensionFacade) can be used to interface non-DS clients with DS implementations - and at the very least this is no more complicated than using the Eclipse Registry API to interface with extension implementation (and I would argue probably less complicated).

Assuming we agree to the above, then:

1. we agree not to add any new custom extension points to Bndtools;
2. we remove the existing custom extension points and replace them with DS implementations as we have the time/energy/motivation. If we feel it is necessary (as the extension points are technically part of a public API for Bndtools), we do this after formally deprecating them; Perhaps this would require a major version change to Bndtools. However, I doubt there are too many 3rd-party plugins out there that are extending Bndtools using these extension points - they are mainly for internal use (I think), so we might be safe with a minor version increment.

Thoughts?

[pkriens]

• No custom extension points – Fully agree
• Deprecating – After we've got a replacement, we can deprecate them, but we should then have at least one major release in between before removal imho. They cost little and even one annoyed customer is one too many.

[bjhargrave]

As for deprecating for removal. I am not convinced that anyone but ourselves have even implemented one of the Bndtools extension points. I am not sure we need a drawn out process to remove them. I would suggest we deprecate and cease using them immediately and state that they will be removed in the next major release.

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