Manager should expose http.Handlers to allow extension by users

[JoelSpeed]

Based on a conversation we had on the community call on the 18th June 2020:

There are several http.Handlers set up by the manager that are currently only accessible if served by the manager (metrics and webhooks). If the manager exposed these handlers once constructed this would allow extension by authors to customise how these endpoints are served.

For example, an author could use their own server and serve TLS connections directly instead of using the HTTP only server that is currently implemented with controller-runtime

Or the user could add some HTTP middleware to ensure that all requests to the endpoints are logged in a particular way or authenticated and authorized before serving the content

[vincepri]

/milestone Next
/help
/kind design
/priority important-longterm

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/milestone Next
/help
/kind design
/priority important-longterm

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/lifecycle stale

[JoelSpeed]

/remove-lifecycle stale

I still think we should do this at some point

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/lifecycle rotten

[JoelSpeed]

/remove-lifecycle rotten

Still think this would be useful longer term

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/lifecycle stale

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/lifecycle rotten

[JoelSpeed]

/remove-lifecycle rotten

Still think this is useful, this came up recently in a Cluster API conversation about Kube RBAC proxy (which is where this originally came from as well)

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/lifecycle stale

[palnabarun]

/remove-lifecycle stale

[palnabarun]

/lifecycle frozen
(until someone takes this up)

[umangachapagain]

/assign

[alexanderstephan]

@umangachapagain Are you still working on this? Otherwise, I could also work on this.

[alexanderstephan]

/assign

[alexanderstephan]

I see two possible approaches for the API of this.

First, my current attempt, which is exposing/exporting the http.Handlers and do not let the manager serve individual handlers / endpoints by having dedicated flags for each.

Besides, there could be a complete refactoring which decouples the handlers in exported, stand-alone functions. Which would result in bigger, more involved code-changes, but might be more straight forward to use since no flags are necessary.

Any ideas / preferences for this?

[VirrageS]

But I think exposing/exporting http.Handlers is not enough. User might want to also change some configuration of the http.Server (for example to serve TLS traffic instead of just HTTP). It feels like we should be able to have some kind of callback to allow changing configuration for both http.Server and http.Handlers before the server starts.