Revert "Make puma default webserver (#481)"

[Samze]

This reverts commit 781690e.

---

While we would like to see Puma be made the default web server, removing this property with no notice is problematic for us as we have non cf-d deployments that consume this property explicitly. Instead, perhaps for now these property defaults could be changed to true for a period of releases before being removed in its entirety.

We'd also like to workout a more formal deprecation policy for properties. Additionally, going forward, I think this pattern of including experimental in the name of the property only causes problems when we have to eventually remove them and replace them with something else. Perhaps we could have just had enable_puma as a property with a textual description stating it was experimental and then eventually remove that phrasing. We can discuss this on the WG call.

@Samze and @sethboyles

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[Samze]

cc @svkrieger / @jochenehret

[Gerg]

I think it's also useful to have the property around for at least a few releases as an "escape valve" in case operators run into issues after upgrading to capi-release versions that default to Puma.

[philippthun]

My proposal for going forward:

1. We keep cc.experimental.use_puma_webserver, but
   (a) move it to a newly introduced "deprecated" section in the spec file.
   (b) write about this deprecation in the release notes of the next CAPI release.
   (c) remove it in 6 months (we don't have the notion of major releases for CAPI, so this is the best I could think of).
   (d) remove the default value.

2. We introduce cc.temporary_enable_deprecated_thin_webserver with default false.

The coding to determine which webserver to use would look like this:

if_p('cc.experimental.use_puma_webserver') do |prop|
 webserver = prop ? 'puma' : 'thin'
else
 webserver = p('cc.temporary_enable_deprecated_thin_webserver') ? 'thin' : 'puma'
end

A table of config combinations showing that the deprecated config option takes precedence:

cc.temporary_enable_deprecated_thin_webserver	cc.experimental.use_puma_webserver	webserver
false	(unset)	puma
false	false	thin
false	true	puma
true	(unset)	thin
true	false	thin
true	true	puma

So we change the default webserver from thin to puma, but we allow operators to either use the old or the new config option to switch back to thin.

[svkrieger]

Philipp's suggestion sounds reasonable to me. No config change needed for your deployments for now and we only have to remove cc.experimental.use_puma_webserver in the future to clean up the possible configurations.

I'll merge the revert PR for now. Let me know what ya'll think, then I'll incorporate the changes.

[stephanme]

+1 for Philipps proposal.

I put the topic "Deprecation policy for Bosh configuration properties" on the agenda of the next CAPI Open Office Hour.
Maybe we find a simpler solution to introduce new experimental features, make it default and eventually remove the old solution.