proxy: Rewrite the proxy middleware

[mholt]

The proxy middleware needs to be rewritten, building upon the fabulous work already put in by contributors to date. These plans are tentative and subject to change.

It will be rewritten from scratch, but with very few (if any) Caddyfile syntax changes (hopefully). This is a "guided rewrite", meaning that the current proxy code (which is very intricate) will be used as a reference for the rewrite. My hope is that we can accomplish these goals to make Caddy into a more competitive, useful reverse proxy:

• Continue supporting "static" backends (which is the only kind of backend currently supported)
• Add support for "dynamic" backends (backends based on parameters of the request on-the-fly or based on external environment, see Dynamic proxy #564, [Feature] Service Discovery using Consul #1630 (?), Regexp in proxy,and placeholder in proxy from,to #1894, Support to reverse proxy into Docker Containers #199, Proxy upstream depending on hostname #990, and its related PR Allow {host} in proxy upstream #998)
• Make the list of supported protocols extensible by plugins (fastcgi, uwsgi, etc.) -- all protocols share same core proxy code, but each can do the RoundTrip their own way. This could even work for the websocket directive, potentially -- note that this is different from proxy's websocket preset! (proxy: ability to RegisterProtocol #1638)
• Potentially lay the ground work for DNS-based load balancing (?) proxy: Load Balance using multiple A records per single hostname #1545
• Ability to proxy to error pages (Possibility to proxy to error page #753)
• Ability to have the errors directive handle errors from upstream (rather than only errors connecting to upstream)
• Potentially custom health checks (Custom proxy upstream health check #324)
• Straighten out the retry logic (proxy: Improve failover logic and retries #1135, also see proxy: Unbuffered request optimization #1314 and conditionally disable response buffering #1460) - ideally we never buffer the response unless explicitly configured/required to do so
• Cf. proxying http2 explicitly  #1772 and Implement the equivalent of nginx proxy_redirect directive #606

This may take a few weeks once work commences.

/cc @abiosoft @nemosupremo - I will probably have questions as I go through this, so expect to hear from me. :)

[ghost]

would this help achieve #1460 too?

[mholt]

@jrobeson Quite likely, yes. If I can do it right.

[Stonedestroyer]

Would this be implemented as well? #606 considering it pertains to the proxy plugin.

[mholt]

@Stonedestroyer Yes, I think it definitely could be.

[lvhuat]

so，what the issue going on now？

[yura8]

This would help #1891 too.

[mholt]

I won't have time to work on this for the coming year or so. If anyone is skilled in writing proxies and would like to take the lead at rewriting it, please chime in!

[mholt]

This rewrite is done in v2. It's not feature-complete but should be able to support all the features that have been requested, one way or another. (Contributions welcomed!)

The feature where a request is proxied to some arbitrary backends seems more suitable for a forward proxy than a reverse proxy. I'm not sure how that would work at all with a fixed number of backends.

[mholt]

Caddy 2 now allows a dynamic address for upstreams, as implemented in 1e31be8.

I think that pretty much finishes up this rewrite. There's still one matter related to error handling that I want to attend to but that's another open issue (#1447).

Go try it out!