reverseproxy: Implement admin API for reporting upstream statuses

[francislavoie]

As discussed in https://caddy.community/t/cli-to-query-for-upstream-server-status/12210

$ curl localhost:2019/reverse_proxy/upstreams
[
  {"address": "localhost:10002", "healthy": true, "num_requests": 0, "fails": 0},
  {"address": "localhost:10000", "healthy": true, "num_requests": 0, "fails": 0},
  {"address": "localhost:10001", "healthy": true, "num_requests": 0, "fails": 0}
]

Tested with a Caddyfile like this:

{
        auto_https off
}

:10000 {
        respond "10000"
}

:10001 {
        respond "10001"
}

:10002 {
        respond "10002"
}

:9999 {
        reverse_proxy localhost:10000 localhost:10001 localhost:10002
}

[francislavoie]

Something funky's going on with github actions... the tests pass but the jobs hard-fail with no results sometimes. I'm very confused. https://www.githubstatus.com/ seems to say everything is fine right now though.

Edit: Re-ran a couple times, finished fine this time 🤷‍♂️

[mholt]

Cool, this is basically exactly what I had in mind. I'll spare a few little nits and just go with it. Thanks! This should be pretty useful.

[ttys3]

I see this not work.

The num_requests always 0, and the healthy is always true even it is not healthy.

about the num_requests, I saw the code:

caddy/modules/caddyhttp/reverseproxy/reverseproxy.go

Line 565 in a779e1b

defer di.Upstream.Host.CountRequest(-1)

don't understand if we need the num_requests, then why decrease it in defer ?

@francislavoie

---

Sorry for the distrub, I've read the document explanation carefully and now see it.
I was wrong, I assume that it means total sum without reading the doc.

But, anyway, the num_requests field is confusing,
I think maybe name it to active_requests better.

[francislavoie]

@ttys3 it depends on how you have your health checking configured. Please ask on the forums! https://caddy.community. Also see the docs, which have an explanation of how it works https://caddyserver.com/docs/api#get-reverse-proxyupstreams

[ttys3]

Sorry for the distrub, I've read the document explanation carefully and now see it.
I was wrong, I assume that it means total sum without reading the doc.

But, anyway, the num_requests field is confusing,
I think maybe name it to active_requests better.

[francislavoie]

The admin API matches the field names in the internal structs. Changing it now would be a breaking change. The documentation explains it sufficiently IMO, so no need to change it.

[ttys3]

Ok. got it.