reverseproxy: allow upstream to have a name or route

[ottenhoff]

Currently, a reverse proxy config looks like this:

reverse_proxy 
  to https://server101.local:8443 https://server102.local:8443
}

It would be nice if the upstreams had a name like server101 and server102. A few reasons:

a) Land on a specific upstream by adding GET param lb=server101
b) Land on a specific upstream by setting header lb: server101
c) Land on a specific upstream by parsing cookie, e.g., JSESSIONID=uuid.server101

Here is a sample config from another load-balancer that shows the idea:

upstream backend {
    server backend1.example.com route=a;
    server backend2.example.com route=b;
    sticky route $route_cookie $route_uri;
}

[francislavoie]

Prior art: #4350

[mholt]

I would suggest using a map directive in conjunction with a placeholder for the upstream address; that way you can map an arbitrary string to a backend address, then use that. 👍

And if you need to load balance to two backends you can map to multiple placeholders and then specify both in the reverse proxy config.

[ottenhoff]

Ultimately, the goal here for me is to support a new lb_policy that routes based on the application's cookie instead of Caddy's inserted cookie.

Tomcat is a popular Java application server and often sends cookies like JSESSIONID=xxxxx-yyyyy-zzzzzzz.appserver1094. It would be easier to write a new lb_policy if there was a name for each upstream. I suppose I could just check for the presence of a map in a custom lb_policy, but it seemed like a common enough use case supported by other loadbalancers that I should write it up.

[mholt]

Can you just use the map directive with the {cookie.JSESSIONID} placeholder? Then you could set an upstream that way. I guess it's not clear to me why that isn't a sufficient solution.