Relation's need config too

[Robert Collins]

On Thu, Jul 19, 2012 at 12:44 PM, Clint Byrum <clint at ubuntu.com> wrote:
> I"ve run into this problem a few times, and I think there's a hole in
> the current model that needs filling.
>
> Recently Juan Negron and Tom Haddon submitted a really awesome set of
> enhancements for the haproxy charm.
>
> https://code.launchpad.net/~mthaddon/charms/precise/haproxy/mini-sprint-sf/+merge/114951
>
> You can see there a lot of it was some nice abstraction for juju commands
> which I think belongs in its own package. But thats not what I'm writing
> about.
>
> Tom and Juan felt that there needed to be a way to assign units of
> a backend web service to servics, which were created by dumping raw
> haproxy service configs into a config option.
...

So one thing that may need clarifying: LP runs about 6 vhosts on a
single backend. Would what you're proposing would entail generating 6
metadata relations per backend? (launchpad.net, code.launchpad.net
etc)

The endpoint host is implied by the relation, the reason that custom
end points are needed is cluster management for LP - the topology we
run in production is this:
internet-> 2x apache (SSL unwrap), IP round robin -> 2xhaproxy using
apache LB -> (the same) 2xhaproxy using fail-over mode -> 90 or so LP
backends spread over 4 hosts.

The double bounce via haproxy is so that one haproxy has a precise
view of the work each backend is doing; IFF it goes down does the
other haproxy end up forwarding traffic to backends. This works very
well in working around python GIL limitations. [becase any (C)python
appserver isn't really threaded, no matter what the packaging on the
box says].

So I wonder if there are really three distinct factors here:
 - running multiple backends on a single node
 - determining the double-layer config for haproxy.
 - the vhosts that each backend is serving

AFAIK we don't use Host: in the haproxy or LP configs - each lp
appserver serves all the domains that all the other appservers do, so
the third point, at least for LP, is entirely moot. Thats not to say
we don't have regular vhosts, but often as not we have dedicated ips
for them too.

-Rob