I want to change the http interface

David Cheney david.cheney at canonical.com
Thu Oct 10 04:31:49 UTC 2013

Right, now I have your attention, let me explain my problem.

I've been assigned to improve the HAProxy charm and the specific issue
I have is the current implementation will place any http requires
relation into the same configuration block [1]. The short summary of
this bug is, if I do this

   juju deploy wordpress w1
   juju deploy wordpress w2
   juju deploy haproxy p1
   juju add-relation w1 p1
   juju add-relation w2 p1

then w1 and w2 will be randomly chosen as backends for any request
hitting p1 even though they represent separate services.

Now, there is no reason that the HAProxy charm has to do this, it's
probably just a combination that

1. there is no limit setting on the relation, which for a requires
relation implies a limit of 1
2. juju doesn't respect that limit

So, right now, whether we like it or not, it is completely valid, from
the POV of some using a charm to relate HA proxy two or more services
providing the http interface and to expect it to work (for undefined
values of work)

Ok, so why doesn't it just work ? HAProxy supports vhosts, and we can
put each http relation into it's own vhost.

Well, while that is true, the http interface as i *believe* it is
specified today, only includes these two atoms, hostname, and port.
This is not sufficient to reverse proxy more than one service as
individual vhosts inside a single HAProxy charm.

At a minimum, to solve my problem, I would need the http interface to
define two more atoms, vhost_name (or something equivalent) and

vhost_name is self explanatory, but cannot be inferred from the relation name
proxy_path is also important as you may use HAProxy to combine several
different http providers each of who manage a different path prefix of
a single vhost.

Thoughts ? And while we're at it, why isn't there a schema for
interfaces as there is for the configuration of a provider ?


[1] There are also other problems with this charm, but I will save
them for later novella.

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