Using Openstack scheduler hints with juju

[William Reade]

On Fri, 2013-02-01 at 15:23 +0100, William Reade wrote:

For the purposes of argument, how well would something like the following work to fulfil the goals that led to the os-scheduler-hints work?

>   * Units of service X should be deployed to instances running on
>     different physical machines ("I want my mongos to be separated").

juju set-constraints -s mongo avoid-services=mongo

>   * Units of service X should be deployed to instances that are
>     running on the same physical machine as some unit of service Y
>     ("I want rabbitmq near mysql").

juju set-constraints -s rabbitmq seek-services=mysql
juju set-constraints -s mysql seek-services=rabbitmq

These constraints have an obvious application with respect to
colocation; but they expose the user's desires in such a way that a
sufficiently smart provider with appropriate support could use them to
ensure that (for example) new units of rabbitmq are at least placed in
the same zone/cell as -- and ideally on the same host as -- some
existing unit of mysql.

I'm also somewhat keen on the idea that the ec2-zone constraint (which
was always kinda crackful) could be dropped and replaced with seek/avoid
handling that used in-same-availability-zone as its distance metric...
so, while this is not a proposal as such, it does reflect my current
thoughts on the matter; I would appreciate criticism.

Cheers
William