Open Stack

Prior to the introduction of local.conf, the only way to configure
OpenStack components was to introduce code directly into DevStack, so
that DevStack would pick it up then inject it into the configuration
file.

This was because DevStack writes out new configuration files on each
run, so it wasn't possible for you to make changes to any configuration
file (nova.conf, neutron.conf, ml2_plugin.ini, etc..).

So, someone who wanted to set the Linux Bridge Agent's
physical_interface_mappings setting for Neutron would have to use
$LB_INTERFACE_MAPPINGS in DevStack, which would then be invoked by
DevStack[1].

The local.conf functionality was introduced quite a while back, and
I think it's time to have a conversation about why we should start
moving away from the previous practice of declaring variables in
DevStack, and then having them injected into the configuration files.

The biggest issue is: There is a disconnect between the developers
using DevStack and someone who is an operator or who has been editing
OpenStack conf files directly. So, for example I can tell you all about
how DevStack has a bunch of variables for configuring Neutron (which is
Not a Good Thing™), and how those go into DevStack and then end up coming
out the other side in a Neutron configuration file.

Really, I would like to get rid of the intermediate layer (DevStack)
and get both Devs and Deployers to be able to just say: Here's my
neutron.conf - let's diff mine and yours and see what we need to sync.

Matt Kassawara and I have had this issue, since he's coming from the
OSAD side, and I'm coming from the DevStack side. We both know what the
Neutron configuration should end up as, but DevStack having its own set
of variables and how those variables are handled and eventually rendered
as a Neutron config file makes things more difficult than it needs to
be, since Matt has to now go and learn about how DevStack handles all
these Neutron specific variables.

The Neutron refactor[2] that I am working on, I am trying to configure
as little as possible in DevStack. Neutron should be able to, out of the
box, Just Work™. If it can't, then that needs to be fixed in Neutron.

Secondly, the Neutron refactor will be getting rid of all the things
like $LB_INTERFACE_MAPPINGS - I would *much* prefer that someone using
DevStack actually set the apropriate line in their local.conf

Such as:

    [[post-config|/$Q_PLUGIN_CONF_FILE]]
    [linux_bridge]
    physical_interface_mappings = foo:bar


The advantage of this is, when someone is working with DevStack, the
things they are configuring are the same as all the other OpenStack documentation.

For example, someone could read the Networking Guide, read the example
configuration[3] and the only thing they'd need to learn is our syntax
for specifying what file the contents go in (the "[[post-config|/$Q_PLUGIN_CONF_FILE]]" piece).

Thoughts?

[1]: https://github.com/openstack-dev/devstack/blob/1195a5b7394fc5b7a1cb1415978e9997701f5af1/lib/neutron_plugins/linuxbridge_agent#L63

[2]: https://review.openstack.org/168438

[3]: http://docs.openstack.org/liberty/networking-guide/scenario-classic-lb.html#example-configuration

-- 
Sean M. Collins