default availability zone is not set based on JUJU_AVAILABILITY_ZONE

I'd suggest we maintain existing behaviour as default for now; would a toggle on nova-compute and neutron-gateway to use the Juju provided JUJU_AVAILABILITY_ZONE make sense? then a deployment can turn on this feature - I also think we should tie a unit of n-ovs to the same az as its parent n-compute otherwise people will get in a twist - so something like

   juju config nova-compute use-juju-availability-zones=True

would reconfigure nova-compute with a preset AZ and propagate the same value down to the neutron-openvswitch agents.

neutron-gateway would need the same option.

What's the earliest release we can support this back to?

FWIW I'm not super keen on a charm level config option to set the AZ - that means fragmentation of applications (i.e one nova-compute application instances per AZ) which is fiddly and sub-optimal IMHO.

I agree about adding the option.

The earliest release would be mitaka where those options were introduced to Neutron.

In most cases I expect Nova node and Neutron agent AZs to match so passing az down to agents seems reasonable.

There are probably some edge cases where we'd want to have different AZs for agents and nova nodes but that's a secondary concern.

Subscribed ~field-high as this affects an ongoing project with a requirement for HA DHCP with AZ-aware scheduling.

Reviews:
https://review.openstack.org/#/q/topic:bug/1796068+(status:open+OR+status:merged)

Reviewed: https://review.openstack.org/608109
Committed: https://git.openstack.org/cgit/openstack/charm-neutron-gateway/commit/?id=71c0120d213e2adbd42df847022d8e36b5df9417
Submitter: Zuul
Branch: master

commit 71c0120d213e2adbd42df847022d8e36b5df9417
Author: Dmitrii Shcherbakov <email address hidden>
Date: Wed Oct 3 14:36:52 2018 +0100

    Allow Juju AZ context information to be used

    The change adds an option to the charm to use JUJU_AVAILABILITY_ZONE
    environment variable set by Juju for the hook environment based on the
    underlying provider's availability zone information for a given machine.

    This information is used to configure the availability_zone setting for
    Neutron DHCP and L3 agents specifically because they support it
    and for other agents (because both neutron.conf and agent-specific
    configuration files are loaded) such as metadata agents and lbaas
    agents.

    Additionally, a setting is added to allow changing the default
    availability zone because 'nova' is a default value coming from the
    Neutron defaults for agents.

    Change-Id: I94303aa70ee3adc6ace0f9af1e7c4f5c0edbcdb5
    Closes-Bug: #1796068

Reviewed: https://review.openstack.org/608030
Committed: https://git.openstack.org/cgit/openstack/charm-nova-compute/commit/?id=4fb124310091008b3f650ac389d010d530b6831e
Submitter: Zuul
Branch: master

commit 4fb124310091008b3f650ac389d010d530b6831e
Author: Dmitrii Shcherbakov <email address hidden>
Date: Thu Oct 4 19:22:43 2018 +0300

    Allow Juju AZ context information to be used

    The change adds an option to the charm to use JUJU_AVAILABILITY_ZONE
    environment variable set by Juju for the hook environment based on the
    underlying provider's availability zone information for a given machine.

    This information is used to configure default_availability_zone for nova
    and availability_zone for subordinate networking charms.

    Change-Id: Idc7112e7fe7b76d15cf9c4896b702b8ffd8c0e8e
    Closes-Bug: #1796068

Fix proposed to branch: stable/18.08
Review: https://review.openstack.org/609034

Fix proposed to branch: stable/18.08
Review: https://review.openstack.org/609035

Change abandoned by James Page (<email address hidden>) on branch: stable/18.08
Review: https://review.openstack.org/609035
Reason: Part of latest charm release.

Change abandoned by James Page (<email address hidden>) on branch: stable/18.08
Review: https://review.openstack.org/609034
Reason: Part of latest charm release.

Marking neutron-openvswitch task as invalid as nova-compute provides the zone value over the subordinate relation.

Added neutron-api charm.

network_scheduler_driver is set to WeightScheduler, not AZAwareWeightScheduler by default in neutron.

AGENTS_SCHEDULER_OPTS = [
    cfg.StrOpt('network_scheduler_driver',
               default='neutron.scheduler.'
                       'dhcp_agent_scheduler.WeightScheduler',
               help=_('Driver to use for scheduling network to DHCP agent')),

neutron.conf needs to contain this in order for this to work:

    network_scheduler_driver = neutron.scheduler.dhcp_agent_scheduler.AZAwareWeightScheduler
    dhcp_load_type = networks

Agent load calculation can also be done based on different criteria (networks, subnets, ports):

    cfg.StrOpt('dhcp_load_type', default='networks',
               choices=['networks', 'subnets', 'ports'],

AZAwareWeightScheduler is based on WeightScheduler

https://github.com/openstack/neutron/blob/mitaka-eol/neutron/scheduler/dhcp_agent_scheduler.py#L92-L98
class AZAwareWeightScheduler(WeightScheduler):

The spec also mentions that by default all agents belong to the 'nova' AZ so the scheduler change should be backwards-compatible.
http://specs.openstack.org/openstack/neutron-specs/specs/mitaka/availability-zone.html#upgrade-impact

https://review.openstack.org/#/c/632090/

Reviewed: https://review.openstack.org/632090
Committed: https://git.openstack.org/cgit/openstack/charm-neutron-api/commit/?id=1e6430f9c6158ca048764866b9e985b5dc92d685
Submitter: Zuul
Branch: master

commit 1e6430f9c6158ca048764866b9e985b5dc92d685
Author: Dmitrii Shcherbakov <email address hidden>
Date: Mon Jan 21 14:39:41 2019 +0200

    Switch to AZAwareWeightScheduler as of Mitaka

    AZAwareWeightScheduler is based on WeightScheduler and provides a way to make
    DHCP agent scheduling be AZ-aware. This is used in conjunction with
    dhcp-agents-per-network config option and per-network agents (such as dnsmasq)
    will be distributed across neutron-dhcp-agents that have availability_zone
    configuration (based on dhcp-load-type for placement calculation).

    bp: https://blueprints.launchpad.net/neutron/+spec/add-availability-zone

    Upgrade impact is mentioned here:
    specs.openstack.org/openstack/neutron-specs/specs/mitaka/availability-zone.html

    The spec mentions that by default all agents belong to 'nova' AZ so
    the scheduler change should be backwards-compatible.

    Change-Id: I4d948efa157573fdbc0fbfd3b1efb21b69a713ef
    Closes-Bug: #1796068