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Group and host variables overriding in OpenStack-Ansible

group-overrides

You probably know it already, Ansible has different means of overriding a variable, and also different levels of precedence. OpenStack-Ansible is no different, and ships with different levels itself. I'll talk about the group and host variables here.

If you manage your own Ansible configurations, you probably have roles (inside their defaults and vars folders) variables, playbook variables, and you probably tie all of these up with group and host vars. You probably have noticed that these variables are often modified, and are the core of your Ansible installation.

Editing those is something highly inconvenient for an Ansible based project that ships its own group/host variables: it means your users have to edit the code of your project (for example, OpenStack-Ansible), to match their needs.

We still have ways to override group and host vars in OpenStack-Ansible, but it depends on what you need, and which branch you are on. Let's talk about that.

For Pike

During Pike cycle, we moved the group variables into a different folder. This allowed us to use our variables plugin, loading group and host variables from different folders, with the precedence completely up to the deployer (wiring can be seen here, if you want to replicate this in your own environment)

This series of commits allows you to override any group or host variable, surgically. See also our upstream documentation.

As usual, you could still use OpenStack-Ansible user variables, if you want to set a variable for all your nodes. This has the highest level of precedence, because it passes the user variable file in the CLI, like you would if you were using ansible-playbook <...> -e @filename.yml.

For Newton, and Ocata

There was no consensus (yet) on whether or not OpenStack-Ansible community should allow this backport of the variables into the different folder. Newton and Ocata being stable branches, it's unlikely that you'll be able to override OpenStack-Ansible variables surgically, like you would in Pike.

The variable plugin is still loaded, which means you can still have extra group variables and extra host variables, if need be. It's a convenient way to add new variables, properly scoped, but that won't help you if you want to override, for example lxc_container_config_list, for one or a group of hosts.

Let's see what is possible.

The lxc_container_config_list example

For this example, we want to override lxc_container_config_list for cinder_api nodes.

The task creating the container and using this list uses two variables you can technically override: lxc_container_default_config_list and lxc_container_config_list.

The simplest case

If you want to keep the lxc_container_config_list (the existing group variable override), and add something on top, you could simply redefine your lxc_container_default_config_list into your /etc/openstack_deploy/group_vars/cinder_api.yml (please double check that the vars plugin is loaded for you, by having a look at your in your /usr/local/bin/openstack-ansible.rc).

Overriding the group variable, elegantly.

Overriding the lxc_container_config_list would be different, because the precedence would force you to use our user variables.

You could add into your own user variable file (let's say /etc/openstack_deploy/user_cinder.yml):

lxc_container_config_list: "{{ properties.lxc_container_config | default(['lxc.aa_profile=lxc-openstack']) }}"

So, you will be updating the lxc_config_list for all containers to the lxc-openstack aa profile, but at the same time, you allow a container property to override it!

You can then create your /etc/openstack_deploy/env.d/cinder.yml with the following content:

container_skel:
  cinder_api_container:
    properties:
      lxc_container_config: [ "lxc.aa_profile=unconfined" ]

However, you still need to ship env.d content for each of the groups requiring a different value than lxc-openstack. In other words, in this case, you need to override the env.d content for the cinder_volume and neutron_agent groups.

Overriding the group variable, centrally.

There is an alternative:

You could directly use group membership inside your user variables!

Example:

cinder_lxc_container_config_list: [ "lxc.aa_profile=override" ]
others_lxc_container_config_list:  [ "lxc.aa_profile=lxc-openstack" ]
lxc_container_config_list: "{% if 'cinder_api' in group_names %}{{ cinder_lxc_container_config_list }}{% else %}{{ others_lxc_container_config_list }}{% endif %}"

Or even better, if you want to add the cinder config list on top of the existing ones:

lxc_container_config_list: |-
  {% set config_list =  [] %}
  {% set _ = config_list.extend(others_lxc_container_config_list) %}
  {% if 'cinder_api' in group_names %}
  {% set _ = config_list.extend(cinder_lxc_container_config_list) %}
  {% endif %}
  {{ config_list }}

Overriding a host variable.

What would you do if you need to override the cinder backends per host on the storage hosts, and define their limit_container_types amongst other data?

For that you can configure your conf.d/cinder.yml:

storage_hosts:
  aio1:
    ip: 172.29.236.100
    container_vars:
      cinder_backends: "{{ cinder_backends_per_host.get(inventory_hostname) }}"

After that, you have to configure a user variable, where cinder_backends_per_host is a dictionary, and each host of your inventory would be a key:

cinder_backends_per_host:
  aio1_aodh_container-c065ea73:
    limit_container_types: cinder_volume
    lvm:
      volume_group: cinder-volumes
      volume_driver: cinder.volume.drivers.lvm.LVMVolumeDriver3
      volume_backend_name: LVM_iSCSI
      iscsi_ip_address: "172.29.236.100"

Conclusion

It is possible to override your group variables and host variables in every version of OpenStack-Ansible. The more recent your version is, the more streamlined and user-friendly your experience will be. Stay up to date, because Ansible 2.4 will probably bring changes improving things even further...

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