openstack-ansible/doc/source/install-guide/app-advanced-config-affinity.rst

Affinity

OpenStack-Ansible's dynamic inventory generation has a concept called affinity. This determines how many containers of a similar type are deployed onto a single physical host.

Using shared-infra_hosts as an example, consider this openstack_user_config.yml:

shared-infra_hosts:
  infra1:
    ip: 172.29.236.101
  infra2:
    ip: 172.29.236.102
  infra3:
    ip: 172.29.236.103

Three hosts are assigned to the shared-infra_hosts group, OpenStack-Ansible ensures that each host runs a single database container, a single memcached container, and a single RabbitMQ container. Each host has an affinity of 1 by default, and that means each host will run one of each container type.

You can skip the deployment of RabbitMQ altogether. This is helpful when deploying a standalone swift environment. If you need this configuration, your openstack_user_config.yml would look like this:

shared-infra_hosts:
  infra1:
    affinity:
      rabbit_mq_container: 0
    ip: 172.29.236.101
  infra2:
    affinity:
      rabbit_mq_container: 0
    ip: 172.29.236.102
  infra3:
    affinity:
      rabbit_mq_container: 0
    ip: 172.29.236.103

The configuration above deploys a memcached container and a database container on each host, without the RabbitMQ containers.