Affinity Policy

Affinity Policy

The affinity policy is designed for senlin to leverage the server group API in nova. Using this policy, you can specify whether the nodes in a cluster should be collocated on the same physical machine (aka. “affinity”) or they should be spread onto as many physical machines as possible (aka. “anti-affinity”).

Currently, this policy can be used on nova server clusters only. In other words, the type name of the cluster’s profile has to be os.nova.server.


A typical spec for an affinity policy looks like the following example:

type: senlin.policy.affinity
version: 1.0
    name: my_server_group
    policies: affinity
  availability_zone: nova
  enable_drs_extension: false

The affinity policy has the following properties:

  • An optional string that will be used as the name of server group to be created.
  • servergroup.policies: A string indicating the policy to be used for the server group.
  • availability_zone: Optional string specifying the availability zone for the nodes to launch from.
  • enable_drs_extension: A boolean indicating whether VMware vSphere extension should be enabled.


When creating an affinity policy, the Senlin engine checks if the provided spec is valid:

  • The value for servergroup.policies must be one of “affinity” or “anti-affinity”. The default value is “affinity” if omitted.
  • The value of availability_zone is the name of an availability zone known to the Nova compute service.

Server Group Name

Since the os.nova.server profile type may contain scheduler_hints which has server group specified, the affinity policy will behave differently based on different settings.

If the profile used by a cluster contains a scheduler_hints property (as shown in the example), the Senlin engine checks if the specified group name (“group_135” in this case) is actually known to the Nova compute service as a valid server group. The server group name from the profile spec will take precedence over the value in the policy spec.

type: os.nova.server
version: 1.0
  flavor: m1.small
    group: group_135

If the group value is found to be a valid server group name, the Senlin engine will try compare if the policies specified for the nova server group matches that specified in the affinity policy spec. If the policies don’t match, the affinity policy won’t be able to be attached to the cluster.

If the profile spec doesn’t contain a scheduler_hints property or the scheduler_hints property doesn’t have a group value, the Senlin engine will use the value from the affinity policy spec, if provided. If the policy spec also failed to provide a group name, the Senlin engine will try to create a server group with a random name, e.g. “server_group_x2mde78a”. The newly created server group will be deleted automatically when you detach the affinity policy from the cluster.

Availability Zone Name

The spec property availability_zone is optional, no matter the value for enable_drs_extension is specified or not or what value it is assigned. However, if the availability_zone property does have a value, it will have an impact on the placement of newly created nodes. This subsection discusses the cases when DRS extension is not enabled.

In the case that DRS extension is not enabled and the availability_zone property doesn’t have a value. Senlin engine won’t assign an availability zone for newly created nodes.

By contrast, if the availability_zone property does have a value and it has been validated to be name of an availability zone known to Nova, all newly created nodes will be created into the specified availability zone.

DRS Extension

The property enable_drs_extension tells Senlin engine that the affinity would be enforced by the VMware vSphere extension. In this case, the value of the availability_zone property will be used to search for a suitable hypervisor to which new nodes are scheduled.

All newly created nodes in the cluster, when an affinity policy is attached and enabled, will be scheduled to an availability zone named <ZONE>:<HOST> where <ZONE> is the value of availability_zone and <HOST> is the hostname of a selected DRS hypervisor.

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