## Managing Node Affinity

updated: 2020-10-17 18:54

# Managing Node Affinity

When deploying multiple nodes running identical instances of the same service (or application) for the sake of load-balancing or high-availability, it is very likely you don’t want all nodes deployed onto the same physical machine. However, when you have a cluster with some nodes playing one role (e.g. Application Server) and other nodes playing another role (.e.g. Database), you may want to collocate these nodes onto the same physical machine so that inter-node communication can be faster.

To meet these intra-cluster node collocation requirements, you have different choices.

## Use Server Group in Profile

For the purpose of managing cluster node affinity, you may choose to create a server group by invoking nova command line, e.g.:

    $ openstack server group create sg01 --policy affinity
    +--------------+------+------------+---------+---------------+---------+----------+
    | Id           | Name | Project Id | User Id | Policies      | Members | Metadata |
    +--------------+------+------------+---------+---------------+---------+----------+
    | 54a88567-... | sg01 | ...        | ...     | [u'affinity'] | []      | {}       |
    +--------------+------+------------+---------+---------------+---------+----------+

Then when you create a nova server profile, you can input the name of the server group into the `scheduler_hints` property as shown below:

    $ cat web_cluster.yaml
    type: os.nova.server
    version: 1.0
    properties:
      name: web_server

      <... other properties go here ...>

      scheduler_hints:
        group: sg01

Later, when you create a cluster using this profile, the server nodes will be booted on the same physical host if possible. In other words, the affinity is managed directly by the nova compute service. If there are no physical hosts satisfying the constraints, node creation requests will fail.

## Use Same-Host or Different-Host in Profile

When adding nodes to an existing cluster, the new nodes can reference a different profile object of the same profile type (i.e. `os.nova.server`). If a new node is expected to be launched on the same/different host from a set of server nodes, you can specify the constraint as a `scheduler_hints` as well.

Suppose you have two server nodes in a cluster with UUID “UUID1” and “UUID2” respectively, you can input the scheduling constraints in a profile as shown below:

    $ cat standalone_server.yaml
    type: os.nova.server
    version: 1.0
    properties:
      name: web_server

      <... other properties go here ...>

      scheduler_hints:
        different_host:
          - UUID1
          - UUID2

When adding a node that uses this profile into the cluster, the node creation either fails (e.g. no available host found) or the node is created successfully on a different host from the specified server nodes.

Similarly, you can replace the `different_host` key above by `same_host` to instruct that the new node collocated with the specified existing node(s).

## Managing Affinity using Affinity Policy

Another option to manage node affinity is to use the affinity policy (see Affinity Policy). By creating and attaching an affinity policy to a cluster, you can still control how nodes are distributed relative to the underlying hosts. See the above link for usage of the policy.

## See Also

- Managing Policies

- Builtin Policy - Affinity Policy

updated: 2020-10-17 18:54

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