Extend Compute Model Attributes

https://blueprints.launchpad.net/watcher/+spec/extend-compute-model-attributes

Problem description

When creating a solution to achieve a goal, Watcher commonly builds an action plan composed of multiple actions, which are mostly based on Server Migrations. These computed migrations are not always valid from Nova’s point of view since they can violate some placement constraints defined for a server, which Watcher is not aware of. Watcher could be improved to include additional constraints and other compute resources in its models, to assist strategies on building action plans. This blueprint proposes extending Watcher’s compute model to include additional compute attributes.

Use Cases

As an OpenStack Admin I would like to optimize my deployment, balancing the workload across compute nodes, without breaking any placement constraints associated with its instances like: server groups, affinity and anti-affinity policies, pinned availability zones, etc.

Proposed change

This blueprint proposes to extend the current compute model to include additional instance’s attributes, available in the new API versions of GET /servers/detail API:

• Scheduler Hints: provided by the user at server creation time, it defines specific constraints that impact where the server is going to be placed. E.g: server affinity or anti-affinity with another server or list of servers, server groups, etc.

• Pinned Availability Zone: defines that an instance should be pinned to an availability zone and this parameter should also be considered when migrating it.

• Server Groups: a list of identifier of server groups to which the server belongs. Each server group has its own policies and rules.

Note

If the configured compute API microversion is lower than the required ones, these new attributes will be left as empty, and strategies will not take the advantage of this proposed extension.

The Server Group model object currently doesn’t exist in the Nova Cluster Data Model and is not part of this implementation to add it. Server Group’s policies and rules doesn’t change after its creation, only the list of members gets updated together with server create and delete operations. The Nova collector doesn’t need to keep polling for changes of this resource since Servers already provide this information in server_groups and in scheduler_hints details.

Note

When consuming new attributes from the compute model, Strategies will need to retrieve details about Server Group’s policies and rules when calculating server migrations. This will only be possible by adding a new Nova API request, GET /os-server-groups, but it is not in the scope of this specification to implement it.

A new configuration option will be added to the collector, to allow Admin to enable or disable the collection of these additional attributes. It will be set to disable by default, but can be changed in the next cycles once Strategies start to consume its content.

The API microversion will increased in favor of the new attributes to be added in the response body of GET /v1/data_model call.

Alternatives

In order to improve the host selection, Watcher could rely on Nova to provide the list of valid destination hosts for a server migration. However, this API does not exist in Nova and implementing it can also be a challenge, since Nova’s Scheduler is not ready to answer to these requests, without reserving the resources associated with them. This alternative would also increase the amount of API requests to Nova, since some strategies calculates multiple solutions before selecting the best one.

Data model impact

New fields will be added to the Instance Element class, which is an element of the Nova Cluster Data Model. The new fields are the following:

"server_pinned_az": wfields.StringField(),
"server_hints": wfields.JsonField(),
"server_groups": wfields.ListOfStringsField(),

REST API impact

A new API microversion will introduce new attributes to the response body of the GET /v1/data_model method. Example of the new attributes in the response json:

{
    "server_pinned_az": "us-west",
    "server_hints": {
      "same_host": [
          "48c0e433-3d90-4056-a865-c8f555320aef",
      ]
    },
    "server_groups": [
      "d4e75852-8358-44c6-b20e-da3cc4853b19",
    ],
    ....

Security impact

None.

Notifications impact

None.

Other end user impact

None.

Performance Impact

• A minimal impact is expected since the in-memory model will be extended and have more fields. A configuration option will allow users to disable extended Instance fields, reducing the impact to almost zero.

Other deployer impact

None.

Developer impact

When developing a new Strategy or updating an existing one, developers can consider these new constraints when selecting a destination host for a server migration.

Implementation

Assignee(s)

Primary assignee:

dviroel

Work Items

• Extend Nova Cluster Data Model to include new Instance attributes.

• Update the Nova Collector to parse additional Instance attributes.

• Extend Nova Notifications processing to update new attributes.

Dependencies

• Some attributes are only available in newer versions of Nova’s API: * Server Groups: microversion 2.71 * Pinned Availability Zone: microversion 2.96 * Scheduler Hints: microversion 2.100 To achieve better results, it is expected that deployed Nova supports most of the above microversions.

Testing

Unit tests will cover different scenarios when collecting data from Nova service. A new tempest test will create instances with additional attributes and validate that this info is available in the new model. A devstack job will be modified to enable the additional attributes collection and run new tempest tests.

Documentation Impact

Update documentation that mention model collectors, to include information about additional attributes in newer microversions.

References

None.

History

Revisions

Release Name	Description
2025.2	Introduced