Mirror nova host aggregates in placement


This spec proposes to ensure that the placement service has a placement aggregate record for each nova host aggregate.

Problem description

Nova host aggregates are useful for grouping collections of nova-compute service workers together. Certain scheduler filters such as the AggregateMultiTenancyIsolationFilter filter look at metadata stored against a nova host aggregate when evaluating whether a hypervisor host meets certain filtering criteria.

In order to replace some of these filters with similar filter parameters to the GET /allocation_candidates placement REST API, we first need to ensure that nova host aggregates that tie collections of nova-compute service workers together are represented in the placement API as placement aggregates and associated with the resource provider records that correspond to the compute node that the nova-compute service manages.

Use Cases

As a deployer I want to be able to leverage placement aggregates for my normal host-aggregate groupings and thus I want nova to keep the changes I make mirrored to placement for me.

Proposed change

We propose to modify the implementation of the Compute API to call to the placement service (via the scheduler reportclient) when a “host” member is added or removed from a nova host aggregate.

Similarly, when a nova host aggregate is deleted, there will be a call to the placement service to remove any resource provider to placement aggregate associations for the nova host aggregate in question.

We will not be communicating with the placement service for either the creation or update of a nova host aggregate, since the placement service does not need to create a record for an aggregate until such time as a resource provider is associated with it.

A new data migration command sync_placement_aggregates will be added to synchronize existing nova host aggregate records using the nova-manage tool.


If there is a failure in nova-api communicating with the placement service, we will log a warning but not return an error to the end user. The nova-manage command will allow reconciling aggregate information at a later time.


We could have adapted the nova-compute service workers to do this syncing behaviour, but that would have required an upcall from the computes to the nova api layer.

We could have had an external agent do the mirroring/syncing behaviour.

We could use a periodic task in the nova-scheduler service instead of a nova-manage command.

Data model impact


REST API impact


Security impact


Notifications impact


Other end user impact


Performance Impact

There will be a slight negative impact to the add_host and remove_host operations for the Compute API since it will now need to communicate with the placement service.

Other deployer impact

The nova-api service’s nova.conf file will now need to contain placement service authentication credentials in order for the nova-api service to communicate with placement. We will make the lack of placement auth credentials a warning in Rocky and required in Stein.

In addition, the operator will want to run the nova-manage sync_placement_aggregates command periodically to ensure nova and placement have reconciled views of aggregate information.

Developer impact


Upgrade impact

The nova-api service will now depend on being configured to authenticate with the placement service, similar to the nova-compute and nova-conductor services. We will make this a soft failure (warning in logs) in Rocky and a hard failure in Stein.



Primary assignee:


Work Items

  • Add initialization check to nova-api to auth with placement. Make this a non-hard failure for Rocky and a note about failing hard in Stein

  • Add methods to the scheduler reportclient for adding an aggregate association to a resource provider by name, since the os-aggregates Compute API uses a non-UUID host name parameter for identifying the nova-compute service worker

  • Modify the nova.compute.api.AggregateAPI.add_host_to_aggregate() and remove_host_from_aggregate() methods to call out to the reportclient to add or remove a resource provider to aggregate association by provider name

  • Create new sync_placement_aggregates command in the nova-manage tool




Normal testing as well as full functional tests of the new nova-manage sync_placement_aggregates command.

Documentation Impact

A release note describing the mirroring process, requirement of the nova-api’s nova.conf to contain placement credentials and inclusion of the sync_placement_aggregates command in nova-manage should be done. In addition, the placement API reference should be updated to describe how the nova host aggregates are mirrored to placement.


  • Enables these blueprints: