CellsV2 - Instance Groups API DB migrations


Instance group tables that currently reside in the cell database must be migrated to the API database. Instance groups are exposed in the API and generally accessed in the scheduler.

Problem description

Use Cases

Users wish to create instance groups that operate globally across deployments without worrying about cells implementation.

Proposed change

New instance_groups and instance_group_policy and instance_group_member tables will be created in the API database. The models for these will closely match the existing models in the nova database but they will no longer have soft delete.

Methods currently located in db/sqalchemy/api.py will be mirrored in objects/instance_group.py and modified to access the API database. The InstanceGroup and InstanceGroupList objects will be modified to access the API database initially and the fall-back to the cell database if neccessary. InstanceGroupList methods will return items from both the cell and API databases. These methods will, if-neccessary remove duplicates from the returned items.

Migration methods will be created to move data from the cell to API database. These migration methods will be added to the online_data_migrations nova manage command.

The Flavor tables have already been migrated to the API db. In general the proposed changes will follow those methods. [1]


It may be possible to leave the instance_group_member table in the cell database as this table will grow with the number of instances.

The InstanceGroup function get_hosts accesses the instance_group_member table. It is used in both the scheduler and the compute manager for affinity. As this table is generally accessed outside of the cells it is likely that there would be a greater performance hit from placing this in the cell database.

Data model impact

There will be a large data model impact as many new tables will be created in the API database. The data models have been omitted as they are essentially unchanged from those currently in the cell database.

REST API impact


Security impact


Notifications impact


Other end user impact


Performance Impact


Other deployer impact

Deployers must be aware of the nova-manage command that will perform one time data migration for the tables mentioned.

Developer impact




Primary assignee:


Other contributors:


Work Items

  • Create a new database table and database migration for instance_groups , instance_group_policy and instance_group_members.

  • Modify functions in InstanceGroup and InstanceGroupList to access the API database.

  • Create migration functions for the affected tables and add these to online_data_migrations command.




  • Unit tests for API database access functions.

  • Functional tests for data migration of instance group tables.

Documentation Impact

Documentation must mention the one time data migration tool in the nova-manage command and the data that is migrated.




Release Name