Count resources to check quota in API for cells

For cellsv2, quota tables are moving to the API database as data global to a deployment. Currently, for instance delete, quota reservations are made in the API and then committed in compute. This is a disconnect which couples compute cells with the API cell. In cellsv2, we endeavor to decouple compute cells from the API cell as much as possible – ideally, cells should not need to have the API database connection in their configuration.

We propose a new approach of counting consumed resources and checking the count against the quota limits in the API instead of the current reserve/commit model where a reservation record is created, quota usage records are created and marked as “in_use” when they are committed, and the reservation record deleted.

Problem description

The current quota design consists of reservations and commits/rollbacks. A simplified explanation of how it works during a create is: “reserve” creates a reservation record and a usage record indicating resources are “reserved.” “Commit” updates the usage record to modify the “reserved” field, the “in use” field, and deletes the reservation record. “Rollback” updates the usage record to modify the “reserved” field and deletes the reservation record.

For instance delete, resources are first reserved in the API when a request is received and then the reservation is later committed in compute when the resources are freed. In cellsv2, this means compute cells will write to the API database for the quota commit if the current quota model is kept. If we instead count resources in the API to check quota, it will be possible in the future [*] to decouple compute cells from the API cell completely.

[*]Quota reads cannot be completely eliminated in compute cells in a special case: nova-compute de/allocating fixed IPs from nova-network during de/provisioning. Nova-network will need to read quota limits from the API database to check quota. This special case can be removed when nova-network is fully removed.

Use Cases

  • Operators want to partition their deployments into cells for scaling, failure domain, and buildout reasons. When partitioned, coupling between the API cell and compute cells should be minimized.

Proposed change

Consumed resources will be counted to check quota instead of the current reserve/commit/rollback model.

  • “Reserve,” “commit,” and “rollback” calls will be removed everywhere. Quota checks will instead consist of reading the quota limits from the API database and comparing the limit with the resource count.
  • “Reserve” calls will be replaced with something like “check_resource_count” which will query the databases for consumed resources, count them, and raise OverQuota if quota limits for the project can’t accomodate the request.


The initial proposal for this work was to commit quota immediately in the API wherever possible and is an alternative approach to this one. The drawback to committing quota immediately in the API is that it can’t be entirely avoided for a failed resize scenario. If a resize fails, resource consumption must be updated accordingly in the quota_usages records whereas with a resource counting approach, no such update would be needed.

Data model impact

We could drop the reservations and quota_usages tables from the API database as they won’t be used.

REST API impact


Security impact


Notifications impact


Other end user impact

With the resource counting approach, it will be possible for a project to consume more resources than they have quota if they are racing near the end of their quota limits. This is because we must aggregate consumed resources across instances in separate databases. So it would be possible for a quota check Y to pass at the API and shortly after a racing request X also passed quota check will have consumed the remaining resources allowed for the project, and then request Y will consume more resources than the quota limit afterward.

Performance Impact

Performance will be adversely affected in the case of counting resources for cores and ram. This is because there is currently no project/user association with allocations in placement. In the absence of an efficient method through the placement API, to count cores and ram, the following approach is required:

  • Get all instances by project per cell and add up the vcpus and memory_mb counts. For example:

    instance_get_all_by_filters(filters={'deleted': False,
                                         'project_id': myproj})

This approach also has the caveat that counting of instances, cores, and ram is not possible when a cell is down. This means when a cell is down, the usage for instances, cores, and ram will not contain the resources in the down cell, enabling the possibility of allocating new resources in available cells, and going over quota when the down cell returns. In practice, until multicell is possible, this should not be a problem in the single cell case.

The plan is to add project/user associations for allocations in placement and add the ability to query for allocations based on project/user. When the query is available, the counting of cores and ram in cells will be replaced with a call to the placement API for allocations which is resilient to down cells.

All other resources should be able to be counted in one step:

  • instances (ReservableResource): This can be obtained from instance_mappings table in API DB. We may be able to create a tally from the aforementioned cores/ram query and use that instead of doing a new query of instance_mappings.
  • security_groups (ReservableResource): Deprecated in 2.36 and not checked in Nova with Neutron. This is checked in the API with nova-network. security_groups are in the cell database so this would be a cell DB read from the API to check.
  • floating_ips (ReservableResource): Deprecated in 2.36 and not checked in Nova with Neutron. This is checked when auto_assign_floating_ip allocates a floating ip with nova-network. floating_ips are in the cell database so this would be a local DB read until nova-network is removed.
  • fixed_ips (ReservableResource): Not checked in Nova with Neutron. This is checked when nova-compute de/allocates a fixed_ip with nova-network. fixed_ips are in the cell database so this would be a local DB read until nova-network is removed.
  • metadata_items (AbsoluteResource): This is a limit on allowed number of image metadata items and is checked when image metadata requests come in. No counting of resources in the database is necessary.
  • injected_files (AbsoluteResource): Similar to metadata_items.
  • injected_file_content_bytes (AbsoluteResource): Similar to metadata_items.
  • injected_file_path_bytes (AbsoluteResource): Similar to metadata_items.
  • security_group_rules (CountableResource): Similar to security_groups.
  • key_pairs (CountableResource): This can be obtained from key_pairs table in API DB.
  • server_groups (ReservableResource): This can be obtained from instance_groups table in API DB.
  • server_group_members (CountableResource): This can be obtained from instance_group_member table in API DB.

Here is an explanation of the resource types, taken from a ML post [1]:

  • ReservableResource: Can be used with reservations, resources are stored in the DB.
  • AbsoluteResource: Number of resources are not stored in the DB.
  • CountableResource: Subclass of AbsoluteResource except resources are stored in the DB. Has a counting function that will be called to determine the current counts of the resource. Not intended to count by project ID.

With the new approach, it seems like ReservableResources should be changed to CountableResources with a count function provided for each.


Other deployer impact

The “nova-manage project quota_usage_refresh” command can be deprecated as refreshing quotas would no longer be something we do.

Developer impact

Nova developers will no longer call quota “reserve,” “commit,” or “rollback.” Instead, they will call quota “check_resource_count” or similar when adding a new API which will consume quota.



Primary assignee:
Other contributors:

Work Items

  • Add a method in nova/objects/ called check_resource_count that counts consumed resources and raises OverQuota if the request would go over quota limits.
  • Remove reserve/commit/rollback everywhere.
  • Mark “reserve,” “commit,” and “rollback” methods as DEPRECATED in the docstrings to prevent their further use.




New unit tests will be added to cover the new resource counting scenarios.

For the most part, this work should be transparent to end-users, so the existing suite of unit, functional, and integration tests should suffice for testing what is proposed.

There is an outstanding review for a regression test for the “quota out of sync” bug that could be used to verify this proposal solves that problem as a side effect.

Documentation Impact



Release Name Description
Ocata Introduced
Pike Re-proposed