Placement Forbidden Traits

Whereas we have explored the diversity of queries that need to be made against the Placement API to retrieve resource providers and allocation candidates it is known that being able to express “not having traits X, Y and Z” is a worthy pursuit.

Therefore we resolve to add support for additional query syntax (described below) that will allow such queries to both GET /resource_providers and GET /allocation_candidates.

Problem description

Traits can be used to signal that a resource provider is special in some fashion. In those cases, it is useful to be able to say “don’t use this special thing for this non-special workload”. This allows the special resources to be preserved. In order for clients like the Nova scheduler to express this requirement, the Placement API needs additional query syntax.

This is considered a generally useful expression in the API, even if the use cases (below) may be resolvable with other tooling.

Use Cases

As an operator of a baremetal service I wish to preserve my expensive GPU loaded hardware for paying bitcoin miners and would prefer to add traits to my hardware in a positive fashion. That is, I want to trait the hardware with CUSTOM_MASSIVE_GPU and not have to put CUSTOM_NOT_MASSIVE_GPU on everything else. Instead flavors and other forms of workload request would be able to express NOT CUSTOM_MASSIVE_GPU.

Similarly some operators, deployers, and toolmakers wish to be able to trait a branch of a tree of nested resource providers as indicating that the branch belongs to some group or purpose and then be able to express “not that group”.

Proposed change

Adjust the handling of the required parameter so that traits can be expressed as required to be present or required to be not present. Traits which are required to be not present are prefixed with a ‘!’. In the following example we require STORAGE_DISK_SSD and not CUSTOM_GOLDEN_RAID:

GET /resource_providers?resources=DISK_GB&required=STORAGE_DISK_SSD,!CUSTOM_GOLDEN_RAID

This syntax has the advantage of not requiring a new query parameter, which might be a good thing. It, however, has the cost of what might be considered too much encoding and a lack of visibility. ‘!’, however, is frequently understood to mean “not”.

More specifics below.


Although this spec does not enumerate any changes required in nova-scheduler’s handling of flavor extra specs or image metadata to pass not-required traits, we plan that the format will mirror the format used for required traits and be trait:$TRAIT_NAME=forbidden.


The syntax of this change is ripe for bikeshedding, so this list of alternatives is far from complete, but one example:

Add a new query parameter, forbidden, which is the opposite of required and has the same syntax. It means “traits listed here should not be in the results”.

This format could be easier to manage and reason about (for existing users) because the existing syntax in flavor extra specs looks like this:

"trait:$TRAIT_NAME": "required"

This suggests that forbidden would work well as:

"trait:$TRAIT_NAME": "forbidden"

Whether that syntax should be reflected on the placement side or not is unclear. The author of this spec feels that two different query parameters for the same type of thing is confusing, but can certainly understand that this alternative could work well and feelings are often insufficient justification for syntax.

Data model impact

No changes would be required in the data models themselves, however changes would be required in data queries to exclude the resource providers that have the traits that must not be present.

REST API impact

A new microversion will be created which will update the validation for the required parameter on GET /allocation_candidates and GET /resource_providers to accept ! as a prefix on listed traits to express that the prefixed trait is required to be not present in the results.

Those traits which are forbidden will then be passed to the ResourceProviderList.get_all_by_filters or AllocationCandidates.get_by_requests methods as required.

A trait that is prefixed with ! that is a duplicate of a trait listed elsewhere in the required parameter is an error and will result in a 400 Bad Request response. No whitespace is allowed between the ! and the trait. Whitespace before or after the phrase of the ! and the trait is allowed and will be stripped.

A malformed trait will result in a 400 Bad Request response (this is already the case).

If the alternative format is chosen, the validation (and the associated response codes) of forbidden will be the same as that used for required.

Security impact


Notifications impact


Other end user impact


Performance Impact

Queries to the database will see a moderate increase in complexity but existing table indexes should handle this with aplomb.

Other deployer impact


Developer impact

Developers of clients of Placement (e.g., nova-scheduler) will want to be aware of the new syntax.

Upgrade impact




Primary assignee:


Other contributors:


Work Items

  • Update the ResourceProviderList.get_all_by_filters and AllocationCandidates.get_by_requests methods to change the database queries to filter on “not this trait”. This work can (and should) be done in a patchset separate and prior to the API changes.

  • Update the placement API handlers for GET /resource_providers and GET /allocation_candidates in a new microversion to pass the negative traits to the methods changed in the steps above, including input validation adjustments.

  • Add functional tests of the modified database queries.

  • Add gabbi tests that express the new queries, both successful queries and those that should cause a 400 response.

  • Release note for the API change.

  • Update the microversion documents to indicate the new version.

  • Update placement-api-ref to show the new query handling.




There are two levels of testing required here:

  • Functional tests to confirm that the database changes are correct.

  • Gabbi tests to confirm that the API behaves.

Documentation Impact

Three areas of documentation change:




Release Name