Request traits in Nova

This blueprint aims to standardize the request of required qualitative attributes using resource provider traits.

Problem description

Cloud administrators currently have to deal with free-formed flavor extra_specs and image metadata in order to specify capabilities in a launch request. Without standardization of capabilities, there is no possibility of interoperable OpenStack clouds. We have introduced traits to manage the qualitative parts of ResourceProviders in Placement [1]. Administrators should be able to associate a set of required traits with flavors.

Use Cases

For administrative users:

  • Allow administrators to specify a set of traits that a flavor requires.

Proposed change

We propose to store traits in flavor extra_specs and collect traits from flavor in a boot request. Image metadata association is not in the scope of this blueprint. The scheduler will pass traits to the GET /allocation_candidates endpoint in the Placement API to filter out resource providers without each of the required traits.

The trait syntax in flavor extra_specs looks like:


The trait in the key of extra spec is for avoiding the length limit of the value of extra spec. The only valid value is required. For any other value will be considered as invalid.


Instead of storing traits in flavor extra_specs we could add traits as a fields of flavor model and save them into database. However, as the concept of flavor maybe removed from Nova in the future, adding new fields into flavor should be avoided.

Considering traits preference, we currently have some ideas about “Where to specify preferences” and “How to order preferences”, but they may not be settled in this spec.

Data model impact


REST API impact

There is no direct API change. But when invaid traits in the flavor extra spec or the invalid value in the trait extra spec, the server booting with that flavor will fail. The server will get into the error status, just same as the scheduling failed. This needn’t new microversion, since it considers as scheduling failed case which is current API behavior.

Security impact


Notifications impact


Other end user impact


Performance Impact


Other deployer impact


Developer impact




Primary assignee:

Lei Zhang <>

Other contributors:

Alex Xu <> Ed Leafe <> cyx1231st <>

Work Items

Extract traits from flavor extra_specs.


Dependent on a blueprint, Add trait support in the allocation candidates API [2], which enables querying resource providers based on traits for Placement service.


Unit tests and functional tests for building up requests shall be added. The functional test will be the end-to-end test for the trait integration between nova and placement, the test should include the boot and migration cases.

Documentation Impact

The user guide for specifying traits in flavor needs to be documented.




Release Name