Flavor Extra Spec Validator


Introduce a mechanism to describe and validate flavor extra specs.

Problem description

Flavor extra specs are one of the Wild West aspects of nova. There are a number of issues we’d like to address:

  • Lack of documentation for many flavor extra specs [1]. While Glance has metadefs [2], those are generally out-of-date, incomplete, and not consumable from nova and our user-facing documentation.

  • No warnings or errors if there is a typo in your extra spec, resulting in different behavior to that expected.

  • No defined way to do things like deprecate a flavor extra spec, resulting in continued reinvention of the wheel.

Use Cases

  • As a deployer, I’d like to know what flavor extra specs and image metadata properties are available and why I’d want to use them.

  • As a deployer, I’d like nova to tell me when I’ve used a flavor extra spec that doesn’t exist or has a typo in it.

  • As a developer, I’d like an easy way to deprecate flavor extra specs, which is something that will only become more common if we do things like model NUMA in placement.

  • As a documentation writer, I’d like to be able to cross-reference the various flavor extra specs and image metadata properties available.

Proposed change

A flavor extra spec is a key-value pair. For example:


Different solutions are needed to validate the value part of an extra spec compared to the key part. This spec aims to tackle validation of both, however, the following are considered out-of-scope for this change:

  • Enforcement of extra spec dependencies. For example, if extra spec A requires extra spec B be configured first. We will document the dependency but it won’t be enforced.


    In most cases this is already handled by virt drivers, though this does occur much later in the process that what this spec proposes.

  • Enforcement of virt driver dependencies. Unfortunately, while flavor extra specs should be generic, this isn’t always the case. As above, we will document this dependency but it won’t be enforced.

This change builds upon Flavor extra spec image metadata validation, which covers some of these issues for us.

Value validation

Value validation is the easier of the two issues to tackle. It will resolve issues like the following in a generic manner:


For a generic extra spec, a definition of a validator will need to contain the following:

  • Name or key of the extra spec, e.g. hw:cpu_policy for the above example. This must be patternable to handle e.g. hw:numa_cpus.{id}.

  • Description of the extra spec.

  • Support status of the extra spec.

  • Valid values; whether it’s an integer, a boolean, a string matching a given regex or pattern, an enum, or something else entirely

  • Virt driver dependencies; this is only for documentation purposes and will not be enforced

  • Extra spec dependencies; this is only for documentation purposes and will not be enforced

For any extra specs defined in in-tree code, we propose also maintaining the definitions in-tree. To do this, we propose adding a new module, nova.api.validation.extra_specs, which will contain definitions for flavor validators. These will be defined using Python objects.

For example:

numa_node = base.ExtraSpecValidator(
        'The number of virtual NUMA nodes to allocate to configure the '
        'guest with. Each virtual NUMA node will be mapped to a unique '
        'host NUMA node. Only supported by the libvirt virt driver.'
        'type': int,
        'description': 'The number of virtual NUMA nodes to allocate',
        'min': 1,

numa_cpu = base.ExtraSpecValidator(
        'A mapping of **guest** CPUs to the **guest** NUMA node '
        'identified by ``{id}``. This can be used to provide asymmetric '
        'CPU-NUMA allocation and is necessary where the number of guest '
        'NUMA nodes is not a factor of the number of guest CPUs.'
            'name': 'id',
            'type': int,
            'description': 'The ID of the **guest** NUMA node.',
        'type': str,
        'description': (
            'The guest CPUs, in the form of a CPU map, to allocate to the '
            'guest NUMA node identified by ``{id}``.'
        'pattern': r'\^?\d+((-\d+)?(,\^?\d+(-\d+)?)?)*',

In addition to the extra specs defined in-tree, it is also possible for operators to define their own extra specs that would be used by e.g. custom scheduler filters. For these, we propose providing an entry point through which operators can define their own custom definitions. This entry point should point to a list of extra spec validators. These will have lower precedence than in-tree definitions. This is not expected to be a large burden since operators already need to provide a package for the custom scheduler filters and documentation will be provided to help users add these.

For example:

nova.extra_spec_validators =
    custom_scheduler = custom.scheduler.extra_spec_validators:VALIDATORS
        description='A custom, out-of-tree validator'
            'type:' bool,
            'description' 'Whether to allow the instance to do something',

Regardless of the source of the extra spec validator, they will be used by the flavors/{flavor_id}/os-extra_specs API. A microversion will be introduced for this API to avoid breaking existing tools that are inadvertently setting the wrong values.

Key validation

We also want to be able to catch invalid extra specs themselves. It will resolve issues like the following in a generic manner:


This involves maintaining a registry of all valid extra specs. Given that we’re using a regex to define extra spec names and provide custom extra spec validators via the entry point, we expect to have enough power to achieve this. However, there may be a scenarios where an operator wishes to disable or bypass this validation. To this end, we will add a new validation query parameter to the flavors/{flavor_id}/os-extra_specs API. This will accept three possible values:

strict (default)

Requests for extra specs with invalid values or extra specs that we do not have a validator for will be rejected with a HTTP 400 response.


Requests for extra specs with invalid values will be rejected with a HTTP 400 response. Requests for unregistered extra specs will be logged but permitted.


Validation is disabled. No logging will occur.

All other values will be rejected.

Other changes

We also propose adding tooling to (a) render reStructuredText documentation from the definitions and (b) convert the definitions into Glance metadata definition files. Both of these tools will live within the nova tree, allowing us to remain the single source of truth for these things.


  • We could ignore some of the above issues and try to solve others in a piecemeal fashion. This will likely be far more tedious and time consuming as modifications will be needed in far more places.

  • We could introduce a configuration option to toggle strict API validation instead of or in addition to the API microversion. This introduces a new example of config-driven API behavior, which is something we’re trying to remove from nova. It is also unnecessary because the use of microversions or the validation query parameter allows users to continue using the older behavior when absolutely necessary.

  • We could initially log warnings for invalid keys and introduce the API change in a later release. This is unnecessary because the use of microversions or the validation query parameter allows users to continue using the older behavior when absolutely necessary.

  • We could introduce a new API microversion each time a new extra spec is introduced. This would be extremely tedious, would only be possible for in-tree extra specs, and is on the whole rather unnecessary.

  • We could not add the validate query parameter and instead insist that all extra specs be registered. However, this validation is intended to help operators, not hurt them, and there are reasons people might want to bypass this.

  • We could use a YAML file to describe out-of-tree extra specs rather than custom Python objects. However, this is prone to inadvertent tampering and forces people to learn multiple ways of configuring things.

Data model impact


REST API impact

We will add an API microversion to the flavors/{flavor_id}/os-extra_specs API to return HTTP 400 on invalid flavor extra specs. We will also add support for a validation query parameter to partially or fully disable this behavior, if necessary.

Security impact


Notifications impact


Other end user impact

End users will have better documentation for the available flavor extra specs and image metadata properties.

Performance Impact


Other deployer impact

Operators will now need to describe any custom flavor extra specs used in their deployment using custom validators or will they will see errors when using the new API microversion without the validation parameter.

Developer impact

Developers should now add new flavor extra specs to the nova.api.validation.extra_specs module to take advantage of the validation available.

Upgrade impact

Operators with out-of-tree scheduler filters or virt drivers may need to add extra spec validators to their package.



Primary assignee:


Other contributors:


Feature Liaison


Work Items

  1. Produce extra spec definitions for all in-tree flavor extra specs.

  2. Add entry point-based loading mechanism for custom extra specs and document how operators can and should use this.

  3. Add a new API microversion and code to validate user-provided flavor extra specs and these definitions.

  4. Add a Sphinx extension to render this spec into documentation and another tool to convert the spec into Glance metadata definitions.

  5. Add a tool to generate glance-metadef compatible JSON files that can be consumed by the glance metadata definitions catalog API.




Unit tests.

Documentation Impact

There will be better docs, through the power of Sphinx. We will need to document how operators can develop validators for their custom extra specs.




Release Name





Re-proposed with a simpler name and signficant modifications