Node Traits

Add more granular Nova placement of ironic nodes by allowing operators to set a list of traits associated with each node.

Problem description

While the recent addition of Resource Class on every node has helped improve the way ironic resources are scheduled in Nova, using the new Placement API, there are many use cases that need more granular control.

An example use case is dedicating a pool of ironic nodes for a specific project. Another is allowing flavors that best fit to the available hardware, you might want smaller flavors to build on larger machines if you are out of smaller machines.

In a similar way, you might have a pool of hardware that are all the same except for the chosen RAID configuration. Longer term, when you provision the node via Nova boot it would be good to be able to automatically reconfigure the hardware with the chosen RAID configuration. For now this is considered out of scope.

Proposed change

The Placement API has the ability to tag a resource provider with a trait:

The nova ironic driver currently ensures a Resource Provider exists for every Ironic Node. It already populates the inventory with the correct quantitative resources (i.e the Resource Class).

To get the extra granularity of scheduling we depend on adding traits to the resource provider, and the implementation of the following Nova specs that ensure the traits requested in flavors are honored by the scheduling process:

The current proposal is for Ironic to store a list of traits for each node. That list can then be synced into the appropriate resource provider in the Placement API by Nova’s ironic virt driver, in a similar way to how the RESOURCE_CLASS of each ironic node is reported today.

Lets talk about the use case of dedicating specific Ironic nodes for use only by a specific set of projects. The remainder of the hosts are for general use. If a user has a dedicated pool of resources, they have the ability to pick if they create an instance in their dedicated pool or in the general pool. Other users are only able to build in the general pool. One way to bisect the nodes like this is assigning traits such as CUSTOM_IRONIC_NODE_PROJECT_B and CUSTOM_IRONIC_NODE_GENERAL_USE to the appropriate ironic nodes. Then there is a public flavor to target the general pool of hosts, and a private project specific flavor that targets their dedicated pool. By taking this approach it is easy to add additional pools of nodes for other sets of projects. It is easier because you don’t need to modify any of the existing nodes and flavors when you add an additional pool of nodes.

Note that inspection rules can be used to set the initial value for a node’s Resource Class. It is expected the initial set of traits for a node can be set in the same way.


There is already a Nova spec proposed discussing the Nova side of how we can assign traits to the resource providers, and on Nova boot send the chosen traits back to ironic:

When provisioning a node, the Ironic virt driver in Nova will now send additional flavor extra_specs to Ironic. Currently only extra_specs starting with capabilities are set in instance_info. After the above spec, the ironic virt driver will also include the flavor extra specs that define what traits have been requested, storing them instance_info[‘traits’].


Note there is no change to how capabilities are used as part of this spec.

Ironic needs to validate that instance_info[‘traits’] is consistent with the list of traits currently associated with Ironic node, i.e. we must check that instance_info[‘traits’] only includes traits that are already in the list of traits set on the Ironic node. This is particularly useful in preventing strange behaviour due to races between the update of an Ironic node’s trait list and getting that list copied into the Placement API by the Ironic virt driver. This validation is probably best integrated as part of driver.deploy.validate or similar, to ensure it is triggered by a call to deploy a node and a call to validate a node.


The ironic virt driver in Nova will sync the list of traits reported by each Ironic Node into Nova’s Placement API, in a similar way to how the Resource Class is done today. Should an operator talk directly to Placement to adjust the traits, this process will remove all those modifications when the next sync occurs. Operators must use the Ironic API to change the list of traits on a given node. Ironic is assumed to be the source of truth for the list of traits associated with a Resource Provider that is representing an Ironic node.


In the future, it is expected that a driver may need to validate some well known traits to see if they are supported. In addition it may be that some drivers automatically report some traits. Following on from that, it could be that drivers read the traits specified in instance_info to reconfigure the node. This is all out of scope here.

Looking at what is in scope, there are alternative approaches:

  • Make operators (and ironic-inspector) talk directly to the Placement API to set the node traits. This would be very odd given the Nova sync of the Resource Providers using the ComputeNode uuid as the uuid for the Resource Provider. You can’t set the traits until that sync has completed. Right now placement is largely an internal API with few policy controls, so that would all need to change to allow the above. It all seems very messy.
  • Keep traits APIs in Ironic, but make it a pass-through proxy to Placement. This is would make Ironic hard depend on placement, which would be a strange requirement for things like Bifrost, and would complicate upgrades.

The current approach keeps Ironic from needing to depend on placement in any way, which works well.

The REST API described below allows fine grained control of setting traits, following the patterns from the API-WG and existing placement APIs. We could instead have just extended the existing ironic node PATCH interface.

The suggested CLI approach below follows the new API, but as an alternative we could have extended the existing CLI interface around the PATCH API call. It would have looked something more like:

  • openstack baremetal node set –trait CUSTOM_FOO <node-uuid>
  • openstack baremetal node unset –trait CUSTOM_FOO <node-uuid>

Data model impact

Following a similar pattern to the existing table for tags, we will need to add this new table to store the traits associated with a node:

CREATE TABLE node_traits (
    node_id INT(11) NOT NULL,
    PRIMARY KEY (node_id, trait),
    KEY (trait),
    FOREIGN KEY (node_id)
      REFERENCES nodes(id)

A new ironic.objects.traits.NodeTraitList object will be added to the object model. The ironic.objects.traits.NodeTraitList field in the python object model will be populated on-demand (i.e. not eager-loaded).

A trait should be defined in a way that matches the placement API definition, as a Unicode string no longer than 255 characters.

State Machine Impact

No impact.

REST API impact

The placement API defines a set of standard traits in the os-traits library. Any traits that are not defined in that library must start with the prefix of CUSTOM_. Any trait set in Ironic must follow these rules, else the ironic Nova virt driver will be unable to add the traits in Placement. For similar reasons there is a limit of 50 traits on any node, to match the limit in Placement. A request to add a badly formatted trait should get a response with the status code 400.

Note at no point does Ironic talk to the Placement API. The above validation depends only on access to the python library os-traits. As such, this validation poses little restriction on how traits can be used in standalone Ironic to assign arbitrary traits on particular Ironic nodes. Any non-standard traits simply need to have a prefix of CUSTOM_ added. For more details on os-traits please see:

For convenience, it will be possible to get the full list of nodes and the traits associated with each node by extending the existing API in the following way (when requesting a high enough microversion that includes these details):

GET /v1/nodes/detail

  "nodes": [
      "traits": ['CUSTOM_FOO', 'CUSTOM_BAR', 'CUSTOM_BAZ'],

In a similar way to other fields, we will also support a request to get just this field (in part to make the Nova virt driver polling more efficient):

GET /v1/nodes/?fields=uuid,traits

  "nodes": [
      "uuid": "uuid-1",
      "traits": ['CUSTOM_FOO', 'CUSTOM_BAR', 'CUSTOM_BAZ']

The manipulation of node traits will follow the patterns established by both the placement API and API WG tags spec:

To start with there will be a new traits resource that follows the above patterns.

Example request for all node traits:

GET /nodes/{node_ident}/traits


    "traits": ['CUSTOM_FOO', 'CUSTOM_BAR', 'CUSTOM_BAZ']

Example request to set all node traits to given list:

PUT /nodes/{node_ident}/traits
    "traits": ['CUSTOM_FOO', 'CUSTOM_BAR', 'CUSTOM_BAZ']


    "traits": ['CUSTOM_FOO', 'CUSTOM_BAR', 'CUSTOM_BAZ']

The response on success is status code 200. On failure to validate (using the os-traits library) we return the status code 400 (Bad Request), matching the HTTP Guidelines from the API-WG.

Note that unlike with Resource Class, we are allowing the trait to be updated at any time. This is mostly because placement allows such updates and because although the Resource Class and Ironic node are used in the allocations in placement, traits are not used in allocations.

In a similar way the following API removes all the traits:

DELETE /nodes/{node_ident}/traits

The response on success is status code 204, with an empty body.

To add or remove an individual trait use:

PUT /nodes/{node_ident}/traits/CUSTOM_FOO
<no body>

DELETE /nodes/{node_ident}/traits/CUSTOM_FOO

Filtering the node list by traits should work as expected:

GET /nodes?not-traits=CUSTOM_RED,CUSTOM_BLUE&traits=CUSTOM_FOO
GET /nodes?traits-any=CUSTOM_RED,CUSTOM_BLUE
GET /nodes?not-traits-any=CUSTOM_RED,CUSTOM_BLUE

As mentioned above, the final change that is made is to ensure instance_info/traits is a subset of the traits set on the Ironic node. This should be part of the existing driver.deploy.validate() call (or similar) such that the traits will be checked both before a deploy starts and on an explicit node validate call.

Client (CLI) impact

“ironic” CLI

No changes, it is deprecated.

“openstack baremetal” CLI

You can list the traits on a node:

  • openstack baremetal node list –fields uuid name traits
  • openstack baremetal node show <node-ident> –fields uuid name traits
  • openstack baremetal node trait list <node-ident>

You can update the list of traits on a node:

  • openstack baremetal node add trait <node-ident> CUSTOM_FOO CUSTOM_BAR
  • openstack baremetal node remove trait <node-ident> CUSTOM_FOO CUSTOM_BAR
  • openstack baremetal node remove trait –all <node-ident>

This is roughly copying the command syntax of consistency groups:

It is common to use set and unset for key value pairs, but add and remove seems a better fit for in-place modifications of a list. It stops any ambiguity of set meaning either an addition of a list of traits or replacing the whole list of traits. Another alternative is to add trait operations into the existing openstack baremetal node set operation, but we are instead following the structure of the API.

You can query the list of nodes using traits:

  • openstack baremetal node list –trait CUSTOM_RED –not-trait CUSTOM_BLUE
  • openstack baremetal node list –trait-any CUSTOM_RED CUSTOM_BLUE
  • openstack baremetal node list –not-trait-any CUSTOM_RED CUSTOM_BLUE

RPC API impact

No impact.

Driver API impact

No impact.

Nova driver impact

Need to ensure the correct flavor extra specs are passed back when starting a node.

Ramdisk impact


Security impact

There will be a hard coded limit of 50 traits for any Node to prevent misuse of the API. This prevents denial of service attack where the database is filled up by a rogue user setting lots of traits. Really the limit is in place to match the limit applied in the placement API.

Other end user impact


Scalability impact


Performance Impact


Other deployer impact


Developer impact




Primary assignee:
John Garbutt (johnthetubaguy)
Other contributors:
Dmitry Tantsur (dtantsur) Mark Goddard

Work Items

  • Add table to store traits for nodes
  • Add object to expose the table
  • Add new traits API
  • Add openstack cli support for the new API
  • Follow up with Nova driver work


Nova functional tests are planning on covering the scheduling aspects of the integration. As part of this spec we will focus on ensuring the API works correctly to persist the traits for given nodes, and query resources using traits.

Upgrades and Backwards Compatibility

Longer term, capabilities and other APIs may be phased out. We are not considering that as part of this spec. There is much more work needed before we have feature parity between the old and new scheduling mechanisms.

Documentation Impact

Need to update the API-REF and the admin doc to cover how to use the new API.