Placement Usage

Tracking Resources

The placement service enables other projects to track their own resources. Those projects can register/delete their own resources to/from placement via the placement HTTP API.

The placement service originated in the Nova project. As a result much of the functionality in placement was driven by nova’s requirements. However, that functionality was designed to be sufficiently generic to be used by any service that needs to manage the selection and consumption of resources.

How Nova Uses Placement

Two processes, nova-compute and nova-scheduler, host most of nova’s interaction with placement.

The nova resource tracker in nova-compute is responsible for creating the resource provider record corresponding to the compute host on which the resource tracker runs, setting the inventory that describes the quantitative resources that are available for workloads to consume (e.g., VCPU), and setting the traits that describe qualitative aspects of the resources (e.g., STORAGE_DISK_SSD).

If other projects – for example, Neutron or Cyborg – wish to manage resources on a compute host, they should create resource providers as children of the compute host provider and register their own managed resources as inventory on those child providers. For more information, see the Modeling with Provider Trees.

The nova-scheduler is responsible for selecting a set of suitable destination hosts for a workload. It begins by formulating a request to placement for a list of allocation candidates. That request expresses quantitative and qualitative requirements, membership in aggregates, and in more complex cases, the topology of related resources. That list is reduced and ordered by filters and weighers within the scheduler process. An allocation is made against a resource provider representing a destination, consuming a portion of the inventory set by the resource tracker.


The placement API service provides a well-documented, JSON-based HTTP API and data model. It is designed to be easy to use from whatever HTTP client is suitable. There is a plugin to the openstackclient command line tool called osc-placement which is useful for occasional inspection and manipulation of the resources in the placement service.


The placement API uses microversions for making incremental changes to the API which client requests must opt into.

It is especially important to keep in mind that nova-compute is a client of the placement REST API and based on how Nova supports rolling upgrades the nova-compute service could be Newton level code making requests to an Ocata placement API, and vice-versa, an Ocata compute service in a cells v2 cell could be making requests to a Newton placement API.

This history of placement microversions may be found in the following subsection.