The placement API service was introduced in the 14.0.0 Newton release within the nova repository and extracted to the placement repository in the 19.0.0 Stein release. This is a REST API stack and data model used to track resource provider inventories and usages, along with different classes of resources. For example, a resource provider can be a compute node, a shared storage pool, or an IP allocation pool. The placement service tracks the inventory and usage of each provider. For example, an instance created on a compute node may be a consumer of resources such as RAM and CPU from a compute node resource provider, disk from an external shared storage pool resource provider and IP addresses from an external IP pool resource provider.

The types of resources consumed are tracked as classes. The service provides a set of standard resource classes (for example DISK_GB, MEMORY_MB, and VCPU) and provides the ability to define custom resource classes as needed.

Each resource provider may also have a set of traits which describe qualitative aspects of the resource provider. Traits describe an aspect of a resource provider that cannot itself be consumed but a workload may wish to specify. For example, available disk may be solid state drives (SSD).