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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 <provider-tree>
.
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.
provider-tree
REST API
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.
Microversions
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.
../placement-api-microversion-history