To use oslo.limit in a project:

from oslo_limit import limit

Conceptual Overview

This library is meant to aid service developers performing usage checks for resources managed by their service. It does this by clearly defining what is being claimed, where resources are being claimed, and encapsulating enforcement logic behind easy-to-use utilities. The following subsections introduce common terms and concepts useful for communicating within the context of usage enforcement for distributed systems.


Usage is the real-time allocation of resources belonging to someone or something. They are assumed to be present, or created, and thus accounted for.

With respect to OpenStack being a distributed system, the service responsible for the resource is considered the usage authority for that resource. Ensuring accurate usage almost always requires the service to perform a lookup and possibly aggregate results to definitively provide usage information.


A limit is the total number of resources someone or something should have.

With respect to OpenStack, the service which owns a particular resource may also own that resource’s limit. Conversely, limit information may be centralized in a shared service. The latter is the pattern implied by the usage of this library. The justification for decoupling resource limits from individual services is to make it easier to provide a consistent experience for users or operators setting and enforcing limits, regardless of the resource.


A claim represents the quantity of resources being asked for by someone. Claims are constrained by the relationship between resource usage and limits. Successful claims are aggregated into usage.

Within the OpenStack ecosystem, claims can be made against specific targets depending on the resource. For instance, a user may request two additional servers for her project. This resulting claim might be two instances, the total number of cores between both instances, the total memory consumed by both instances, or all three. The claim is also targeted to a specific project, which affects how this library asks for usage information.


Enforcement is the process of collecting usage data, limit information, and claims in order to make a decision about whether a user should be able to obtain more resources.