Unified Limits

Unified Limits


The unified limits API is currently labeled as experimental and can change in backwards incompatible ways. After we get feedback on the intricacies of the API and no longer expect to make API breaking changes, the API will be marked as stable.

As of the Queens release, keystone has the ability to store and relay information known as a limit. Limits can be used by services to enforce quota on resources across OpenStack. This section describes the basic concepts of limits, how the information can be consumed by services, and how operators can manage resource quota across OpenStack using limits.

What is a limit?

A limit is a threshold for resource management and helps control resource utilization. A process for managing limits allows for reallocation of resources to different users or projects as needs change. Some information needed to establish a limit may include:

  • project_id
  • API service type (e.g. compute, network, object-storage)
  • a resource type (e.g. ram_mb, vcpus, security-groups)
  • a default limit
  • a project specific limit
  • user_id (optional)
  • a region (optional depending on the service)

Since keystone is the source of truth for nearly everything in the above list, limits are a natural fit as a keystone resource. Two different limit resources exist in this design. The first is a registered limit and the second is a project limit.

Registered limits

A registered limit accomplishes two important things in order to enforce quota across multi-tenant, distributed systems. First, it establishes resource types and associates them to services. Second, it sets a default resource limit for all projects. The first part maps specific resource types to the services that provide them. For example, a registered limit can map vcpus, to the compute service. The second part sets a default of 20 vcpus per project. This provides all the information needed for basic quota enforcement for any resource provided by a service.

Project limits

A project limit is a limit associated to a specific project and it acts as an override for a registered limit. A project limit still requires a resource type and service, both of which must exist as a registered limit. For example, let’s assume a registered limit exists for vcpus provided by the compute service. It wouldn’t be possible for a project limit to be created for cores on the compute service for a specific project. Project limits can only override limits that have already been registered. In a general sense, registered limits are likely established when a new service or cloud is deployed. Project limits are used continuously to manage the flow of resource allocation.

Together, registered limits and project limits give deployments the ability to restrict resources across the deployment by default, while being flexible enough to freely marshal resources across projects.

Limits and usage

When we talk about a quota system, we’re really talking about two systems. A system for setting and maintaining limits, the theoretical maximum usage, and a system for enforcing that usage does not exceed limits. While they are coupled, they are distinct.

Up to this point, we’ve established that keystone is the system for maintaining limit information. Keystone’s responsibility is to ensuring that any changes to limits are consistent with related limits currently stored in keystone.

Individual services maintain and enforce usage. Services check for enforcement against the current limits at the time a resource allocation is requested by a particular user. A usage reflects the actual allocation of units of a particular resource to a consumer.

Given the above, the following is a possible and legal flow:

  • User Jane is in project Foo
  • Project Foo has a default CPU limit of 20
  • User Jane allocated 18 CPUs in project Foo
  • Administrator Kelly sets project Foo CPU limit to 10
  • User Jane can no longer allocate instance resources in project Foo, until she (or others in the project) have deleted at least 9 CPUs to get under the new limit

The following would be another permuation:

  • User Jane is in project Foo
  • Project Foo has a default CPU limit of 20
  • User Jane allocated 20 CPUs in project Foo
  • User Jane attempts to create another instance, which results in a failed resource request since the request would violate usage based on the current limit of CPUs
  • User Jane requests more resources
  • Administrator Kelly adjust the project limit for Foo to be 30 CPUs
  • User Jane resends her request for an instance, which successed since the useage for project Foo is under the project limit of 30 CPUs

This behavior lets administrators set the policy of what the future should be when convenient, and prevent those projects from creating any more resources that would exceed the limits in question. Members of a project can fix this for themselves by bringing down the project usage to where there is now headroom. If they don’t, at some point the administrators can more aggressively delete things themselves.

Enforcement models

Project resources in keystone can be organized in hierarchical structures, where projects can be nested. As a result, resource limits and usage should respect that hierarchy if present. It’s possible to think of different cases where limits or usage assume different characteristics, regardless of the project structure. For example, if a project’s usage for a particular resource hasn’t been met, should the projects underneath that project assume those limits? Should they not assume those limits? These opinionated models are referred to as enforcement models. This section is dedicated to describing different enforcement models that are implemented.

It is important to note that enforcement must be consistent across the entire deployment. Grouping certain characteristics into a model makes referring to behaviors consistent across services. Operators should be aware that switching between enforcement models may result in backwards incompatible changes. We recommend extremely careful planning and understanding of various enforcement models if you’re planning on switching from one model to another in a deployment.

Keystone exposes a GET /limits-model endpoint that returns the enforcement model selected by the deployment. This allows limit information to be discoverable and perserves interoperability between OpenStack deployments with different enforcement models.


The API to expose the limit model configuration in keystone will be implemented when keystone supports more than one enforcement model. Until then, keystone only supports the following enforcement model.



The flat enforcement model is currently the only supported enforcement model. Future work will add more enforcement models which will be configurable via configuration files and discoverable via the API.

Flat enforcement ignores all aspects of a project hierarchy. Each project is considered a peer to all other projects. The limits associated to the parents, siblings, or children have no affect on a particular project. This model exercises the most isolation between projects because there are no assumptions between limits, regardless of the hierarchy. Validation of limits via the API will allow operations that might not be considered accepted in other models.

For example, assume project Charlie is a child of project Beta, which is a child of project Alpha. A default is set on a limit, all projects get that effective default, which is 10. The following operations would be possible:

  • UPDATE LIMIT on Alpha to 20
  • UPDATE LIMIT on Charlie to 30

This is allowed with flat enforcement because the hierarchy is not taken into consideration during limit validation, even though a child project has a higher limit than a parent project.

Creative Commons Attribution 3.0 License
