Many OpenStack services have some mechanism for ensuring that a single user or project doesn’t take over your entire cloud and starve out other users. This requires every service:
has some default limits encoded in the project for resources
keeps track of data relating to project_id / user_id when limits are overridden for particular projects/users.
provides a REST API for changing those, and implements a CLI to do the same.
Ensure that projects / users limits stored are for valid projects / users (often not actually done) 
Clean up orphaned data for projects / users that are deleted (often not actually done) 
That’s just on the definition of limits side. Once limits are defined.
Count resources that are allocated.
Enforce that allocated resources don’t exceed limits in the system.
Requests that fail half way through due to going over limit are rolled back and do not leave garbage lying around.
And this is only for a flat project structure. The moment a hierarchical project structure is introduced, all this has to be done potentially in the context not only of the current project, but taking into account limits and usage by ancestors, siblings, and children.
What is a limit?¶
A limit is the following information:
API service type (e.g. compute, network, object-storage)
a resource type (e.g. ram_mb, vcpus, security-groups)
a default limit (the max resources allowed if no project specific override is in place)
the project specific limit
user_id (optional, and hopefully can be deprecated)
Some current quota implementations today enforce on
project_id for some types of resources. It would be
ideal if that could be deprecated as part of this transition.
This information has to be accurate and consistent in order for any quota calculations to be valid. This is even more true if we talk about the notion of hierarchical quotas, because if data is stored in limits that violates some basic constraint of the system (for instance, project A has child project B, and child B vcpu quota is > A vcpu quota), then all the logic to sensibly calculate usage and quotas at the service will be buggy and hugely more complex.
Where should this be?¶
The current approach to limits puts all this data at the service level (i.e. in nova in the API database). The service type is known, because it’s the API server in question. The resource types are enforced because they are in code. The default limits are in config. The per project allowed are in the database, and project_id/user_id are accepted unvalidated over the API, and may or may not be valid projects or users. This gets so wildly complicated for the hierarchical case. The net effect is that many teams, like the Nova team, have postponed tackling the problem. 
An alternative approach is to move this into keystone. Keystone is the authoritative source of project_id and user_id information. It is the authoritative source of any hierarchical relationship between projects. It is an authority about API services types and regions in a system.
So all that is required is to teach keystone, in a generic way, about what resource types and default limits each service type has (possibly distinct per region), and what project specific overrides might exist.
This could be done with new administrative API calls to keystone to set these.
a facility to CRUD a resource type, and default limit, and associated service and region. This creates a strong definition of all the allowed limits in a cloud, so that limit overrides can be strictly validated (no ability to set the limit on
discwhen it should have been
a facility to CRUD the limit override for a project / user for a resource type. This would be strongly validated on existing service types, region, project/user, preregistered resource type, and limit being an integer.
It is assumed that resource type default definition would happen during service install / upgrade through an administrative command in a similar way to how services and endpoints are defined before services get used.
In the hierarchical case, when creating/updating/deleting a project override, the rules for the hierarchical limits would be enforced before the change is made. We want to guarantee that the hierarchical limits structure is consistent at all times.
Limits vs. Usage Enforcement¶
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.
In this proposal, Keystone maintains limits. 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 Baobab
Project Baobab has a default CPU limit of 20
User Jane allocated 18 CPUs in project Baobab
Administrator Kelly sets Project Baobab CPU limit to 10
User Jane can no longer allocate instance resources in project Baobab, until she (or others in the project) have deleted at least 9 CPUs to get under the new limit
This is the behavior that most administrators want, as it lets them 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. Note, today some projects prevent limits from being set lower than existing allocations. That API behavior will not be honored in this new system. 
Users in projects 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.
Common behavior between projects¶
When we get to an N level project hierarchy, this is going to get complicated. Doing back of the envelope modeling for different quota models  shows that there are a lot of different ways this can be modeled.
Because of this, it’s going to be assumed that we’re going to need some common library with both checking that a limit change to an existing hierarchy is valid, as well as a resource allocation does not exceed quota. While valid limit checking will be in keystone only, and quota checking in projects only, having the same algorithms in common code will ensure that limit changes for Garbutt Model are consistent with quota changes for Garbutt Model.
The exact interfaces will need to be hammered out as this gets implemented.
Access to limits¶
Limit information should be accessed over a REST API call. This is potentially extremely cachable information. Only explicit updates to Limits, made via Keystone API, will invalidate this information. Keystone should be able to implement efficient HTTP caching for this information.
Users in a project will have visibility to all the project limits, as well as limits in child projects. Depending on the quota system model used, they may also have visibility to higher levels of the hierarchy (especially if their allocations only make sense in the context of higher levels of allocations) . There should be a principle of self service here, that users in a project which is over quota should always be able to figure out why that project is.
Service/Administrative users will have this read access for all projects.
This information will be fetched whenever a quota calculation is needed. The service enforcing quotas should always assume it’s calling keystone to fetch the limit every time, even if this just turns into a fast 304 HTTP NOT MODIFIED from keystone.
Items beyond scope¶
During the Limits discussion at the Pike PTG, there was also interest in another kind of system limitation, around rate limits that ensure a healthy environment. The Swift team presented the perspective that they were less interested in project level limits, but more in limiting things to ensure the health of the cluster. Most of these metrics included a time component (like iops).
While this is a very interesting question, and clearly a future need around fairness and cluster health, when we talk about limits in the context of this work we’re only talking about fixed, integer amounts, of resources.
Concrete path forward¶
The following is my best estimate on a path to move forward.
Get general agreement on Keystone ownership of limits
Define keystone get / set limit interfaces (separate spec)
Create os-quotas lib with flat and strict account hierarchical limits
Implement Keystone get / set limit interfaces
plumb unified limits support to one or more service projects (Nova stepping up). Start using by default, including API cutover (lots of bugs are going to fall out of this, we should consider it a whole cycle)
Convert rest of service projects to new limits model.
implement overbooking algorithm in os-quotas lib as experimental.
These are existing sources of information out there around hierarchical limits and quotas. They are included for reference only, and are not meant to mean these will also be implemented.
Existing Proposed Keystone Spec - https://review.openstack.org/#/c/363765/.
This is a lower level specification (with POC code) that includes putting the limits information in the Token. Overuse of token was a primary concern. Tokens, are by definition, stale information, and can be long lived, thus an administrator changing limits would have no idea when the system would start enforcing them. Token bloat is also a concern for projects that have worker daemons and rely heavily on RPC, as it means more load on the RPC bus.
This spec was presented at the Atlanta PTG, and is the spiritual basis for the above agreement.
Proposed Nova Quotas Spec - https://review.openstack.org/#/c/429678.
This is an overview spec that includes a mix of the rationale for putting limits in keystone. The use of quota classes. Some examples on overbooking. Elements of this spec have been carried into this unified spec. The Nova spec is considered defunct.
Mailing list thread - http://lists.openstack.org/pipermail/openstack-dev/2017-March/113099.html