Deployers currently have to maintain policy files regardless if they change the default policy provided from keystone. Maintaining the policy files can be cumbersome and error-prone. In addition to that we don’t use any tooling to generate or communicate policy changes during the release process. By moving policy into code, we can leverage tooling to make maintenance easier for deployers and move to better default policy values.
Today policy exists in a file that deployers are expected to maintain in their deployment. If a deployer needs to change the default policy rules for an operation, they have to make those changes and continuously check to make sure conflicts are resolved with each new release of the policy file. This is cumbersome to maintain, even if a deployment is only using the default policy.
The proposed solution is to check policy into the code base and register it using the oslo.policy library. This is very similar to how projects register and use configuration options using oslo.config. If policies are provided in a policy file on disk, those policies will be registered instead of the in-code default. This provides a way for deployers to override the default we provide.
The registration will need two pieces of data:
Descriptions can also be provided in the registered policy object that help describe the operation and the rule or role that is required to execute it. This description can be used when generating sample policy files from registered rules.
This is the exact same approach nova used to codify policy.
The following are benefits from the approach:
An alternative approach was to pull policy into keystone as an official resource. This would still require some sort of policy override ablility for deployments that do not wish to deploy the default.
None, this change only moves where policy is defined and allows for it to be overridden if necessary.
None. Policy will continue to be evaluated and enforced like it does today.
The performance impact of moving policy in code should be minimal. If the deployment doesn’t have a policy file on disk, the service will not have to fetch it. Instead the default will be registered and used from within code. In the event the deployment is using policy overrides, the combination of the two approaches might cause some performance impact compared to defaults in code, but the overall impact should be negligible.
If a deployer already makes modifications to the default policy file, they will have to continue maintaining those changes. For deployers who modify a subset or none of the policy entries, they can essentially remove their policy file, or the policies that are the default. The end result should be a policy file that purely consists of overrides the deployer wishes to enforce.
Another deployer impact is that deployers no longer need to double check they are protecting all new operations by manually inspecting policy files across releases. Instead, they can be notified about new policies available in a release and then either assume the well documented default or choose to override. The current equivalent to this is to compare operations across policy files without much help from tooling.
Any policies added to the code should be registered before they are used. While the code is switching checks over to context.can() it will be possible to use policy checks that have not been registered. At some point a hacking check should be added to disallow the use of oslo_policy.Enforcer.enforce(). There should also be checks and tests added that make sure new policy entries are accompanied with a release note.
None, we should be able to start working on this today.
Documentation for deployers about the policy file will be updated to mention that only policies which differ from the default will need to be included.