Split-up Assignments, making the Role-Assignment piece pluggable

bp pluggable-assignment

Modify the assignment component by splitting out (and making pluggable) the parts that relate to actual assignments, in order to allow alternative assignment solutions to be provided.

Problem Description

The assignment controller and backend are really misnomers - there are effectively the “rest-of-identity”, that was left over when we split out the actual identity piece (i.e. users and groups). Within this “assignment” component we have the store for domains, projects, roles….and the actual assignments themselves. Domains, projects and roles are the lingua franca used throughout OpenStack for policy control to APIs. The actual assignment relationships between these entities (which are stored in Keystone) are not shared outside of Keystone - rather these assignments are used to determine which roles are included in a token for a given user for a given scope of target entity (i.e. project or domain). Hence in reality what we call the “assignment” component of Keystone today, consists of:

  1. Storage and APIs for defining domains, projects and roles.

  2. Storage and APIs for defining assignment relationships.

  3. A set of APIs that evaluate the assignments, in order that the correct set of roles can be placed in a scoped token.

The fact that the above are all lumped together in one component leads to a number of issues.

First, the component is overly complex. This has been shown to be true in terms of number of new defects being found in this area recently. Further we want to make significant changes in this area: e.g. hierarchical projects and restructuring of the list_role_assignents() method for performance reasons. These changes can only make things worse.

Second, for developers and cloud providers that would like to offer alternative types of assignments (e.g. ABAC, enhanced relationship models etc.), all they really need to provide is their own version of 2 (above) and honor the API of 3. However, since the “assignments” backend contains all of the three items above, it is not simple to provide just the actual assignments piece. The advantage of enabling alternative access control models in this way is that the rest of OpenStack will continue to work - i.e. no change to token formats or policy files in other projects. The OpenStack community as a whole will benefit if we can let Keystone be a platform through which new forms of access control can be brought into play, with the market eventually deciding which of those best matches customer requirements.

Proposed Change

This change would split the current “assignment” controller and backend into two:

  • resources - containing domains, projects and roles

  • assignments - containing current assignment model

This change will both simplify the support for the core entities on which the rest of OpenStack depends, while making it easier to normalize the various (slightly different) manager/driver methods for listing assignment relationships, e.g.:

  • list_role_assignments

  • list_grants

  • get_roles_for_groups

  • get_roles_for_user_and_domain

  • list_projects_for_user

  • list_domains_for_groups

This is particularly important when inheritance is considered - there is a general lack of conformity of how assignment inheritance is handled across the various flavors of driver calls - that is currently leading to a number of defects.

A further side benefit of this change is that deployers could choose different backend technologies (e.g. LDAP vs SQL) for resource entities and assignments that match better the scale & update profiles of the respective entities.

Making this split will not, in and of itself, have any functional change, but will enable more reliable support for the current capabilities, while providing a better base to support the changes in-flight, such as hierarchical multi-tenancy and role assignment performance improvements. It also provides a platform that enables the experimentation of new assignment models, while ensuring their expression of assignments can be still be used by the rest of OpenStack.



Data Model Impact

Other than splitting the data models into two backends, the only change is that assignments would no longer use a foreign key to track the role id. Although this does, or course, reduce referential integrity, this is considered a low risk change given that we have already removed the foreign keys for the user, group, domain and project objects.



Security Impact


Notifications Impact


Other End User Impact


Performance Impact

There is likely to be a mild performance impact from this change for list assignment API calls that return the refs of projects, domains or roles since we can no longer execute the extraction of assignment ids and their subsequent turning to refs in one go. The impact of this will be mitigated by supporting bulk loads of refs from a list of ids in the new base entity drivers. See the references section for details of early WIP code that shows how this would be done.

Other Deployer Impact

Deployers would be able to set the backends for resources and assignments separately, although by default they would be the same (to keep with existing configuration settings).

Developer Impact




Primary assignee:


Work Items

  • Split the controller and backends

  • Split up the test cases into relevant files

  • Provide a sample alternative controller and backend

  • Document how to provide a new assignment controller and backend





Documentation Impact

Changes to the configuration.rst and developing.rst


An early WIP of what would be a first phase of implementing this bp is available at: https://review.openstack.org/#/c/130954/