Cross-backend IDs for User and Group Entities

bp multi-backend-uuids

Enable Keystone to generate and maintain unique IDs for user and group entities that are keystone-installation-wide, irrespective of whether the entity is local to Keystone, in LDAP or federated.

Problem Description

Originally Keystone was responsible for creating all its user and group entities, along with assigning them a unique ID. With the advent of LDAP and federation, this is no longer the case. However, the Keystone API still assumes it can uniquely identify a user and group from just the ID provided. Further, for Openstack installations that support multiple customers, where each customer is represented as a domain with its own LDAP, it is simply impractical to just search for the entity across all backends.

What is required is for Keystone to continue to be able to provide an ID via its API for all entities it encounters (however they were created) and to be able to find that entity again efficiently when it receives an API call to manipulate that entity using the ID the caller provided. Clients of Keystone should be unaware of any such changes below the API.

A key requirement is that the introduction of any such support should not invalidate or change any IDs already issued by Keystone in existing installations, since such IDs may be persistently stored by clients of Keystone (e.g. Swift).

Proposed Change

The proposal is that Keystone will build an identity mapping of Public IDs to underlying backend identifiers, so that Keystone can route API calls to the correct driver. Clients of Keystone will only see the Public ID and the generation of the Public ID will happen on-the-fly within Keystone. This change was originally proposed (and was close to being integrated) for the IceHouse release, but by mutual agreement was deferred to allow discussion of this topic at the Juno summit, which endorsed the general direction outlined in this proposal.

This mapping is only relevant to the identity component of Keystone (i.e. users and groups) and will be implemented as a pluggable mapping backend, with the only storage backend initially provided as SQL.

As was stated in the summary, one of the main drivers for this requirement is the support of domain specific backends (e.g. LDAP) for Openstack installations that support multiple customers (or even multiple divisions with an enterprise). The framework for such support was added to the Keystone codebase in the Havana release within the identity/ module. This support is marked as experimental, however, since it does not support a concept of ID mapping - and instead tried to deduce the domain scope of the API from the context of the call (e.g. to which domain is the token scoped). While this works in many cases, it is an incomplete solution - examples of where it cannot deduce the correct domain include:

  • Authentication via user ID

  • Cross-domain role assignment

This change therefore builds on the multi-domain framework in the current codebase, but removes the “domain deduction” code in favor of ID mapping.

There are a number of design items that need to be taken into account in implementing this change:

  • Ensuring compatibility with existing entity IDs. There are two kinds of user and group entity IDs in Keystone installations today:

    • UUID IDs for SQL backed identities, or for those entities created by Keystone in a RW LDAP backend (the uuid is stored in an LDAP attribute)

    • Various LDAP attributes as selected by operators (e.g. email address) for LDAP backed identities created outside of Keystone

    Since (ignoring the current experimental support for domain-specific backends) all IDs can only be related to the single chosen SQL or LDAP identity driver, upgrading to Juno with multi-backend UUIDs should not affect these IDs that have already been exposed. The proposal is that by default no identity mapping entries will be created, since there is only the single standard identity driver. The only case where entity IDs from the standard identity driver will be mapped is if the cloud provider wants to ensure that the whatever LDAP attribute is being used as the entity ID is not exposed as thePublic ID. This is achieved by setting the backward_compatible_ids configuration option to False.

  • Public ID generator algorithm. While the obvious choice is simply another regular uuid4 hex string, it is proposed to use a hash algorithm so that Public IDs are regeneratable from the underlying local identifier components. This has a number of advantages:

    • Loss of entire mapping table. If the mapping table was ever lost, then an additional recovery mechanism (as opposed to reverting to a backup) would be to simply allow the table to regenerate on-the-fly as user and group entities were encountered by Keystone. It should be noted, however, that if the assignments table was also lost, then such a facility could only be part of recovery.

    • Ease of purging mapping table. In the case where the identities are managed outside of Keystone (e.g. LDAP or federation), then it is likely that stale entries representing users and groups that have been deleted from the underlying identity store will remain stored in the mapping table. Whilst benign, there will clearly be a need for some kind of periodic cleanup. If the table was regeneratable then simply purging the entire table (or all entries for a given domain) using an option in keystone-manage would be a simple cure (albeit incurring a slight performance degradation each time a given entity was next encountered). In addition, an option will be provided to delete individual entries based on the local entity information.

    • Ensuring consistent Public ID generation when using multiple Keystones. For some large installations, multiple Keystones may be running, backed by a replicated database. In such circumstances it is possible that two different Keystones might encounter an entity for the first time and both create a mapping for it (due to delays in database replication). In the regeneratable case, they would both create the same Public ID and avoid a table row clash. While it should be noted that this issue already exists in the regular RW SQL backend (for example, two users of the same name could be created with different UUIDs via two different Keystones), the case where a RO LDAP backend is providing the identity store via a mapping is far more likely to generate a clash (since it only requires two requests to read the same entity at roughly the same time while running multiple Keystones).

    The selection of a hash algorithm to use is a balance between resulting key size (and hence column width of any client that will need to store the generated ID), probability of collision and security of the encoded data. It should be noted that for most installations, the data being hashed here (local ID and domain ID) is not likely to considered secret - hence the actual security of the hash involved is less important. However, to provide flexibility across all use cases, a pluggable generator backend will be implemented, with a sha256 generator provided as the default. The key produced by generators will be limited to a maximum string size 64 bytes (and any generator producing more than that will cause an exception to be thrown).

    There are also several complexities that are created by creating an identity mapping layer:

    • In the codebase today the controllers generate the UUID ID for an entity they are creating, and pass this to the manager/driver layer. All our unit testing of the manager/driver layer also assumes that the caller specifies the ID. Now that the manager layer is dynamically building a Public ID mapping table, it seems inappropriate that the controller layer thinks it is in control of ID generation. The proposal is, therefore, to remove the ID generation from the User and Group controllers, and let the identity manager carry out this role. Note that this only affects the controller-manager interface, not the driver interface itself. This change, while only affecting a few lines of production code, will result in a large set of mechanical changes to our unit tests.

    • If the underlying driver supports UUIDs (for example the current SQL backend), then there seems little advantage in taking a hash of the UUID. The proposal therefore is to use the local UUID as the Public ID for such drivers, and load a null mapping into the mapping table.


An alternative approach was discussed during IceHouse development and at the Juno summit of creating the mapping within the ID itself - i.e. encoding all the details needed to find the entity in the backend in the ID string. One such proposal was:

<local ID>@@<domain-name>

The problem with this proposal is that, as it stands today, both domain name and any local ID can both be 64 bytes long - and the entity ID Keystone needs to return is also just 64 bytes. A discussion on the dev list explored the option of increasing the size of the entity ID being returned by Keystone, which resulted in strong objections to this from other projects (that are consumers of these entity IDs). The discussion thread can be found here:

The above solution was, in fact, prototyped during IceHouse development as part of the development of this change and can be found here:

A further refinement to this alternative idea was also discussed in terms of compressing the number of bytes required for the domain info, while also restricting the number of bytes allocated to the local ID, so as to fit the while identifier within the currently spec of 64 bytes. While this may work in many practical cases, the general consensus is that this would restrict us unduly in terms of what information we could store to uniquely identify the entity in question in its local backend. A simple example is that some backend stores may store user and group IDs in different namespaces, and hence the mapping should also store what type of entity this is. Further, for Federation, there may be additional information we might wish to store.

If in the future we did want to implement some kind of scheme along these lines, then the currently proposed architecture for this change would allow a mapping backend to be implemented that simply provided the encoding to and from the Public ID rather than actually storing the mapping attributes in a table.

An earlier version of this proposal suggested providing an option for the algorithm for Public ID generation, e.g. choosing between a regular UUID and a hash. This was dropped as there was little advantage in the use of regular UUIDs.

Data Model Impact

The data model changes involve the creation of a new table that provides the mapping:

class IDMapping(sql.ModelBase, sql.ModelDictMixin):
    __tablename__ = 'id_mapping'
    public_id = sql.Column(sql.String(64), primary_key=True)
    domain_id = sql.Column(sql.String(64), nullable=False)
    local_id = sql.Column(sql.String(64), nullable=False)
    type = sql.Column(
        sql.Enum(map.EntityType.USER, map.EntityType.GROUP,
    __table_args__ = (sql.UniqueConstraint('domain_id', 'local_id', 'type'),

The unique constraint is defined to ensure two mappings to the same Public ID cannot be stored in the table.

No further indexes are suggested, since (except for keystone-manage) there are only two real access patterns: By Public ID (which is the prime key) and by specifying all the local identifiers (which should already be indexed due to the unique constraint). Given that keystone-manage is unlikely to be time-critical, the trade-off of further indexes is unlikely to be worth it.


There are no new API calls for this proposal. Some existing API calls have added restrictions placed upon them.

  • “List users” and “List groups” In the case when Keystone is configured for domain-specific backends (via the configuration file) these API calls require a domain scope to be specified. This can be done explicitly by using the already supported domain_id filter or implicitly by using a domain scoped token. If neither of these are provided, the call will return a 401 (Unauthorized) error code.

  • “Add user to group” Since group membership is considered a function of identity and the underlying driver backend, membership across different domain-specific backends is not supported and will return a 403 (Forbidden) error code. This does not affect role assignment across domains and backends which remains unrestricted.

Further, if an unsupported identity mapping generator algorithm has been specified in the Keystone configuration file, then any identity API is likely to generate a 500 (Internal Server Error) return code.

Security Impact

The identity mapping function described in this proposal does not store user data - it simply maps a Public ID to the local identifier. Although in general the local ID (even for LDAP) is not considered sensitive data, one benefit of this proposal is that the LDAP local ID does not escape Keystone (since only the Public ID is exposed). This is in contrast to the current single-domain LDAP implementation that exposes the local ID defined by the LDAP driver as the publicly visible entity ID.

Notifications Impact

The existing identity notification will continue to function, although this references the Public ID rather than any local identifier information.

Other End User Impact


Performance Impact

The introduction of a mapping layer will obviously have some impact. However, since the mapping layer is only used, by default, in domain-specific backend situations which are not supported in production today, there will be no impact on a default or existing installation. When it is used the following additional database calls will be made:

  • A non-PK table lookup for each entity returned by a “List” call for users and groups (in order to map to the Public ID)

  • A mapping entry table is created the first time a user or group item is encountered (to create the mapping)

  • A PK lookup for every user and group API used to manipulate an entity (to lookup the mapping)

Only the first of these has any chance of having any noticeable performance impact. If this proves to be the case, then the optimization listed above in the section of ID generation using hashing could be implemented in a follow-on patch.

Other Deployer Impact

The two main impacts on a deployer will be:

  • Choice of Public ID generator algorithm This was discussed earlier in this specification.

  • Periodic purging of stale entries from the mapping tables As also described earlier, for backend entity stores that are managed outside of Keystone, in general there is no reliable notification mechanism that Keystone could use to auto-purge stale entries from the mapping table. To enable manual purging, keystone-manage will support a new option mapping_purge which will allow the operator to specify the following options:

    • keystone-manage mapping_purge --all– This will purge all mappings

    • keystone-manage mapping_purge --domain-name <name>– This will purge all mappings for the named domain

    • keystone-manage mapping_purge --domain-name <name> --local-id <ID> --type <user|group>– This will purge the mapping for the named local identifier

    • keystone-manage mapping_purge --public-id <ID>– This will purge the mapping for the named public ID

Developer Impact

There are no changes to the identity driver interface with this proposal, although there are two changes to the controller-manager interface:

  • The (now unused) optional parameter domain_scope will be removed (this was the “domain deduction” from the earlier Havana implementation.

  • The explicit user_id and group_id parameters in the Create call for those entities, since the manager layer will now generate the ID. The manager will then pass this ID to the driver layer.



Primary assignee:


Work Items

The set of items required are:

  • Removal of the “domain deduction” parameter from the identity controller- manager interface.

  • Ensure a domain is either explicitly or implicitly defined for the List user and group entities in the controller.

  • Removal of the user_id and group_id parameters from the identity manager Create calls for those entities.

  • Implementation of the backend mapping layer.

  • Provide the two ID generators, UUID and hash, controller by a configuration option.

  • Modify the identity manager layer to call the identity mapping layer to ensure only Public IDs are exposed to the controller.

  • Modify keystone-manage to provide options for purging the mappings.

  • Amend the existing ldap backend unit testing to cover the cases of backward compatible and non backward compatible IDs as well as to provide better coverage for the multi-backend scenarios.

  • Provide specific unit testing for the identity mapping layer.

  • Modify existing unit testing that calls the Create user and group APIs to support the manager generating the ID.

A full implementation that matches the above spec is already available at:




No additional tempest testing is proposed since the existing tests are sufficient to catch potential anomalies in the Public ID.

Documentation Impact

Since there is no change to the API, the only documentation changes required are to the configuration guide.


  1. Juno Etherpad:

  2. NIST high level policy information on hashing algorithms and use