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Group Membership

Jackrabbit API

The Jackrabbit API extensions provide various methods to edit and explore the member relationship of users and groups:

Characteristics of the Default Implementation

Member Representation in the Repository

Behavior in Jackrabbit 2.x

With the default configuration Jackrabbit 2.x stores the group members as weak references in a rep:members multi value property in the group node. If the groupMembershipSplitSize configuration parameter is set and valid, the group memberships are collected in a node structure below rep:members instead of the default multi valued property. Its value determines the maximum number of member properties until additional intermediate nodes are inserted. Valid parameter values are integers > 4. The node structure is a balanced b-tree where only the leave nodes carry the actual values in residual properties which name is the principal name of the member.

Behavior as of OAK 1.0

As of Oak the user manager automatically chooses an appropriate storage structure depending on the number of group members. If the number of members is low they are stored as weak references in a rep:members multi value property. This is similar to Jackrabbit 2.x. If the number of members is high the user manager will limit the size of the multi value properties and create overflow rep:MemberReferences nodes below a rep:membersList node to hold the extra members.

Relevant new and modified node types
[rep:Group] > rep:Authorizable, rep:MemberReferences
  + rep:members (rep:Members) = rep:Members multiple protected VERSION /* @deprecated */
  + rep:membersList (rep:MemberReferencesList) = rep:MemberReferencesList protected COPY

  - rep:members (WEAKREFERENCE) protected multiple < 'rep:Authorizable'

  + * (rep:MemberReferences) = rep:MemberReferences protected COPY
Example Group with few members

(irrelevant properties excluded)

    "jcr:primaryType": "rep:Group",
    "rep:principalName": "contributor",
    "rep:members": [
Example Group with many members

(irrelevant properties excluded)

    "jcr:primaryType": "rep:Group",
    "rep:principalName": "employees",
    "rep:membersList": {
        "jcr:primaryType": "rep:MemberReferencesList",
        "0": {
            "jcr:primaryType": "rep:MemberReferences",
            "rep:members": [
        "341": {
            "jcr:primaryType": "rep:MemberReferences",
            "rep:members": [

Note: The exact threshold value that determines the storage strategy is an implementation detail and might even vary depending on the underlying persistence layer. In Oak 1.0 the threshold value is set to 100.

Upgrading Groups from Jackrabbit 2.x to Oak content structure

Upon upgrade from a Jackrabbit 2.x repository to OAK the group member lists that adjusted to reflect the new content structure as created by the OAK user management implementation; this means that the group member node structure will be changed by the migration process. Applications that rely on these implementation details of Jackrabbit 2.x user management instead of use the corresponding API calls will need to be modified accordingly.

XML Import of Group Members

Importing group members through the import methods in javax.jcr.Session or javax.jcr.Workspace is storage agnostic and supports both, property based and node based, strategies and is backward compatible to content exported from Jackrabbit 2.x. The group member lists that are modified during an import are internally processed using the normal user manager APIs. This implies that the node structure after the import might not be the same as the one represented in the input.

Add and Remove Group Members by Id

Since Oak 1.3.4 the default user management implementation also allows to modify group membership by specifying the member id(s) (see JCR-3880 and OAK-3170). The following details are worth mentioning:

  • a null or empty String id will immediately fail the operation with ConstraintViolationException; changes already made will not be reverted,
  • an attempt to make the same group member of itself will list that id in the return value but will not fail the operation,
  • duplicate ids in the parameter list will be silently ignored,
  • cyclic membership validation is postponed to the validator called upon Root.commit and will only fail at that point; the cyclic membership then needs to be manually resolved by the application (see OAK-3170 and below)
  • whether or not a non-existing (or not accessible) authorizable can be added or removed depends on the configured ImportBehavior:
    • ABORT: each id is resolved to the corresponding authorizable; if it doesn't exist ConstraintViolationException is thrown immediately; changes already made will not be reverted.
    • BESTEFFORT: the specified ids are not resolved to the corresponding authorizables and are silently added|removed to|from the set of members; ids that were not successfully added|removed are listed in the return value.
    • IGNORE: each id is resolved to the corresponding authorizable; if it doesn't exist it will be returned as failed in the return value.

Invalid Membership

Invalid Authorizable

Adding a different implementation of Authorizable is not allowed. This is always verified when calling Group.addMember(Authorizable).

Same Group as Member

Adding the target group as member of itself will not succeed. When adding members by ID (Group.addMembers(String...)) the violation is spotted by simple ID comparison.

Everyone Group and Everyone as Member

The group representing the EveryonePrincipal is specially handled. Due to it's dynamic nature adding members to this group is not allowed and adding it as a member to any other group would cause a cyclic membership.

Note however, that this validation is omitted in case of Group.addMembers(String...) with ImportBehavior.BESTEFFORT (see above).

Cyclic Membership

Since Oak 1.7.0 the explicit check for cyclic group membership has been moved from the Validator to the Group implementation. As before cyclic membership might not be spotted and the membership resolution will log the cycle upon collection of all members/groups.

The following scenarios may leave the cycle unnoticed upon adding members:

  • Group.addMember(Authorizable) when the editing Session cannot read all groups included in the cycle.
  • Group.addMembers(String...) with ImportBehavior.BESTEFFORT where the member ID is not resolved.

See OAK-3170 for additional information.


Note that as of Oak 1.0 the implementation is responsible for defining the content structure and will expand the multi-valued rep:members property accordingly. Consequently, the following configuration option groupMembershipSplitSize present with Jackrabbit 2.x is not supported anymore.