7.3. Linking Attributes to Manage Attribute Values

A class of service dynamically supplies attribute values for entries which all have attributes with the same value, like building addresses, postal codes, or main office numbers. These are shared attribute values, which are updated in a single template entry.
Frequently, though, there are relationships between entries where there needs to be a way to express linkage between them, but the values (and possibly even the attributes) that express that relationship are different. Red Hat Directory Server provides a way to link specified attributes together, so that when one attribute in one entry is altered, a corresponding attribute on a related entry is automatically updated. (The link and managed attributes both have DN values. The value of the link attribute contains the DN of the entry for the plug-in to update; the managed attribute in the second entry has a DN value which points back to the original link entry.)

7.3.1. About Linking Attributes

The Linked Attributes Plug-in, allows multiple instances of the plug-in. Each instance configures one attribute which is manually maintained by the administrator (linkType) and one attribute which is automatically maintained by the plug-in (managedType).
Basic Linked Attribute Configuration

Figure 7.4. Basic Linked Attribute Configuration

Note

To preserve data consistency, only the plug-in process should maintain the managed attribute. Consider creating an ACI that will restrict all write access to any managed attribute. See Section 18.8, “Adding an ACI” for information on setting ACIs.
A Linked Attribute Plug-in instance can be restricted to a single subtree within the directory. This can allow more flexible customization of attribute combinations and affected entries. If no scope is set, then the plug-in operates in the entire directory.
Restricting the Linked Attribute Plug-in to a Specific Subtree

Figure 7.5. Restricting the Linked Attribute Plug-in to a Specific Subtree

When configuring the Linked Attribute Plug-in instance, certain configurations are required:
  • Both the managed attribute and linked attribute must require the Distinguished Name syntax in their attribute definitions. The linked attributes are essentially managed cross-references, and the way that the plug-in handles these cross-references is by pulling the DN of the entry from the attribute value.
    For information on planning custom schema elements, see Chapter 12, Managing the Directory Schema.
  • Each Linked Attribute Plug-in instance must be local and any managed attributes must be blocked from replication using fractional replication.
    Any changes that are made on one supplier will automatically trigger the plug-in to manage the values on the corresponding directory entries, so the data stay consistent across servers. However, the managed attributes must be maintained by the plug-in instance for the data to be consistent between the linked entries. This means that managed attribute values should be maintained solely by the plug-in processes, not the replication process, even in a multi-master replication environment.
    For information on using fractional replication, see Section 15.1.7, “Replicating a Subset of Attributes with Fractional Replication”.

7.3.2. Looking at the Linking Attributes Plug-in Syntax

The default Linked Attributes Plug-in entry is a container entry for each plug-in instance, similar to the password syntax plug-ins or the DNA Plug-in in the next section. Each entry beneath this container entry defines a different link-managed attribute pair.
To create a new linking attribute pair, then, create a new plug-in instance beneath the container entry. A basic linking attribute plug-in instance required defining two things:
  • The attribute that is managed manually by administrators, in the linkType attribute
  • The attribute that is created dynamically by the plug-in, in the managedType attribute
  • Optionally, a scope that restricts the plug-in to a specific part of the directory tree, in the linkScope attribute

Example 7.6. Example Linked Attributes Plug-in Instance Entry

dn: cn=Manager Link,cn=Linked Attributes,cn=plugins,cn=config
objectClass: top
objectClass: extensibleObject
cn: Manager Link
linkType: directReport
managedType: manager
linkScope: ou=people,dc=example,dc=com
All of the attributes available for an instance of the Linked Attributes Plug-in instance are listed in Table 7.2, “Linked Attributes Plug-in Instance Attributes”.

Table 7.2. Linked Attributes Plug-in Instance Attributes

Plug-in Attribute Description
cn Gives a unique name for the plug-in instance.
linkScope Contains the DN of a suffix to which to restrict the function of the plug-in instance.
linkType Gives the attribute which is maintained by an administrator. This attribute is manually maintained and is used as the reference for the plug-in. This attribute must have a DN value format. When the attribute is added, modified, or deleted, then its value contains the DN of the target entry for the plug-in to update.
managedType Gives the attribute which is maintained by the plug-in. This attribute is created and updated on target entries. This attribute must have a DN value format. When the attribute is added to the entry, its value will point back as a cross-reference to the managed entry.