!!! Overview 
Just as subordinate references help maintain connectivity between parent and child [NDS Partitions] stored on different servers, the following other kinds of references also help keep [EDirectory] trees connected and maintain [EDirectory Synchronization]: 
* [{$pagename}]
* [Back Links] 
* [Distributed Reference Links] (DRLs) 
* [Obituaries|NDS Obituaries] 

[EDirectory] stores these types of information in [{$pagename}], which are place holders containing information about entries that the server does not hold. [{$pagename}] are not copies of the complete entries, but rather pointers to the real entry. 

Besides providing connectivity, [{$pagename}] improve system performance by caching frequently accessed information. 

[{$pagename}] are maintained by the [Backlinker or DRL processor|Backlinker] and the [Purger] process. 

!! [eDirectory 8.8.8.0 (20801.46)] and Later
Starting with [eDirectory 8.8.8.0 (20801.46)] the [Distributed Reference Links] ([DRL]) method has been removed eliminating these unnecessary cycles.

Additionally, if all servers in a ring are on [eDirectory 8.8.8.0 (20801.46)], [NDS Obituary State] flag 2 is no longer used in processing delete, rename and move [NDS Obituaries].   

Prior to [eDirectory 8.8.8.0 (20801.46)], the [NDS Obituaries] process would not run on a [NDS Partition] if the server was currently outbounding changes for that partition to other servers.  This would cause [NDS Obituaries] for that partition to be delayed.  In a busy environment with hundreds of changes per second this could lead to [Change Cache] buildup.  Now the [NDS Obituaries] process can run in parallel with outbound synchronization thereby reducing obituary processing delays.

%%warning
[EDirectory 8.8.5.0 (20219.15)] and later [EDirectory Versions] do __NOT support [NetWare]__ and these features are NOT supported.
%%

!! So what is actually maintained? 
That depends on the object and the version of [eDirectory]. The base class, name, and certain attributes are all maintained. Some examples of maintained attributes include Public Key and [GUID] for User objects, Replica for [NDS Partition] Root objects, and Status and NDS Version for NCP objects. 

In order to achieve the [{$pagename}], various attributes relating to the obituary process are maintained, these are 
* [backLink] 
* [usedBy] 
* [notify] 
* [reference] 
These attributes are maintained using the [Backlinker], [Janitor] and [Distributed Reference Links] ([DRLs]). 

!! Creating [{$pagename}]
[EDirectory] creates [{$pagename}] for the following operations: 
* [Authentication] - A user authenticates to a server, and this user does not have an entry stored in a [NDS Partition] on the server. To enable authentication, the server must create an external reference so that a [localEntryID] can be given to the [authentication] process. 
* Browsing - When a user, browsing the [eDirectory] tree, requests information about an entry that is not stored locally, [eDirectory] creates an external reference to the entry. 
* [SecurityEquals] - Users who authenticate to the server can have security equivalence to entries not stored locally. Such entries require external references. 
* Attributes of Local Entries - Some attributes, such as [Member], take a list of entries and can have entries of objects that are not stored locally. Each such entry requires an external reference. 
* [File System] - The file system uses entry IDs to maintain a list of owners and trustees of files and directories. Trustees or owners that are not local entries require external references. 

In addition, [eDirectory] creates external references when a [replica] is removed from the server. [eDirectory] changes all of the entries in the removed replica into [NDS External References] and marks them as expired. 

Before an [{$pagename}] is created in [eDirectory], newer than [NetWare] 5.x, it places a [UsedBy] attribute on a writable copy of the referenced object. The [UsedBy] attribute is also placed on the referenced object's [NDS Partition] root and the object's [NDS Partition] root. 

!! Deleting [{$pagename}] 
On each server, [eDirectory] deletes expired [{$pagename}] if they have not been used within a specified time period when the [Backlinker] process runs. 

The system administrator can use a SET the [n4u.nds.external-reference-life-span] parameter to set a number of hours after which [eDirectory] deletes [{$pagename}] that have: 
* not been used 
* are not needed for another entry's context, 
* do not contain information that the operating system needs. 

To remove expired [{$pagename}], [eDirectory] builds a list of unused [{$pagename}] by checking the life span interval of each external reference. 

The [Backlinker] process checks to see if the file system must access any of the external references. If the file system uses the expired external references, they are not taken off the delete list. The [Backlinker] process deletes the remaining entries on the list. The [Janitor] process is responsible for purging the deleted external references. 

When [eDirectory] updates entries and partitions, it also must update [{$pagename}] created for those entries. Synchronizing external references is usually done by the server receiving the original synchronization request; however, any read/write replica can initiate synchronization if the external reference is being deleted or renamed. 

!! Category
%%category [eDirectory]%%

!! More Information 
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