| Platform SDK: Exchange Server |
The PR_EMS_AB_USN_CHANGED property contains the update sequence number (USN) for an object at the time it was changed.
| Header file: | EMSABTAG.H |
| Applies to: | All directory objects. These objects appear to MAPI as messaging_user objects, which are accessible through the MAPI IMailUser interface. |
| Identifier: | 0x8029 |
| Property type: | PT_LONG |
| Access: | Read-only |
The PR_EMS_AB_USN_CHANGED property on an object is set to the next USN counter when the object is created or modified. This USN is server-specific, and servers are not updated to be alike, so the PR_EMS_AB_USN_CHANGED property for a certain directory object will usually be different on each server.
Each server has an internal USN counter that is incremented each time an object is created or modified in the directory. The updated USN counter is assigned to the PR_EMS_AB_USN_CHANGED property of the created or modified object when the directory needs to be replicated to another server. The highest USN that was sent to the other server on the previous replication is remembered—only USNs higher than that are sent. The result of this action is that only new information is sent to the other server.
This property can be used in a similar manner when writing directory synchronization programs.