Retrieving Changes Made to the Directory Service

The ability to retrieve changes that have been made to the directory service can be useful for building applications such as directory synchronization tools.

One important attribute is an object's Update Sequence Number or USN. When a change is made to an object in the Exchange directory, the local directory service modifies the USN on the object to be the next available USN for the server. Each server maintains it's own USN count. Therefore, when a server changes an object, the directory service takes the current USN, increments it and stamps it on the changed object. If an application knows the USN number of the last changed object it received from the server, the next time it makes a request for changes it asks for all directory objects with USNs greater than the last USN it received.

Here is a simple example.

Dim ADOConn As ADODB.Connection
Dim ADOCommand As New Command
Dim RS As ADODB.Recordset

Set ADOConn = CreateObject("ADODB.Connection")
ADOConn.Provider = "ADSDSOObject"
ADOConn.Open "Active Directory Provider"

Set ADOCommand.ActiveConnection = ADOConn
ADOCommand.CommandText = "<LDAP://localhost/o=kramerica>;(USN-Changed>=1080);rdn;subtree"
Set RS = ADOCommand.Execute

While Not RS.EOF
    Debug.Print (RS.Fields(0))
    RS.MoveNext
Wend

RS.Close
Set ADOConn = Nothing
Set ADOCommand = Nothing
Set RS = Nothing

Note: This example is specific to Exchange Server version 5.5 and below, and is not upwardly compatible with Exchange 6.0. Management and access of Exchange 6.0 Servers should be made through the CDO Exchange Management interfaces instead.