Understanding and Troubleshooting Directory Access

Understanding How Directory Access Works
This section describes each of the Exchange 2000 directory access components and explains how the core technology works in a stable Active Directory environment.
What Is DSAccess?
DSAccess is a core component of Exchange 2000 that is implemented as a DLL file named DSACCESS.DLL. The purpose of DSAccess is to control how other Exchange components access Active Directory. DSAccess discovers the Active Directory topology, detects domain controllers and global catalog servers, and maintains a list of valid directory servers that are suitable for use by Exchange components. If the status of a domain controller or global catalog server changes, DSAccess updates its list. In addition, DSAccess contains a memory cache, which reduces the load on Active Directory by reducing the number of Lightweight Directory Access Protocol (LDAP) requests that individual components must send to Active Directory servers.

The core components of Exchange 2000 require access to the directory. Therefore, they rely on DSAccess to provide a current list of Active Directory servers. For example, the message transfer agent (MTA) routes LDAP queries through the DSAccess layer to Active Directory. The store process uses DSAccess to obtain configuration information from Active Directory in order to connect to databases. The transport process uses DSAccess to obtain information about the connector arrangement in order to route messages. If each of these Exchange 2000 components had a separate mechanism for performing Active Directory lookups, Exchange 2000 would become disjointed and less scalable. In Exchange 2000, DSAccess is the centralized mechanism that determines the Active Directory topology, opens the appropriate LDAP connections, and works around server failures.