The directory system agent (DSA) is a collection of services and processes that run on each Windows 2000 Server and later domain controller and provides access to the data store. The data store is the physical store of directory data located on a hard disk. In Active Directory Domain Services, the DSA is part of the local system authority (LSA) subsystem. Clients access the directory using one of the following mechanisms supported by the DSA. This document provides details about Active Directory Directory System Agent implemenation and practical view of the component. DSA is the primary component for Active Directory LDAP operations and helps Administrators to understand the implementation aspects of the component.
Please follow the below link to download the document , hope it helps !
http://gallery.technet.microsoft.com/Active-Directory-Directory-cca49b03