A directory service keeps details of all the resources in a client server computing system in a database which can be accessed by programs written for the system. This database is replicated over a number of directory servers. When a program requires some details of a resource that is available, say a printer, it communicates with one of the directory servers using a protocol known as the Directory Access Protocol. This was defined as part of the X500 directory standard developed by the International Standards Organization. See also Directory System Protocol.