Directory Services and the Cell Environment

· Cell Directory Service (CDS)

· Global Directory Service (GDS)

· Domain Name System (DNS)

· Global Directory Agent (GDA)

CDS is a high-performance distributed service that provides a consistent, location-independent method for naming and using resources inside a cell (intracell). CDS can also be used for communication between cells (intercell) when cells are connected into a hierarchy.

GDS supports the global naming environment inside cells (intracell) and outside of cells (intercell). GDS is an implementation of a directory service standard known as X.500. This standard is specified by the International Organization for Standardization (ISO) 9594 and the International Telegraph and Telephone Consultative Committee (CCITT) X.500 series. Because it is based on a worldwide standard, GDS offers the opportunity for a universally interoperable global directory.

The following figure represents a hypothetical configuration of two cells that each use GDS to access names in the other cell. Names that are stored directly in GDS also are accessible from each cell. CDS is the directory service within each cell. The same organization administers both cells, which are configured based on geographic location and network topology.

Cell and Global Naming Environments

DNS is a widely used existing global name service for which DCE offers support. Many networks currently use DNS primarily as a name service for Internet host names. Although DNS is not a part of the DCE technology offering, the Directory Service contains support for cells to interoperate through DNS.

The GDA is the DCE component that makes cell interoperation possible. The GDA enables CDS to access a name in another cell through one of the global naming environments (GDS or DNS), or through the CDS of the parent cell, if the cell is part of a hierarchical cell configuration. The GDA is an independent process that can exist on a system separate from a CDS server, although by default the DCE configuration script configures the GDA on the same machine as a CDS server. CDS needs to be able to contact at least one GDA to participate in the global naming environment.

The following figure shows how the GDA helps CDS access names outside of a cell. When CDS determines that a name is not in its own cell, it passes the name to a GDA, which searches the appropriate naming environment (CDS, GDS, or DNS) for more information about the name. The GDA returns information that enables the original CDS server to contact the CDS server in whose cell the name resides. The GDA can help CDS find names in a cell that is registered in DNS (Scenario A), a cell that is registered in GDS (Scenario B), or a cell that is registered in the originating cell's parent cell (not shown). The GDA decides which name service to use based on the syntax of the name. An In-Depth Analysis of DCE Names describes name syntaxes in detail.

Interaction of CDSs, GDAs, and Global Directory Services