Книга: Distributed operating systems

10.5. DIRECTORY SERVICE

A major goal of DCE is to make all resources accessible to any process in the system, without regard to the relative location of the resource user (client) and the resource provider (server). These resources include users, machines, cells, servers, services, files, security data, and many others. To accomplish this goal, it is necessary for DCE to maintain a directory service that keeps track of where all resources are located and to provide people-friendly names for them. In this section we will describe this service and how it operates.

The DCE directory service is organized per cell. Each cell has a Cell Directory Service (CDS), which stores the names and properties of the cell's resources. This service is organized as a replicated, distributed data base system to provide good performance and high availability, even in the face of server crashes. To operate, every cell must have at least one running CDS server.

Each resource has a unique name, consisting of the name of its cell followed by the name used within its cell. To locate a resource, the directory service needs a way to locate cells. Two such mechanisms are supported, the Global Directory Service (GDS) and the Domain Name System (DNS). GDS is the "native" DCE service for locating cells. It uses the X.500 standard. However, since many DCE users use the Internet, the standard Internet naming system, DNS, is also supported. It would have been better to have had a single mechanism for locating cells (and a single syntax for naming them), but political considerations made this impossible.

The relationship between these components is illustrated in Fig. 10-18. Here we see another component of the directory service, the Global Directory Agent (GDA), which CDS uses to interact with GDS and DNS. When CDS needs to look up a remote name, it asks its GDA to do the work for it. This design makes CDS independent of the protocols used by GDS and DNS. Like CDS and GDS, GDA is implemented as a daemon process that accept queries using RPC and returns replies.

Fig. 10-18. Relation between CDS, GDS, GDA, and DNS.

In the following section we will describe how names are formed in DCE. After that, we will examine CDS and GDS.

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