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TCP/IP Tutorial and Technical Overview - IBM Redbooks

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13.3.1 DCE directory service<br />

When working in a large, complex network environment, it is important to keep<br />

track of the locations, names, <strong>and</strong> services (<strong>and</strong> many other details) of the<br />

participants <strong>and</strong> resources in that network. It is also important to be able to<br />

access this information easily. To enable this, information needs to be stored in a<br />

logical, central location <strong>and</strong> have st<strong>and</strong>ard interfaces for accessing the<br />

information. The DCE Cell Directory Service does exactly this.<br />

The DCE directory service has the following major components:<br />

► Cell Directory Service (CDS)<br />

► Global Directory Service (GDS)<br />

► Global Directory Agent (GDA)<br />

► Application program interface (API)<br />

Cell Directory Service<br />

The Cell Directory Service manages a database of information about the<br />

resources in a group of closely cooperating hosts, which is called a cell. A DCE<br />

cell is very scalable <strong>and</strong> can contain many thous<strong>and</strong>s of entities. Typically, even<br />

fairly large corporate companies will be organized within a single cell, which can<br />

cover several countries. The directory service database contains a hierarchical<br />

set of names, which represent a logical view of the machines, applications,<br />

users, <strong>and</strong> resources within the cell. These names are usually directory entries<br />

within a directory unit. Often, this hierarchical set of names is also called the<br />

namespace. Every cell requires at least one DCE server configured with the Cell<br />

Directory Service (a directory server).<br />

The CDS has two very important characteristics: It can be distributed, <strong>and</strong> it can<br />

be replicated. Distributed means that the entire database does not have to reside<br />

on one physical machine in the cell. The database can logically be partitioned<br />

into multiple sections (called replicas), <strong>and</strong> each replica can reside on a separate<br />

machine. The first instance of that replica is the master replica, which has<br />

read/write access. The ability of the cell directory to be split into several master<br />

replicas allows the option of distributing the management responsibility for<br />

resources in different parts of the cell. This might be particularly important if the<br />

cell covers, say, several countries.<br />

Each master replica can be replicated. That is, a copy of this replica can be<br />

made on a different machine (which is also a directory server). This is called a<br />

read-only replica. Read-only replicas provide both resilience <strong>and</strong> performance<br />

enhancement by allowing a host machine to perform lookups to the nearest<br />

available replica.<br />

498 <strong>TCP</strong>/<strong>IP</strong> <strong>Tutorial</strong> <strong>and</strong> <strong>Technical</strong> <strong>Overview</strong>

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