OSF DCE Administration Guide--Core Components

OSF DCE Administration Guide—Core Components
Two types of replicas can exist:
• Master
• Read-only
A replica’s type affects the processing that can be done on it and the way CDS updates it.
The type of replica that CDS uses when it looks up or changes data is invisible to users.
However, it helps to understand how the two types differ.
The master replica is the first instance of a specific directory in the cell’s namespace.
After you make copies of the directory, you can designate a different replica as the
master, if necessary. However, only one master replica of each directory can exist at a
time. (See Chapter 21 for complete information on how to redesignate the master replica
of a directory.)
The master replica is the only directly modifiable replica of a directory. CDS can create,
change, and delete information in a master replica. Because it is modifiable, the master
replica incurs more overhead than read-only replicas, which CDS keeps up-to-date
periodically with changes made to the master replica.
A read-only replica is a copy of a directory that is available only for looking up
information. CDS does not create, modify, or delete names in read-only replicas; it
simply updates them with changes made to the master replica.
Replicas can contain three kinds of entries:
• object entries
• soft links
• child pointers
12.2.1 Object Entries
An object is any real resource—like a disk, application, or node—that is given a CDS
name. When an object name is created, client applications and the CDS software supply
attributes to be stored with the name. An attribute, consisting of an attribute name and
value(s), describes a particular operational property of an object. The name and its
attributes make up the object entry. When a client application requests a lookup of the
name, CDS returns the value of the relevant attribute or attributes.
Object entries are typically created and managed through a client application interface.
For example, the DCE control program and the Name Service Interface (NSI) of the RPC
runtime let users create entries that represent RPC servers, groups, and profiles. These
are special kinds of entries that enable an RPC application to locate and select servers.
(See the OSF DCE Application Development Guide for details on how RPC uses CDS
for this purpose.)
You can also create object entries through the DCE control program (dcecp). (See Part
1 of this document and the OSF DCE Administration Reference for information on the
commands that allow you to create and manage object entries by using dcecp.)
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