Replicas and Their Contents
Directories are the units by which you distribute and replicate names throughout the cell's namespace. Each physical copy of a directory, including the original, is called a replica. When you create a replica of a directory, you replicate all of the entries in it as well.
Replicas are stored in clearinghouses. You can think of a clearinghouse as the collection of directory replicas at a particular server. After you create a directory in one clearinghouse, you can create replicas of it in other clearinghouses to increase availability for looking up information. CDS periodically ensures that the contents of all replicas of a directory remain consistent.
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 Restructuring a Namespace 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
More: