Red Hat DIRECTORY SERVER 8.0, Руководство администратора

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Chapter 8.
227
Managing Replication
Replication
is the mechanism by which directory data is automatically copied from one Red Hat
Directory Server instance to another; it is an important mechanism for extending the directory service
beyond a single server configuration. This chapter describes the tasks to be performed on the master
and consumer servers to set up single-master replication, multi-master replication, and cascading
replication.
8.1. Replication Overview
Replication is the mechanism by which directory data is automatically copied from one Directory
Server to another. Updates of any kind — entry additions, modifications, or even deletions — are
automatically mirrored to other Directory Servers using replication. This section contains information
on the following replication concepts:
•
Section 8.1.1, “What Directory Units Are Replicated”
•
Section 8.1.2, “Read-Write and Read-Only Replicas”
•
Section 8.1.3, “Suppliers and Consumers”
•
Section 8.1.4, “Changelog”
•
Section 8.1.5, “Replication Identity”
•
Section 8.1.6, “Replication Agreement”
•
Section 8.1.8, “Compatibility with Earlier Versions of Directory Server”
8.1.1. What Directory Units Are Replicated
The smallest unit of of the directory which can be replicated is a database. This means that one
can replicate an entire database but not a subtree within a database. Therefore, when creating the
directory tree, consider any replication plans as part of determining how to distribute information.
Replication also requires that one database correspond to one suffix. This means that a suffix (or
namespace) that is distributed over two or more databases using custom distribution logic cannot be
replicated. For more information on this topic, see
Section 3.2, “Creating and Maintaining Databases”
.
8.1.2. Read-Write and Read-Only Replicas
A database that participates in replication is called a
replica
. There are two kinds of replicas: read-
write or read-only. A
read-write replica
contains master copies of directory information and can be
updated. A
read-only replica
services read, search, and compare requests, but refers all update
operations to read-write replicas. A server can hold any number of read-only or read-write replicas.
8.1.3. Suppliers and Consumers
A server that holds a replica that is copied to a replica on a different server is called a
supplier
for that
replica. A server that holds a replica that is copied from a different server is called a
consumer
for
that replica. Generally, the replica on the supplier server is a read-write replica, and the one on the
consumer server is a read-only replica, with two exceptions: