HP-UX Directory Server 8.1 deployment guide

6 Designing the replication process
Replicating the directory contents increases the availability and performance of the directory
service. Chapter 4 “Designing the directory tree” and Chapter 5 “Designing the directory
topology” cover the design of the directory tree and the directory topology. This chapter addresses
the physical and geographical location of the data and, specifically, how to use replication to
ensure the data is available when and where it is needed.
This chapter discusses uses for replication and offers advice on designing a replication strategy
for the directory environment.
6.1 Introduction to replication
Replication is the mechanism that automatically copies directory data from one HP-UX Directory
Server to another. Using replication, any directory tree or subtree (stored in its own database)
can be copied between servers. The Directory Server that holds the master copy of the information
automatically copies any updates to all replicas.
Replication provides a high-availability directory service and can distribute the data
geographically. In practical terms, replication provides the following benefits:
• Fault tolerance and failover — By replicating directory trees to multiple servers, the directory
service is available even if hardware, software, or network problems prevent the directory
client applications from accessing a particular Directory Server. Clients are referred to another
Directory Server for read and write operations.
NOTE:
Write failover is only possible with multi-master replication.
• Load balancing — Replicating the directory tree across servers reduces the access load on
any given machine, thereby improving server response time.
• Higher performance and reduced response times — Replicating directory entries to a location
close to users significantly improves directory response times.
• Local data management — Replication allows information to be owned and managed locally
while sharing it with other Directory Servers across the enterprise.
6.1.1 Replication concepts
Always start planning replication by making the following fundamental decisions:
• What information to replicate.
• Which servers hold the master copy, or read-write replica, of that information.
• Which servers hold the read-only copy, or read-only replica, of that information.
• What should happen when a read-only replica receives an update request; that is, to which
server it should refer the request.
These decisions cannot be made effectively without an understanding of how the Directory
Server handles these concepts. For example, decide what information to replicate, be aware of
the smallest replication unit that the Directory Server can handle. The replication concepts used
by the Directory Server provide a framework for thinking about the global decisions that need
to be made.
6.1.1.1 Unit of replication
The smallest unit of replication is a database. An entire database can be replicated but not a
subtree within a database. Therefore, when defining the directory tree, always consider replication.
6.1 Introduction to replication 73