The more common approach to providing a highly available directory service is to use redundant server components and replication. Redundant solutions are usually less expensive and easier to implement than clustering solutions. These solutions are also generally easier to manage. Note that replication, as part of a redundant solution, has numerous functions other than availability. While the main advantage of replication is the ability to split the read load across multiple servers, this advantage causes additional overhead in terms of server management. Replication also offers scalability on read operations and, with proper design, scalability on write operations, within certain limits. For an overview of replication concepts, see Chapter 4, Directory Server Replication, in Sun Java System Directory Server Enterprise Edition 6.1 Reference.
During a failure, a redundant system might provide poorer availability than a clustering solution. Imagine, for example, an environment in which the load is shared between two redundant server components. The failure of one server component might put an excessive load on the other server, making this server respond more slowly to client requests. A slow response might be considered a failure for clients that rely on quick response times. In other words, the availability of the service, even though the service is operational, might not meet the availability requirements of the client.