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 6.2 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.
The main advantage of a clustered solution is automatic recovery from failure, that is, recovery without user intervention. Disadvantages of clustering are complexity and inability to recover from database corruption.
In a clustered environment, the cluster uses the same IP address for Directory Server and Directory Proxy Server, regardless of which cluster node is actually running the service. That is, the IP address is transparent to the client application. In a replicated environment, each machine in the topology has its own IP address. In this case, Directory Proxy Server can be used to provide a single point of access to the directory topology. The replication topology is therefore effectively hidden from client applications. To increase this transparency, Directory Proxy Server can be configured to follow referrals and search references automatically. Directory Proxy Server also provides load balancing and the ability to switch to another machine when one fails.
In terms of the SPOFs that are described at the beginning of this chapter, redundancy and clustering handle failure in the following ways:
Single hardware failure. In a clustered environment, this kind of failure has no impact on the directory service. Only multiple hardware failures impact the service in a cluster.
A single hardware failure is fatal to a machine that is not in a clustered environment. Therefore, even if you have redundant hardware, manual intervention is required to repair the failure.
Directory Server or Directory Proxy Server failure. In a clustered environment, the server is automatically restarted. Software failure must occur multiple times in quick succession to trigger the service group to switch to another node in the cluster. This handling of a software failure is also true in a redundant environment.
Database corruption. A cluster cannot survive this kind of failure. Depending on the architecture, a redundant solution should be able to survive database corruption.