ensure that no conflicting operations occur. This function, commonly known
as a distributed lock manager (DLM), is included in some cluster technology.
Cluster technology is changing rapidly. Some cluster products support
dozens of systems in a cluster, as well as clustered nodes that are separated
by miles. Many of these improvements are made possible by storage-area
networks (SANs), as described in Section 10.3.3, which allow many systems
to attach to a pool of storage. If the applications and their data are stored on
the SAN, then the cluster software can assign the application to run on any
host that is attached to the SAN. If the host fails, then any other host can take
over. In a database cluster, dozens of hosts can share the same database, greatly
increasing performance and reliability. Figure 1.8 depicts the general structure
of a clustered system.