Other forms of clusters include parallel clusters and clustering over a
wide-area network (WAN) (as described in Chapter 17). Parallel clusters allow
multiple hosts to access the same data on shared storage. Because most
operating systems lack support for simultaneous data access by multiple hosts,
parallel clusters usually require the use of special versions of software and
special releases of applications. For example, Oracle Real Application Cluster
is a version of Oracle’s database that has been designed to run on a parallel
cluster. Each machine runs Oracle, and a layer of software tracks access to the
shared disk. Each machine has full access to all data in the database. To provide
this shared access, the system must also supply access control and locking to