In Conclusion

The cluster's ability to reliably offer business tier applications to the end-user rests upon servers outside the cluster nodes such as data tier servers and a cluster node manager. The data tier servers ensure data integrity by implementing a lock arbitration method that protects the data from changes made by the same business tier object running concurrently on multiple cluster nodes. The cluster node manager also helps you implement security policies and control access to external devices (such as printers and serial devices) that sit outside the cluster. Care must be taken when building data tier servers and the cluster node manager to ensure that they—like the cluster itself—have no single point of failure.

With open source software, you can build an enterprise-class cluster using inexpensive commodity hardware, and if you design the cluster environment properly, you'll eliminate downtime caused by single-component failures.