A cluster is a group of systems that work together in such a way that the whole functions as a single unit. Such clusters can be loosely coupled or tightly coupled. A loosely coupled environment, each system is complete in itself, and can handle all of the tasks any of the other systems can handle. The environment provides mechanisms for redundancy, load sharing, and fail-over between systems, and is often called a High Availability (HA) cluster. In a tightly coupled environment, the systems involved are highly dependent on one another, often sharing memory and disk storage, and all work on the same task together. The environment provides mechanisms for data sharing, avoiding storage conflicts, keeping the systems in synchronization, and splitting up tasks appropriately. This design is often used in super-computing environments.
Qustering is an advanced technique that involves more than just installing and configuring software. It also involves hardware integration, and systems and network design, and implementation. Along with the URLs mentioned below, a good text on the subject is Building Gustered Linux Systems, by Robert W. Lucke, Prentice Hall. Here we will only touch the very basics, along with what tools Debian provides.
Let's take a brief look at each environment, and some of the tools used to create them.
