1.2 Types of Clusters

Originally, "clusters" and "high-performance computing" were synonymous. Today, the meaning of the word "cluster" has expanded beyond high-performance to include high-availability (HA) clusters and load-balancing (LB) clusters. In practice, there is considerable overlap among these-they are, after all, all clusters. While this book will focus primarily on high-performance clusters, it is worth taking a brief look at high-availability and load-balancing clusters.

High-availability clusters, also called failover clusters, are often used in mission-critical applications. If you can't afford the lost business that will result from having your web server go down, you may want to implement it using a HA cluster. The key to high availability is redundancy. An HA cluster is composed of multiple machines, a subset of which can provide the appropriate service. In its purest form, only a single machine or server is directly available-all other machines will be in standby mode. They will monitor the primary server to insure that it remains operational. If the primary server fails, a secondary server takes its place.

The idea behind a load-balancing cluster is to provide better performance by dividing the work among multiple computers. For example, when a web server is implemented using LB clustering, the different queries to the server are distributed among the computers in the clusters. This might be accomplished using a simple round-robin algorithm. For example, Round-Robin DNS could be used to map responses to DNS queries to the different IP addresses. That is, when a DNS query is made, the local DNS server returns the addresses of the next machine in the cluster, visiting machines in a round-robin fashion. However, this approach can lead to dynamic load imbalances. More sophisticated algorithms use feedback from the individual machines to determine which machine can best handle the next task.

Keep in mind, the term "load-balancing" means different things to different people. A high-performance cluster used for scientific calculation and a cluster used as a web server would likely approach load-balancing in entirely different ways. Each application has different critical requirements.

To some extent, any cluster can provide redundancy, scalability, and improved performance, regardless of its classification. Since load-balancing provides greater availability, it is not unusual to see both load-balancing and high-availability in the same cluster. The Linux Virtual Server Project (LVSR) is an example of combining these two approaches. An LVSR server is a high-availability server implemented by distributing tasks among a number of real servers. Interested readers are encouraged to visit the web pages for the Linux Virtual Server Project (http://www.linux-vs.org) and the High-Availability Linux Project (http://www.linux-ha.org) and to read the relevant HOWTOs. OSCAR users will want to visit the High-Availability OSCAR web site http://www.openclustergroup.org/HA-OSCAR/.