Highly available cluster: a case study

The methodology and design of a system that provides highly available data in a cluster is presented. A highly available cluster consists of multiple machines interconnected by a common bus. Data is replicated at a primary and one or more backup machines. Data is accessed at the primary, using a location independent mechanism that ensures data integrity. If the primary copy of the data fails, access is recovered by switching to a backup copy. Switchover is transparent to the application, hence called seamless switchover. The fault model is fail-stop. The entire cluster is resilient to at least single failures. Designating data as highly available is selective in scope, and the overhead of replication and recovery is incurred only by applications that access highly available data. An experimental prototype was implemented using IBM AS/400 machines and a high-speed bus with fiber-optic links.<>