Highly-available services using the primary-backup approach

The authors derive lower bounds and the corresponding optimal protocols for three parameters for synchronous primary-backup systems. They compare their results with similar results for active replication in order to determine whether the common folklore on the virtues of the two approaches can be shown formally. They also extend some of their results to asynchronous primary-backup systems. They implement an important subclass of primary-backup protocols that they call 0-blocking. These protocols are interesting because they introduce no additional protocol related delay into a failure-free service request. Through implementing these protocols the authors hope to determine the appropriateness of their theoretical system model and uncover other practical advantages or limitations of the primary-backup approach