Quantifying and Improving the Availability of High-Performance Cluster-Based Internet Services

Cluster-based servers can substantially increase performance when nodes cooperate to globally manage resources. However, in this paper we show that cooperation results in a substantial availability loss, in the absence of high-availability mechanisms. Specifically, we show that a sophisticated cluster-based Web server, which gains a factor of 3 in performance through cooperation, increases service unavailability by a factor of 10 over a non-cooperative version. We then show how to augment this Web server with software components embodying a small set of high-availability techniques to regain the lost availability. Among other interesting observations, we show that the application of multiple high-availability techniques, each implemented independently in its own subsystem, can lead to inconsistent recovery actions. We also show that a novel technique called Fault Model Enforcement can be used to resolve such inconsistencies. Augmenting the server with these techniques led to a final expected availability of close to 99.99%.