Mutant Apples: A Critical Examination of Cloud SLA Availability Definitions

The paper examines the challenges of defining and measuring availability to support real-word service comparison and dispute resolution through SLAs. We propose a rigorous and unambiguous definition of availability in cloud services. In the light of this, we show that what appear to be apples-for-apples comparisons between real-world SLAs are often based on ambiguous definitions, and even where SLAs are well defined, they differ significantly in their interpretation of availability. We show how two example real-world SLAs, would lead one service provider to report 0% availability while another would report 100% for the same system state history. On the basis of this, the paper concludes by arguing for the importance of standardising availability definitions and examines which elements need to be standardised and, just as importantly, which do not. Many of the results of this paper can be generalised to service level attributes other than availability: in general, such standard service definitions are a key element of a true commodity market in cloud resources, allowing service comparability before purchase, redress in the case of failure to deliver expected value and enhancing accountability in the supply chain.