Abstract
We view the cloud as a marketplace for trading instances of web services, which can be "leased" by web applications. We argue that applications can "buy" diversity by selecting instances of web services from multiple cloud sellers in this market. By diversifying the selection and allocation of web service instances, an application can potentially improve its dependability and reduce risks associated with service level agreement (SLA) violations. We propose a novel, dynamic and adaptive approach for implementing design diversity in the cloud market. The approach uses portfolio theory to construct a diversified portfolio of web service instances, which are traded from multiple cloud providers. We illustrate the applicability of the approach. Controlled experiments are also used to (i) test the approach effectiveness in minimizing the risk of SLA violation; (ii) simulate the dynamic and adaptive behaviour of the approach in responding to changes in the market conditions and risk; (iii) evaluate the sensitivity of the allocation decisions to risk and its correlation with other candidates and (iv) evaluate the scalability of the approach and its ramifications on risk reduction under extreme scenarios.