48 International Journal of Web Services Research, 12(3), 48-77, July-September 2015 Copyright © 2015, IGI Global. Copying or distributing in print or electronic forms without written permission of IGI Global is prohibited. ABSTRACT Cloud computing services have been increasingly considered by businesses as a viable option for reducing IT expenditure. However, there are often associated problems with unmanaged accountability. This paper frst analyses the accountability properties of a cloud service and then proposes the accountable cloud service (ACS) model to address those problems. In addition, the authors argue that from an accountability perspective a cloud service is a proactive system that needs to be modeled differently from the traditional reactive systems. They extend traditional structural operational semantics to cater for modeling of actors as well as scenarios of inaction and exception in state transitions. This leads to the creation of a new form of a process algebra called Accountable Process Algebra (APA). They also propose an Obligation Flow Diagram (OFD) as a simple method for confict resolution and verifcation for the ACS model. The ACS model enables obligation specifcation, validation, decomposition, machine-interpretation, monitoring and reasoning, and ultimately facilitates accountability in cloud service consumption. Using Amazon S3 service as a case study, they show how to address those known accountability problems by using our ACS model. Finally the authors discuss the applicability of their model to cloud services in general. Modeling Accountable Cloud Services Based on Dynamic Logic for Accountability Jun Zou, Department of Computing, Macquarie University, Sydney, Australia Yan Wang, Department of Computing, Macquarie University, Sydney, Australia Mehmet A. Orgun, Department of Computing, Macquarie University, Sydney, Australia Keywords: Accountability, Accountable Cloud Service (ACS), Accountable Process Algebra (APA), BPMN2.0, Dynamic Logic, Proactive System, Process Algebra 1. INTRODUCTION 1.1. Background Recently cloud computing has emerged as a new business model that turns IT capabilities into e-services. The sustainability of this business model largely depends on whether the account- ability of cloud services can match that of the traditional IT services. By accountability, we mean a clear disclosure of service obligations; faithfully honoring disclosed obligations, or oth- erwise assuming the liability for the unsatisfac- tory performance of the obligations (Zou, Wang & Lin, 2010). Traditionally, accountability in service is achieved through the enforcement of a legal and paper-based contract. In a cloud service context, using a paper-based contract is no longer effective. The current practice is for service providers to publish a terms and DOI: 10.4018/IJWSR.2015070103