I. Introduction
Over the past decade, we have witnessed the rapid growth of cloud computing services such as Amazon Web Services, Google Cloud, and Microsoft Azure. Cloud computing services have generated tremendous economic values for various companies. It was reported that the annualized revenue of Microsoft Azure is 18.9 billion [1], and that of Amazon Web Services is 29.13 billion [2]. What is more promising is that the cloud computing service market is still growing [3] and a variety of companies such as Google are investing to expand their cloud computing services [4]. In general, cloud computing serves as a new computing paradigm, where cloud providers supply flexible and on-demand computing resources to cloud users.