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Top1. Introduction
Cloud computing is a new style of computing where scalable and flexible information technology (IT) related capabilities are provided as a service (Lin, Fu, Zhu, & Dasmalchi, 2009; Yeo et al., 2010). Users can access the IT resources anytime and from anywhere using multiple devices such as computer workstations, laptops and smart phones etc. (Park & Ryoo, 2012). Cost saving, dynamic scalability and high availability are some of the most significant advantages of cloud computing. Even though, cloud is a new concept, it evolved from some of the existing technologies. It has come through a long journey of computing phases and changed drastically in last few decades (Voas & Zhang, 2009). The developments in computing can be categorized as described below:
Phase 1: General purpose programmable computer;
Phase 2: Analog computers, non-flexible in nature. They were slow and had to be wired manually;
Phase 3: Digital computers to solve complex problems. These computers were fast in comparison to analog;
Phase 4: Users had dumb terminal and were sharing the powerful mainframes;
Phase 5: Users had autonomous computers with computing functionalities like a processor, memory and storage. Because of their own components, they did not depend on the mainframes;
Phase 6: PCs were connected to other computers and servers to form a network, so that the data sharing could take place. This also led to an improvement in performance;
Phase 7: Local networks were connected to global networks to form internet for global sharing;
Phase 8: Grid computing allowed the sharing of computing power and storage through distributed computing system;
Phase 9: Computing to become commodity and shared resources are provided on the Internet. Resources are scalable to cater the increased need.
At a first instance, comparing these paradigms, it appears that we have reached back to the mainframe computing. However, this is not true as cloud paradigm is significantly different from the mainframe and their differences are depicted in Table 1.
Table 1. Mainframe versus cloud computing
| Mainframe | Cloud Computing |
| Computing Power | Finite computing power | Almost infinite computing power |
| User Interface | Uses dumb terminals | Have powerful PCs as terminal which can perform computation |
| Scalability | On demand scalability is not supported | On demand easy scalability |