A Cloud is a type of parallel and distributed system consisting of a collection of inter- connected and virtualized computers that are dynamically provisioned and presented as one or more unified computing resource(s) based on service-level agreements established through negotiation between the service provider and consumers. virtualization A technique for hiding the physical characteristics of computing resources from the way in which other systems, applications, or end users interact with those resources and reduces the per unit service cost. Because the virtualization system sits between the guest and the hardware, it can control the guest’s use of CPU, memory, and storage, even allowing a guest OS to migrate from one machine to another. Virtualization and scalability are two key properties of Cloud Computing. They make Cloud Computing different from traditional distributed systems. Cloud services and computing platforms could be scaled across geographical locations, software configuration and hardware performance. In other words, companies can scale up or down the IT infrastructure depending on their needs. Moreover, the time to start a business process is not contingent on IT infrastructure establishment time any more through virtualization E-commerce over cloud computing and virtualization

There are five categories of virtualization: full virtualization, hardware assisted, partial virtualization, para-virtualization, hybrid virtualization and operation system-level virtualization. Full virtualization, paravirtualisation and operation system-level virtualization are three commonly using virtualization techniques. Benefits of cloud computing for Cloud consumers Pay as used. Small companies do not need to invest in the initial IT infrastructure and maintain them if they adopt Cloud. They can simply pay as they used. In this way, companies can reduce the waste of underutilization. Reduce operation cost. Since Utility Computing uses virtual machine instead of physical machine, the work of hardware operation is shifted from Cloud consumers to Cloud providers. Organizations can reduce their operation cost in this way. There are also additional benefits which can be offered by Cloud Computing. For instance, start-ups can skip the hardware procurement and capital expenditure phase. They can start their business quickly. What‘s more, most of the Cloud providers will constantly update their software offering and adding new features.

Benefits for end-users End-users of Cloud service actually are the most important users. For the end-users of Cloud Computing, the incentives are similar to those motivating enterprises and organizations. They require easy-to-use interfaces, appropriately reliable and timely service delivery, information about their services, etc.. With Cloud Computing, end- users can access and update information wherever they are, rather than having to run back to their offices. Moreover, Cloud Computing accomplishes a better response time than standard server and hardware in most cases.

Benefits of Virtualization Partitioning Multiple applications and operating systems can be supported within a single physical system. Servers can be consolidated into virtual machines on either a scale-up or scale-out architecture. Computing resources are treated as a uniform pool to be allocated to virtual machines in a controlled manner. Isolation Virtual machines are completely isolated from the host machine and other virtual machines. If a virtual machine crashes, all others are unaffected. Data do not leak across virtual machines, and applications can only communicate over configured network connections. Encapsulation Complete virtual machine environment is saved as a single file; easy to back up, move, and copy. Standardized virtualized hardware is presented to the application, guaranteeing compatibility.

Hardware Hardware virtualization or platform virtualization refers to the creation of a virtual machine that acts like a real computer with an operating system. Software executed on these virtual machines is separated from the underlying hardware resources. For example, a computer that is running Microsoft Windows may host a virtual machine that looks like a computer with the Ubuntu Linux operating system; Ubuntu-based software can be run on the virtual machine. In hardware virtualization, the host machine is the actual machine on which the virtualization takes place, and the guest machine is the virtual machine. The words host and guest are used to distinguish the software that runs on the physical machine from the software that runs on the virtual machine. Different types of hardware virtualization include: Full virtualization: Almost complete simulation of the actual hardware to allow software, which typically consists of a guest operating system, to run unmodified. Partial virtualization: Some but not all of the target environment is simulated. Some guest programs, therefore, may need modifications to run in this virtual environment. Paravirtualization: A hardware environment is not simulated; however, the guest programs are executed in their own isolated domains, as if they are running on a separate system. Guest programs need to be specifically modified to run in this environment.

Memory Memory virtualization, aggregating Random Access Memory resources from networked systems into a single memory pool. Virtual memory, giving an application program the impression that it has contiguous working memory, isolating it from the underlying physical memory implementation. Memory over commitment, allows a computer to grant more memory to the virtual machines running on it than is installed on the physical computer. Network Network virtualization, creation of a virtualized network addressing space within or across network subnets. Software Operating system-level virtualization, hosting of multiple virtualized environments within a single OS instance. Application virtualization and workspace virtualization, the hosting of individual applications in an environment separated from the underlying OS. Application virtualization is closely associated with the concept of portable applications.