According to (NIST, 2011) and ( Chaudhry , 2013), cloud computing exhibits its own essential characteristics such as; on demand service, wide network approach, resource pooling, fast elasticity and measured service which demonstrate their relation to, and differences from, traditional computing approaches:
On-demand self-service means a consumer can unilaterally provision computing capabilities, such as server time and network storage, as needed automatically without requiring human interaction with each service provider. Cloud computing focuses on IT services which are driven by users’ requests. It provides a means of delivering computing services that makes the underlying technology, beyond the user device, almost invisible
Wide network approach permits customers to acquire computing resources IT services anytime, anywhere, and through the user-chosen device (e.g., mobile phones, tablets, laptops, and workstations). Users accessing services via Internet technologies expect a secure, “always-on” computing infrastructure that delivers as easily and reliably as electricity from a wall outlet
When we say resources pooling the provider’s computing resources are pooled to serve multiple consumers using a multi-tenant model, with different physical and virtual resources dynamically assigned and reassigned according to consumer demand. There is a sense of location-independence in that the customer generally has no control or knowledge over the exact location of the provided resources but may be able to specify location at a higher level of abstraction (e.g., country, state, or datacenter). Examples of resources include storage, processing, memory, and network bandwidth.
A cloud service has characteristics of fast elasticity means that provides the quick and self-regulating raise and reduces to the quantity of present computer transforming, storage and network bandwidth as a request by customer requirement. It is assumed that changes in the resource infrastructure needs are announced first to the middleware manager and then that such changes can be maintained automatically.
Cloud computing resource usage can be measured, controlled, and reported providing transparency for both the provider and consumer of the utilized service. Cloud computing services use a metering capability which enables to control and optimize resource use. This implies that just like airtime, electricity or municipality water IT services are charged per usage metrics – pay per use. The more you utilize the higher the bill. Just as utility companies sell power to subscribers, and telephone companies sell voice and data services, IT services such as network security management, data center hosting or even departmental billing can now be easily delivered as a contractual service.
2.3.2. Multi-tenancy
Although not an essential characteristic of Cloud Computing in NIST’s model, CSA(Cloud Security alliance ) has identified multi-tenancy as an important element of the cloud. A tenant is any application either inside or outside the enterprise that needs its own secure and exclusive virtual computing environment. This environment can encompass all or some select layers of enterprise architecture, from storage to user interface. All interactive applications (or tenants) have to be multi-user in nature 13.
Multi-tenancy is a property of a system where multiple customers, so-called tenants, transparently share the system’s resources, such as services, applications, databases, or hardware, with the aim of lowering costs, while still being able to exclusively configure the system to the needs of the tenant 9. Multi-Tenancy occurs when two or more virtual machines (VMs) belonging to different customers share the same physical machine 11.
The advantage of multi-tenancy are reducing operational costs by dividing hardware and software resources among the different tenants, simplifying the maintenance and management effort. All these benefits of multi-tenancy give result in lower application costs and give the major benefit to small and medium enterprises (SME) 10. Multi-tenancy service requirements for cloud services providers are policy-driven enforcement, segmentation, isolation of tenant data, Isolation of the tenant workspace (memory), Isolation of tenant execution, chargeback/billing models for different consumer constituencies ,Tenant-aware security, monitoring, management, reporting and self-service administration, Isolation of tenant customizations and extensions to business logic, tenant-aware version control, Tenant-aware error tracking and recovery.


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