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Cloud Computing Reference, Model, , It is possible to organize all the concrete realizations of cloud computing into a, layered view covering the entire stack from hardware appliances to software, systems., , Cloud infrastructure can be heterogeneous in nature because a variety of resources,, such as clusters and even networked PCs, can be used to build it. Moreover,, database systems and other storage services can also be part of the infrastructure., , The physical infrastructure is managed by the core middleware, the objectives of, which are to provide an appropriate runtime environment for applications and to best, utilize resources. At the bottom of the stack, virtualization technologies are used to, guarantee runtime environment customization, application isolation, sandboxing,, and quality of service. Hardware virtualization is most commonly used at this level., Hypervisors manage the pool of resources and expose the distributed infrastructure, as a collection of virtual machines., , The combination of cloud hosting platforms and resources is generally classified as, a Infrastructure-as-a-Service ([aaS) solution. We can organize the different examples, of IaaS into two categories: Some of them provide both the management layer and, the physical infrastructure; others provide only the management layer (IaaS (M)). In, this second case, the management layer is often integrated with other IaaS solutions, that provide physical infrastructure and adds value to them.
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Cloud Computing Reference, Model, , IaaS solutions are suitable for designing the system infrastructure but provide, limited services to build applications. Such service is provided by cloud, programming environments and tools, which form a new layer for offering users a, development platform for applications. The range of tools include Web-based, interfaces, command-line tools, and frameworks for concurrent and distributed, programming. In this scenario, users develop their applications specifically for the, cloud by using the API exposed at the user-level middleware. For this reason, this, approach is also known as Platform-as-a-Service (PaaS) because the service offered, to the user is a development platform rather than an infrastructure., , In the case of Pure PaaS, only the user-level middleware is offered, and it has to be, complemented with a virtual or physical infrastructure., , The top layer of the reference model depicted in Figure contains services delivered, at the application level. These are mostly referred to as Software-as-a-Service, (SaaS)., , As a reference model, it is then expected to have an adaptive management layer in, charge of elastically scaling on demand. SaaS implementations should feature such, behavior automatically, whereas PaaS and IaaS generally provide this functionality, as a part of the API exposed to users.
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XaaS : Everything as a service, Model, , everything as a Service (XaaS). This is one of the most important elements of, cloud computing: Cloud services from different providers can be combined to, provide a completely integrated solution covering all the computing stack of a, system. IaaS providers can offer the bare metal in terms of virtual machines, where PaaS solutions are deployed. When there is no need for a PaaS layer, it is, possible to directly customize the virtual infrastructure with the software stack, needed to run applications., , Table 4.1 summarizes the characteristics of the three major categories used to, classify cloud computing solutions. In the following section, we briefly discuss, these characteristics along with some references to practical implementations.
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Category, SaaS, , PaaS, , laaS/HaaS, , Characteristics, , Customers are provided with applications that, are accessible anytime and from anywhere., , Customers are provided with a platform for, developing applications hosted in the cloud., , Customers are provided with virtualized, hardware and storage on top of which they, can build their infrastructure., , Product Type, , Web applications and, services (Web 2.0), , Programming APIs and, frameworks, Deployment systems, , Virtual machine, management, infrastructure, , Storage management, Network management, , Vendors and, Products, , SalesForce.com, (CRM), , Clarizen.com (project, management), Google Apps, , Google AppEngine, Microsoft Azure, Manjrasoft Aneka, Data Synapse, , Amazon EC2 and $3, GoGrid, Nirvanix
