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|“||refers to the arrangement, coordination and management of cloud infrastructure to provide different cloud services to meet IT and business requirements.||”|
Figure 15 shows a generic stack diagram of this composition that underlies the provisioning of cloud services.
A three-layered model is used in this representation, representing the grouping of three types of system components cloud providers need to compose to deliver their services.
In the model shown in Figure 15, the top is the service layer, this is where cloud providers define interfaces for cloud consumers to access the computing services. Access interfaces of each of the three service models are provided in this layer. It is possible, though not necessary, that SaaS applications can be built on top of PaaS components and PaaS components can be built on top of IaaS components. The optional dependency relationships among SaaS, PaaS, and IaaS components are represented graphically as components stacking on each other; while the angling of the components represents that each of the service component can stand by itself. For example, a SaaS application can be implemented and hosted on virtual machines from an IaaS cloud or it can be implemented directly on top of cloud resources without using IaaS virtual machines.
The middle layer in the model is the resource abstraction and control layer. This layer contains the system components that cloud providers use to provide and manage access to the physical computing resources through software abstraction. Examples of resource abstraction components include software elements such as hypervisors, virtual machines, virtual data storage, and other computing resource abstractions. The resource abstraction needs to ensure efficient, secure, and reliable usage of the underlying physical resources. While virtual machine technology is commonly used at this layer, other means of providing the necessary software abstractions are also possible. The control aspect of this layer refers to the software components that are responsible for resource allocation, access control, and usage monitoring. This is the software fabric that ties together the numerous underlying physical resources and their software abstractions to enable resource pooling, dynamic allocation, and measured service. Various open source and proprietary cloud software are examples of this type of middleware.
The lowest layer in the stack is the physical resource layer, which includes all the physical computing resources. This layer includes hardware resources, such as computers (CPU and memory), networks (routers, firewalls, switches, network links and interfaces), storage components (hard disks) and other physical computing infrastructure elements. It also includes facility resources, such as heating, ventilation and air conditioning (HVAC), power, communications, and other aspects of the physical plant.
Following system architecture conventions, the horizontal positioning, i.e., the layering, in a model represents dependency relationships — the upper layer components are dependent on adjacent lower layer to function. The resource abstraction and control layer exposes virtual cloud resources on top of the physical resource layer and supports the service layer where cloud services interfaces are exposed to cloud consumers, while cloud consumers do not have direct access to the physical resources.
- NIST Special Publication 500-292, at 12-14.