An introduction to web applications architecture

Broad network access – Capabilities are available over the network and accessed through standard mechanisms that promote use by heterogeneous thin or thick client platforms (e.g. mobile phones, tablets, laptops, and workstations).

Resource 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 data enter). Examples of resources include storage, processing, memory, and network bandwidth.

Rapid elasticity – Capabilities can be elastically provisioned and released, in some cases automatically, to scale rapidly outward and inward commensurate with demand. To the consumer, the capabilities available for provisioning often appear to be unlimited and can be appropriated in any quantity at any time.

Measured service – Cloud systems automatically control and optimize resource use by leveraging a metering capability at some level of abstraction appropriate to the type of service (e.g. storage, processing, bandwidth, active user accounts). Resource usage can be monitored, controlled, audited, and reported, providing transparency for both the provider and consumer of the utilized service.

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NIST (2011) defines three service models:
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  Software as a Service (SaaS) – provides applications designed for end-users, delivered over the web.
  
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  Platform as a Service (PaaS) – provides tools and services for developing and deploying applications.
  
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  Infrastructure as a Service (IaaS) – provides servers, storage, network services and virtual machines.
  

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As these service models offer increasing levels of abstraction they are often depicted as a stack of service layers – infrastructure, platform, and software-as-a-service layers – though they may not be physically implemented by a tiered architecture (Figure 3).
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Figure 3 Cloud computing depicted as a stack of service providing layers
View description - Figure 3 Cloud computing depicted as a stack of service providing layers
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