TYPES OF CLOUD OFFERING

the level of abstractions they export to the cloud users (or programmers) and the level of computing resource management (flexibility) they offer as illustrated in Figure 2. Based on this characterization, cloud offerings can be broadly classified into three categories:
• Infrastructure-as-a-Service (IaaS): These cloud service providers offer computational resources such as servers, network, storage etc. from a shared facility managed by the provider to cloud users on an on-demand basis. Examples of IaaS providers include Amazon Web Services and Flexiscale. Note that IaaS providers allow users to dynamically grow and shrink their resource allocations to match their demands; however, the responsibility of utilizing this elasticity effectively rests with the cloud user.

• Platform-as-a-Service (PaaS): The platform as a service providers export application-development platforms that broadly fall into two categories: (1) those that export application development platforms for certain domains or class of applications (e.g., the Google AppEngine and Force.com), and (2) those that export general-purpose application development platform (e.g., Microsoft’s Azure). In either case, applications developed using these platforms are able to benefit from elasticity offered by Infrastructure-as-a-Service cloud providers. For instance, the Google AppEngine is directed at traditional web applications; applications developed using the AppEngine API can scale automatically with increase in the number of resources as well as achieve high availability through replication. Force.com exports a platform for developing business application that solely interacts with the salesforce.com database. Microsoft's Azure platform, on the other hand, allows application to be developed using .NET libraries and compiled to common runtime language environment. The .NET libraries provide a degree of scalability and fail-over support, but require developers to declaratively specify application characteristics to take advantage of these features. Users do not get control of the operating system or runtime but are allowed choice of programming languages.

• Software-as-a-Service (SaaS): The Software-as-a-service cloud service providers offer specific application services, delivered over the Internet on some form of on-demand payment system. Examples include Salesforce.com and WebEx.


In addition, there are two variants of the cloud computing offerings that describe whether or not its services are available to global or select audience:
 Public Cloud: In this variant of the cloud, services are made available to global audience on a pay-per-use fashion with utility computing being the service sold. Examples include Amazon Web Services, Google AppEngine etc.
 Private Cloud: This variant refers to internal data center of a business or other organizations which is available to members of such entities only.