Platform as a service
Platform as a service (PaaS) is a category of cloud computing services that provides a computing platform and a solution stack as a service. Along with software as a service (SaaS) and infrastructure as a service (IaaS), it is a service model of cloud computing. In this model, the consumer creates the software using tools and/or libraries from the provider. The consumer also controls software deployment and configuration settings. The provider provides the networks, servers, storage, and other services that are required to host the consumer's application.
PaaS offerings facilitate the deployment of applications without the cost and complexity of buying and managing the underlying hardware and software and provisioning hosting capabilities.
There are various types of PaaS vendors; however, all offer application hosting and a deployment environment, along with various integrated services. Services offer varying levels of scalability and maintenance.
PaaS offerings may also include facilities for application design, application development, testing, and deployment as well as services such as team collaboration, web service integration, and marshalling, database integration, security, scalability, storage, persistence, state management, application versioning, application instrumentation, and developer community facilitation.
- Add-on development facilities
These facilities allow customization of existing software-as-a-service (SaaS) applications, and in some ways are the equivalent of macro language customization facilities provided with packaged software applications such as Lotus Notes, or Microsoft Word. Often these require PaaS developers and their users to purchase subscriptions to the co-resident SaaS application.
- Stand alone development environments
Stand-alone PaaS environments do not include technical, licensing or financial dependencies on specific SaaS applications or web services, and are intended to provide a generalized development environment.
- Application delivery-only environments
Delivery-only PaaS offerings do not include development, debugging and test capabilities as part of the service, though they may be supplied offline (via an Eclipse plugin for example). The services provided generally focus on security and on-demand scalability.
- Open platform as a service
This type of PaaS does not include hosting as such, rather it provides open source software to allow a PaaS provider to run applications. For example, AppScale allows a user to deploy some applications written for Google App Engine to their own servers, providing datastore access from a standard SQL or NoSQL database. Some open platforms let the developer use any programming language, any database, any operating system, any server, etc. to deploy their applications.
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- Google angles for business users with 'platform as a service'
- Comparing Amazon’s and Google’s Platform-as-a-Service (PaaS) Offerings | Enterprise Web 2.0 | ZDNet.com
- Platform-as-a-Service Architecture for Real-Time Quality of Service Management in Clouds 
- Chen, Tse-Shih, et al. "Platform-as-a-Service Architecture for Parallel Video Analysis in Clouds." Advances in Intelligent Systems and Applications-Volume 2. Springer Berlin Heidelberg, 2013. 619-626.
- Using the Google Plugin for Eclipse
- AppScale: Open Source Platform
- Interview with inventor of Open Platform as a Service