A document came out of Gaithersburg, Maryland in 2011. Published by the National Institute of Standards and Technology it was simply titled “The NIST Definition of Cloud Computing.” If you attended tech conferences during that period, reciting some version of that definition was pretty much a requirement. The private, public, and hybrid cloud terms were in this document. So were concepts like on-demand self-service and resource pooling. As were the familiar Infrastructure-as-a-Service (IaaS), Platform-as-a-Service (PaaS), and Software-as-a-Service (SaaS) service models.
NIST didn’t invent any of this out of whole cloth. But by packaging up a sort of industry and government consensus about the basics of cloud computing, they regularized and standardized that consensus. And, overall, it worked pretty well. Iaas was about provisioning fundamental computing resources like processing, storage, and networks. SaaS was about providing applications to consumers.
As for PaaS? PaaS was about applications created using programming languages, libraries, services, and tools supported by the provider.
Arguably, this PaaS definition was never as neat as the others. IaaS resources were easy to understand; they were like the resources you have on a server, except cloudier. And SaaS was just an app on the Web—application service providers (ASPs) reimagined, if you would. PaaS was sort of everything that was above infrastructure but below an application an end-user could run directly. Cloud-enabled middleware, hooks to add features to a single online service like Salesforce.com, single-purpose hosted programming environments (as Google App Engine and Azure were initially), and open extensible environments like OpenShift that could also be installed on-premise. Most fell broadly under the PaaS rubric.
The NIST definition also didn’t really capture how the nature of the service model depends upon the audience to an extent. Thus, Salesforce.com is primarily a SaaS as far as the end-user is concerned but it’s a PaaS in the context of developers extending a CRM application.
Today, I’d argue that the lines NIST drew probably still have some practical significance but the distinctions are increasingly frayed. IaaS platforms have almost universally moved beyond simple infrastructure. OpenStack has compute (Nova), storage (Swift and Cinder), and Networking (Neutron) components but it also includes database projects (Trove), identity management (Keystone), and the Heat orchestration engine to launch composite cloud applications.
In many cases these higher-level functions can be either used standalone or replaced/complemented by more comprehensive alternatives. For example, in a hybrid cloud environment, a cloud management platform like Red Hat CloudForms (ManageIQ is the upstream project) provides multi-cloud management and sophisticated policy controls. The IaaS+ term is sometimes used to capture this idea of more than base-level infrastructure but less than a comprehensive developer platform.
In the case of SaaS, today’s APIs everywhere world means that most things with a UI also can be accessed programmatically in various ways. In other words, they’re platforms—however limited in scope and however tied to a single application.
But, really, the fraying is broader than that. I’ve argued previously that we’re in the process of shifting toward a new style of distributed application infrastructure and of developing applications for that infrastructure. It won’t happen immediately—hence, Gartner’s bimodal IT model—but it will happen. In the process, traditional specialties/silos (depending upon your perspective) are breaking down. This is true whether you’re talking enterprise buyers/influencers, IT organizations, industry analysts, or vendors.
As a result, it's hard to separate PaaS--in the relatively narrow sense that it was first discussed--with the broader idea of an application development platform with middleware integration,messaging, mobile, etc. services. Red Hat's doing a lot of work to bridge those two worlds. For example, Red Hat’s JBoss Middleware portfolio of libraries, services, frameworks, and tools is widely used by developers to build enterprise applications, integrate applications and data, and automate business processes. With JBoss xPaaS Services for OpenShift, these same capabilities are being offered integrated with OpenShift. This lets developers build applications, integrate with other systems, orchestrate using rules and processes, and then deploy across hybrid environments.
The advantage of the xPaaS approach is that it doesn’t merely put middleware into the cloud in its traditional form. Rather, it effectively reimagines enterprise application development to enable faster, easier, and less error-prone provisioning and configuration for a more productive developer experience.Eventually all of the JBoss Middleware products will have xPaaS variants. In each case, the core product is exactly the same whether used in a traditional on-premise manner or as xPaaS, so apps can be moved seamlessly between environments. In the xPaaS environment, JBoss Middleware developers experience benefits from OpenShift-based user interface enhancements, automated configuration, and a more consistent experience across different middleware products.
Then DevOps  comes along to blur even more lines because it brings in a whole other set of, often, open source tooling including CICD (e.g. Jenkins), automation and configuration management (e.g. Ansible), collaboration, testing, monitoring, etc. These are increasingly part of that new distributed application platform as well as is the culture around iteration and collaboration that DevOps requires.
I have trouble not looking at this breakdown of historical taxonomies as a positive. It offers the possibility of more complete and better integrated application development platforms and more effective processes to use those platforms. It’s not the old siloed world any longer.
 I just published this white paper that gives my/Red Hat’s view of DevOps.
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