The services offered by cloud computing are further divided into different service models—infrastructure as a service (IaaS), platform as a service (PaaS), and software as a service (SaaS). This classification is done to segregate the different types of services a user can purchase to meet their business needs in a cloud computing environment.

IaaS is the cloud service model that enables the user provision virtualized hardware resources on demand. Physically, these resources could be spread across multiple data centers, which the service provider maintains. These resources include the virtual storage, network connections, and load balancers for the provisioned hardware resource. The user can use the resource on demand and pay per use. If the user needs more resources, the provider has the ability to automatically scale up the hardware according to the need and vice versa. A good example of an IaaS provider is the Amazon Web Services (AWS—http://aws.amazon.com). It is the most popular IaaS provider in the cloud. Rackspace (http://www.rackspace.com) is another such example.

PaaS is the cloud service model that provides the tools to build software applications on the cloud. A close analogy would be to look at PaaS as an operating system and middleware of the cloud environment. PaaS provides developers with the underlying platform to use to develop their apps. It takes care to support a specific language or technology that the stack developers want to use. Many PaaS providers also enable on-demand scaling of the underlying computer and storage resources, automatically, to free the cloud user from the job of allocating resources manually. In PaaS, the consumer of the service controls deployment and configuration. The PaaS provider provisions the servers, network, and the computational needs of the software application. The PaaS model also enables a multitenant architecture so that multiple users could use the web application in a secure, scalable, concurrent, and fail-safe manner. Sophisticated PaaS solutions also provide an integrated web application development environment, which facilitates collaborative coding, source control, and deployment. Heroku (http://www.heroku.com) and Google App Engine (http://cloud.google.com/AppEngine) are two examples of successful PaaS platforms.

The SaaS model of the cloud provides software you can consume from the purview of your web browser. There is no need for complex and time consuming installations. Open a browser, point to a URL, and use the app pointed to by the URL. What happens behind the scenes is all hidden from the user. SaaS has evolved considerably in the last decade. Many SaaS providers have made desktop or locally-hosted software obsolete. All you need is a browser and you are all set to use any app to do anything. No headaches of software upgrades, version incompatibility, or software portability. Google's Gmail (http://gmail.com) service is one of the most successful and widely known SaaS implementations. The SaaS component has grown exponentially with companies leveraging the underlying infrastructure and platform to build cloud versions of most of their software product offerings. By 2013, almost every company worth its salt has had a SaaS version of its popular software apps available to online customers.

While PaaS is inherently more stable compared to the SaaS component, PaaS has evolved tremendously in the last few years and provided the developer community with amazing tools to work with and deploy distributed apps in virtually no time. The Heroku PaaS is the subject of this book.