Platform-as-a-Service Reference Model

Platform-as-a-Service (PaaS) solutions provide a development and deployment platform for running applications in the cloud. They constitute the middleware on top of which applications are built.

Application management is the core functionality of the middleware. PaaS implementations provide applications with a runtime environment and do not expose any service for managing the underlying infrastructure. They automate the process of deploying applications to the infrastructure, configuring application components,provisioningandconfiguringsupportingtechnologiessuchas load balancers and databases, and managing system change based on policies set by the user. Developers design their systems in terms of applications and are not concerned with hardware (physical or virtual), operating systems, and other low-level services. The core middleware is in charge of managing the resources and scaling applications on demand or automatically, according to the commitments made with users. From a user point of view, the core middleware exposes interfaces that allow programming and deploying applications on the cloud. These can be in the form of a Web-based interface or in the form of programming APIs and libraries.

The specificdevelopmentmodeldecidedforapplicationsdeterminestheinterfaceexposedto the user. Some implementations provide a completely Web-based interface hosted in the cloud and offer a variety of services. It is possible to find integrated developed environments based on 4GL and visual programming concepts, or rapid prototyping environments where applications are built by assembling mash-ups and user-defined components and successively customized. Other implementations of the PaaS model provide a complete object model for representing an application and provide a programming language-based approach. This approach generally offers more flexibility and opportunities but incurs longer development cycles. Developers generally have the full power of programming languages such as Java, NET, Python, or Roby, with some restrictions to provide better scalability and security. In this case, the traditional development environments can be used to design and develop applications, which are then deployed on the cloud by using the APIs exposed by the PaaS provider. Specific components can be offered together with the development libraries for better exploiting the services offered by the PaaS environment. Sometimes a local runtime environment that simulates the conditions of the cloud is given to users for testing their applications before deployment. This environment can be restricted in terms of features, and it is generally not optimized for scaling.

PaaS solutions can offer middleware for developing applications together with the infrastructure or simply provide users with the software that is installed on the user premises. In the first case, the PaaS provider also owns large data centers where applications are executed; in the second case, referred to in this book as Pure PaaS, the middleware constitutes the core value of the offering. It is also possible to have vendors that deliver both middleware and infrastructure and ship only the middleware for private installations.

The PaaS umbrella encompasses a variety of solutions for developing and hosting applications in the cloud. Despite this heterogeneity, it is possible to identify some criteria that are expected to be found in any implementation. As noted by Sam Charrington, product manager at Appistry.com, 2 there are some essential characteristics that identify a PaaS solution: 

• Runtime framework.  This framework represents the “software stack” of the PaaS model and the most intuitive aspect that comes to people’s minds when they refer to PaaS solutions. The runtime framework executes end-user code according to the policies set by the user and the provider. 
• Abstraction. PaaS solutions are distinguished by the higher level of abstraction that they provide. Whereas in the case of IaaS solutions, the focus is on delivering “raw” access to virtual or physical infrastructure, in the case of PaaS the focus is on the applications the cloud must support. This means that PaaS solutions offer a way to deploy and manage applications on the cloud rather than a bunch of virtual machines on top of which the IT infrastructure is built and configured. 
• Automation. PaaS environments automate the process of deploying applications to the infrastructure, scaling them by provisioning additional resources when needed. This process is performed automatically and according to the SLA made between the customers and the provider. This feature is normally not native in IaaS solutions, which only provide ways to provision more resources. 
• Cloud services. PaaS offerings provide developers and architects with services and APIs, helping them to simplify the creation and delivery of elastic and highly available cloud applications. These services are the key differentiators among competing PaaS solutions and generally include specific components for developing applications, and advanced services for application monitoring, management, and reporting.

Another essential component for a PaaS-based approach is the ability to integrate third-party cloud services offered from other vendors by leveraging service-oriented architecture. Such integration should happen through standard interfaces and protocols. This opportunity makes the development of applications more agile and able to evolve according to the needs of customers and users. Many of the Paas offer provide this facility, which is naturally built into the framework they leverage to provide a cloud computing solution.

The impact of the vendor lock-in on applications obviously varies according to the various solutions. Some of them, Such as Force.com, rely on a proprietary runtime framework, which makes there targeting process very difficult. Others, such as GoogleAppEngine and Microsoft Azure, rely on industry-standard routines but utilize private data storage facilities and computing infrastructure. In this case it is possible to find alternatives based on PaaS solutions implementing the same interfaces, with perhaps different performance. Others, such as Appistry Cloud IQ Platform, Heroku, and Engine Yard, completely rely on open standards, thus making the migration of applications easier.

Happy Exploring!