In IT, infrastructure encompasses the computing devices that you use to run software as well as all the other sup-porting components for those computing devices. The computing device can be a smart phone, a tablet such as an iPad, a laptop, or a desktop computer. Take email as an ex – ample. In order to deliver emails to your computing device,a mail server is employed that sits in a data center. This mail server receives mail from the Internet and routes it to you via the local area network; it is an application that is hosted on a computing device called a server. The server serves client devices, such as your tablet or laptop, and this architecture is referred to as the client–server model. A network is needed to connect the server to its clients, and so the network also is part of the IT infrastructure as are the network devices such as firewalls and switches. Your emails are stored on disks in the data center. These disks are linked in arrays, and they need to be very fast in order to serve a number of email accounts at any one time. Usu-ally the disk arrays are configured as Storage Area Network (SAN) or as Network Attached Storage (NAS) devices. For our purposes, it is not important to examine these devices further, so we will refer to them generally as storage de-vices. These too form part of the IT infrastructure. Thus IT infrastructure is comprised of end-user computing de-vices (laptops, smart phones, tablets, etc.), the servers in the data center that serve those end-user devices, and the storage, the network, and networking devices. Applica-tions that perform mundane, day-to-day tasks, such as routing emails, are also part of IT infrastructure. These applications sit just above the operating system and be-low the software layer. They form the integration layer between the operating system and the other applications, and are considered the “glue” between applications by en-abling communication and management of data among them; these integration applications are commonly cat-egorized as middleware and are, as such, components of the IT infrastructure.
In cloud computing, the IT infrastructure is grouped into two distinct granularities. All the physical devices such as the server, network, storage, and computing device, are denoted “infrastructure.” When the servers and comput-ing devices are combined with their operating system and middleware, they are referred to as “platform.” Hence both infrastructure as a service (IaaS) and platform as a service (PaaS) are part of the IT infrastructure stack. Below we consider some of the principal use cases for both of these abstraction levels. Remember, you can build a SaaS service by using PaaS and writing an application on it that has multi-tenancy. Similarly you can build upon PaaS and SaaS by creating an information dissemination service (INaaS) or a business process service (BPaaS), with the addition of information or business processes, respectively. In the same manner, PaaS builds upon IaaS through the addition of an operating system and middleware to the hardware.