Can Terminal Services be considered Virtualization?
Virtualization is a hot topic and at the moment very hyped up. Manufacturers would like to use that hype to boost their products by linking it to the virtualization market. In this craze Terminal Services was also labeled as a “Virtualization product”. In this article let’s look at the facts and I’ll also give my opinion about this virtualization label.
Although virtualization techniques were mentioned a long time ago (around 1960), within the ICT market the launch of VMWare caused the big success of the virtualization market. Their server virtualization product, which made it possible to run multiple servers on one physical system, started the virtualization space. After server virtualization other virtualization products and fields followed quickly like application virtualization, operating system virtualization and desktop virtualization. Products which were already available before the virtualization market want to hitch a ride on the virtualization craze. I was a bit surprised when both Microsoft and Citrix determined that Terminal Services and Citrix Presentation Server are virtualization products.
Before we can start determining whether Terminal Services can be labeled as a virtualization product, we need to first find out what the definitions of virtualization and terminal services are.
Virtualization is a broad term that refers to the abstraction of computer resources. Virtualization hides the physical characteristics of computing resources from their users, be they applications, or end users. This includes making a single physical resource (such as a server, an operating system, an application, or storage device) appear to function as multiple virtual resources; it can also include making multiple physical resources (such as storage devices or servers) appear as a single virtual resource (Source: Wikipedia ).
Terminal Services is one of the components of Microsoft Windows (both server and client versions) that allows a user to access applications and data on a remote computer over any type of network, although normally best used when dealing with either a Wide Area Network (WAN) or Local Area Network (LAN), as ease and compatibility with other types of networks may differ. Terminal Services is Microsoft's implementation of thin-client terminal server computing, where Windows applications, or even the entire desktop of the computer running terminal services, are made accessible to a remote client machine. (Source: Wikipedia).
Terminal Services Virtualization?
Both Microsoft and Citrix are using the virtualization space to position their Terminal Services/Citrix Presentation Server/XenApp product features. Microsoft calls it presentation virtualization, while Citrix used the term session virtualization. Microsoft also describes Terminal Service virtualization as follows:
Microsoft Terminal Services virtualizes the presentation of entire desktops or specific applications, enabling your customers to consolidate applications and data in the data center while providing broad access to local and remote users. It lets an ordinary Windows desktop application run on a shared server machine yet present its user interface on a remote system, such as a desktop computer or thin client. (Source: Microsoft.com).
If we go a bit deeper, Microsoft is describing their interpretation of presentation virtualization as follows: Presentation virtualization isolates processing from the graphics and I/O, making it possible to run an application in one location but have it controlled in another. It creates virtual sessions, in which the executing applications project their user interfaces remotely. Each session might run only a single application, or it might present its user with a complete desktop offering multiple applications. In either case, several virtual sessions can use the same installed copy of an application (Source: Microsoft.com).
Ok, now we have the definitions of virtualization, terminal services and the way Microsoft explains why terminal services are a virtualization technique, it is time to determine if Microsoft is right with their assumption.
Terminal Services is virtualization!
Reading the explanation of virtualization, two important definitions are mentioned: abstraction and hiding the physical characteristics.
From the user perspective the application is not available on his workstation/thin client, but is running somewhere else. Using the definition of hiding physical characteristics, Terminal Services can be seen, from a user perspective, as virtualization. Because the application is not installed locally the user does not have any physical identification with the application.
With the IT perspective in mind Terminal Service can also be seen as virtualization based on the definition that (physical) resources can function as multiple virtual resources. Traditionally, installed applications on a local workstation can be started by one user at a time. By installing the application on a Terminal Server (in combination with a third party SBC add-on) applications can be used by more users at the same time. Although an application cannot be seen as a 100% physical resource, you can see Terminal Services as a way of offering a single resource that will be shown as multiple virtual resources.
In summary, Terminal Services can be seen as virtualization because the application is abstracted from the local workstation and the application appears to function as multiple virtual resources.
Terminal Services is not virtualization!
However, let’s take a closer look at the physical resources. Hardware virtualization, application virtualization and OS virtualization really do separate from the physical resource. With application virtualization the application is not physically available on the system, OS virtualization does not need a hard disk to operate, and with hardware virtualization the virtual machine does not communicate (directly) with real hardware. However Terminal Services, from an IT perspective, still needs physical resources. Terminal Services is not really virtualizing anything, only the location where the application/session is started and the methodology of displaying the application to the user are different. In other words, as Microsoft describes in their explanation, Terminal Services isolates processing from the graphics and I/O, but this is still done using another device without an additional layer in between.
Back to the main question: is Terminal Services virtualization? And the answer is …… it depends. It depends how you look at the concept of virtualization and your point of view on Terminal Services. Terminal Service can be seen as virtualization if you check it from the user perspective view (the application is not running physically on the workstation or thin client) or the view that a single application/session can be used at once by more than one user. If you look at how other virtualization techniques work, Terminal Services does not function the same way and physically nothing is running in a separate layer.
So there is no clear answer and the answer is subjective depending on how you look at virtualization and Terminal Services. My personal opinion is that Terminal Services cannot be labeled as virtualization, because it is not comparable with other virtualization techniques. Through my eyes Terminal Services is not adding an additional (virtualization) layer, but is only dividing the processes between two systems. I think both Microsoft and Citrix are using the “virtualization” term to gain advantages through the current boom of the virtualization market, but both know that if you look at the IT techniques it is not “real” virtualization.