Citrix XenApp: A SaaS Hallmark

by Dave Nowicki, Director of Technology at DP Solutions

Anyone in the IT field most certainly has heard of “Citrix,” but unless you have firsthand experience with a Citrix system, it’s not technology that the mass majority of IT resources may necessarily be familiar with.

Citrix was founded in 1989 by a group of ex-IBM employees that wanted to capitalize on the multiuser environment present in UNIX mainframe technologies. The team initially used current OS/2 technology as the basis for their software; however, it quickly became apparent that OS/2 had a limited shelf life. With this in mind, Citrix transitioned their code to a Novel/DOS technology base, which became a huge success.

With the popularity of this multiuser Novel software, Citrix went public and the software later matured into WinView and finally to WinFrame after Microsoft first licensed the technology for the NT operating system.

Today, Citrix’s core technological offering is called XenApp, which has transformed over the years into a very mature and elegant software suite that is a hallmark of SaaS (software as a service) solutions.  In a nutshell, XenApp allows for a hosted desktop experience in a remote datacenter (or large scale customer environment) that is simple from a user standpoint, and provides excellent economy of scale from an infrastructure standpoint.

Be it a public or private cloud, remote datacenter colocation, or a small Wintel server environment for developers, XenApp refines the Microsoft Terminal Services experience. The solution allows server farms to scale easily and economically with less administrative overhead than comparable native terminal server environments.

Citrix also has a virtualization hypervisor, as well as a line of load balancer appliances.

The heart of any XenApp deployment includes a license server, web interface server, database instance, and the actual XenApp multiuser servers. Properly built Windows Server 2008R2 servers with 8GB of RAM can accommodate about 45 users per server. Utilizing a hypervisor to virtualize the XenApp servers can equate to about 500 users per 96GB hardware chassis.

It is important to note that with any terminal server implementation (XenApp or otherwise) it’s better to ‘scale out’ with more servers than to ‘scale up’ to form fewer monolithic multiuser hosts. There is a point of diminishing returns as additional memory and processors are added and the concurrent user count is no longer linear at certain levels. Additionally, this basic terminal server dogma allows N+1 configuration with relative ease.

One of the important technical points of Citrix is that it works in conjunction with Microsoft Terminal Services. In fact, before an engineer can start the base install of XenApp, the RDC (Remote Desktop Connection) role needs to be present on the servers. Citrix uses a proprietary ICA (Independent Computing Architecture) protocol for connections between the client and the hosted server. ICA protocol offers powerful features such as SpeedScreen, HDX, and various others in sundry presentation-level technologies, as well as off-line sessions, roaming technologies, and streaming application options. Distilled down, this allows a LAN experience over WAN connections by sending only the images of the windows, printers, and applications to the client. This is in stark contrast with native terminal services technology where the user is presented with an entire desktop of the hosted server using little to no optimizations.

All client ICA connections are handled via a web interface server. Options are scalable and modular and can include an https-only option called Secure Gateway to further simplify firewall and proxy rule sets. Additionally the Windows web server role can be completely replaced by one of the aforementioned hardened Citrix web appliances. Legacy administrators will be pleased to know that Program Neighborhood is end of life and no longer able to be used with XenApp farms.

As mentioned earlier, Microsoft RDC/Terminal Services is required for XenApp implementations. This also means that RDC licensing must be used in addition to XenApp licensing. However, the following benefits offset this licensing issue:

  • Less bandwidth is needed than a similar RDC connection.
  • User experience for the same amount of bandwidth is greatly improved.
  • Less memory per user session on the XenApp server. This equates to more bodies-in-seats for a given hardware platform.
  • Intelligent farm load balancing. Metrics range from basic user count all the way to XenApp aware OS performance counters.
  • Intelligent fault tolerance within the farm. Servers can be gracefully removed from service without the idiosyncrasies that are present in native RDC farms.

Citrix XenApp is one of DP Solutions’ core competencies, and is used extensively throughout our hosted and managed services portfolio. It often negates the requirement of complex VPN’s, extended Active Directory domains, and other complicated Windows infrastructure configurations to make a hosted platform in the cloud well within reach of any size customer.

To learn more about Citrix solutions for your business, contact DP Solutions at or 800-679-4377.