Citrix - XenApp

XenApp Architecture

Citrix XenApp is an end-to-end application delivery solution that manages applications in the datacenter and delivers them to users on any device. Applications can either be installed, streamed to the XenApp server or streamed directly to the client device, resulting in simplified management and reduced application operating costs. XenApp is a critical component in any Citrix virtualized environment; therefore it is necessary that Citrix engineers are able to identify how XenApp works and how it fits into a virtual solution.

Application Delivery

When delivering an application with XenApp, it is important for an engineer to understand the different application delivery methods available and to determine which method best fits the needs of the environment. Depending on the type of application, the users accessing the application and the delivery method, applications can be:

• Installed on a XenApp server and accessed as an online application
• Streamed from a file or web server to a XenApp server and accessed as an online application
• Streamed from a file or web server to the client device and accessed as an offline application
• Accessed as a XenApp server-hosted desktop 

Online Applications Installed on a XenApp Server

An online application is processed on the XenApp server and only virtual channel information is sent to the client.

The Citrix Online plug-in is required to access online applications installed on XenApp servers.

Benefits

Considerations

Applications are installed centrally on XenApp servers, which can ease application maintenance. This method makes use of resources available to the host server operating system, which allows for scaling of application performance regardless of the endpoint device in use. Only virtual channel information is sent to the client, which minimizes bandwidth and increases security. Applications requiring significant client and server bandwidth are good candidates for online applications.

Application resources need to be installed at the server level because online applications are processed on the XenApp servers. XenApp server resources must be increased in order to scale the number of concurrent online applications in use. Applications that will not run on a server OS or on a multi-user kernel should not be installed directly on a server.

Online Applications Streamed to a Server

An application that is streamed to a XenApp server allows users to access it as if it were installed directly on the XenApp server.

Benefits	Considerations
This method allows for a centralized update process across multiple XenApp servers. This method allows for multiple versions of an application to be delivered without using separate XenApp servers. For example, Microsoft Word 2003 and 2007 can be delivered from the same XenApp server.	It is recommended to pre-cache the streamed application on the XenApp servers to enhance performance and reduce the application launch time on the client.

Offline Application

An application that is streamed to the client is streamed from a file or web server and launches on the client device as a local application. This offline application uses the client device resources as opposed to using XenApp server resources.

Benefits

Considerations

Endpoint device resources can be used for an application that requires large amounts of processing power. Applications do not need to be installed into the virtual desktop image. Application compatibility with the desktop operating system can be better than running from a server-based computing model. Multiple versions of the same application can be streamed and run at the same time on a virtual desktop An application that requires advanced video capabilities are good candidates for offline applications.

The Citrix Offline plug-in must be installed on client devices to access offline application. Clients that do not support application streaming, such as non-Windows clients, cannot launch offline applications Applications are processed within an isolated virtual environment when launched on a client device. Some applications are not suited for offline application packaging, such as those that install drivers and services.

Server-hosted Desktop

Publishing a desktop provides users access to the desktop of a XenApp server and the resources available on the server.

Benefits

Considerations

Provides users with a familiar interface. Allows thin clients to replace full PCs or desktops. Allows remote or home users to access a full corporate workspace.

Provides users access to the resources on a server which can result in configurations and settings being changed causing server vulnerabilities. Requires, as a recommendation, limiting the use of server-hosted desktops and locking down access to server resources is highly recommended. Consumes more resources on a XenApp server because an entire desktop is provided. for each connected user and the desktop interactions must be transferred to the XenApp server.