Why published desktop starts but not seamless application ?

On a XenApp 6.5 farm, when a seamless app is launched, everything seems to be OK but the app never appears. On the other hand, when a desktop is launched, everything is ok without any problem. In addition, sometimes, seamless apps are launched without error.

I write this article because the way we investigate is far away the solution and it takes us a couple of days to find out where the problem is.

Hope it hepls…

The facts

We have a XenApp 6.5 farm, running on VMWare ESX farm.

Most of the time, when we try to launch seamless app, the Receiver progress bar reach the end of the process but the app is never launched. On the server side, we can’t see the user session. No event seems to be helpful (server-side nor client-side).

When we launch a Desktop, everything runs fine without error.

The investigation

What is the difference between seamless apps and desktops? In our case, everything is similar: same server, same GPO, same scripts, etc. We also use the same user and client computer for the tests.

Is something different in the logon process? Something is different, but not in the logon process. Actually it’s more in the way to logoff.

When you close a seamless application, you don’t really logoff. Citrix does it for you. One process is responsible for logoff users when your application is closed. This is WfShell.exe.

This links may help to understand: http://support.citrix.com/article/CTX891671, http://www.virtualizationadmin.com/articles-tutorials/terminal-services/general/seamless-application-explained.html.

Now, we think we find out where the problem is, but we don’t really know why it happens.

The Solution

Actually, the problem is not only at the Citrix level. It is caused by an incompatibility between XenApp and VMware EVC. EVC is a feature that allows you to have different CPU in a single VMWare Cluster (see here for details). When EVC is enable, you may encounter the problem. More or less frequently. In our case, we saw the problem more frequently on our dev environment than in our production environment.

http://support.citrix.com/article/CTX131628

http://support.citrix.com/article/CTX128953

Of course, we can’t disable EVC because we want to migrate VM across physical hosts. So, we just set the LogoffCheckerStartupDelayInSeconds as 10, and the problem disappear.

To do that, just set the following registry key :

• Key : HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Control\Citrix\wfshell\TWI
• Value : LogoffCheckerStartupDelayInSeconds
• Type : DWord (32bit)
• Value : 10

According to Citrix KB, the problem happens in those conditions:

• Client USB plug and play device redirection.
• File explorer publication.
• Print Manager Publication.
• Control Panel publication.