If all Delivery Controllers in a XenApp site fail, we can configure the VDA to operate in high availability mode so that users can continue to access and use their desktops. In high availability mode, the VDA will accept direct ICA connections from users, rather than connections brokered by the Controller.

When VDA lost its communication with the Controller, high availability mode is initiated only after a set period of time has elapsed. By default, this is 5 minutes (300 seconds). We can also configure the time period. HA mode is enabled for 30 days.

Once in HA mode, the VDA attempts to register with a Controller for up to 30 days, while the user continues to use the desktop in this mode. When the Controller later becomes available before 30 days, the desktop registers and the user’s session continues uninterrupted, but any subsequent connection is brokered by the Controller as normal. If after 30 days the desktop is unable to register with the Controller, the desktop stops listening for connections and is no longer available.

High availability mode is suitable only for use with dedicated desktops, where the mapping between the user and the VDA is known. We cannot configure high availability mode for users with pooled desktops.

To enable high availability mode in the VDA machine:

1. Set the HighAvailability and HaRegistrarTimeout values in the registry keys.
2. Provide users with an ICA launch file that will enable them to make direct ICA connections. We have to create an ICA file for each user who requires this feature.

To configure the VDA to operate in HA mode when necessary, add the following registry keys. You must do this after the VDA has been installed.

1. Add the following registry entry

HKEY_LOCAL_MACHINE\SOFTWARE\Citrix\VirtualDesktopAgent:

Name: HighAvailability

Type: REG_DWORD

Values: 1/0

1 = enables high availability mode

0 (zero) = disables high availability mode

2. Change the VDA registering time period with the Controller before initiating HA mode:

Name: HaRegistrarTimeout

Type: REG_DWORD

Value: 300. The default is 300 seconds.

3. Restart the virtual desktop.

Prepare an ICA launch file

To establish a direct ICA connection to desktops, provide users with a Minimal ICA launch file that they can use should communication with the Controller fail. We must create an ICA launch file for each user who requires this feature and users should only use this ICA launch file when it is appropriate.

Open Notepad on the client computer and copy and paste the below Minimal ICA Launch file content:

High availability mode limitations

High availability mode is suitable for use only with dedicated desktops; you cannot configure this for use with pooled desktops.

Policies originating on the Controller, such as those governing client drive mapping and access to the clipboard, will not function as there is no connection to the Controller. Policies originating from the Domain Controller and Local Group Policy are unaffected.

As it is directly connecting there will not be a communication with NetScaler Gateway and Remote Access.