Understand Citrix High Availability Service when DDC loses connection with Database

Instructions

By Design:

During the period directly after the database connection is lost, resource (Application/Desktop) enumerations in StoreFront and Session launches may fail. In practice it takes ~2 minutes before the site starts working (Resources enumerating and session launching). The site will not be fully functional until all the machines have re-registered which may take up to 10 minutes. This will also depend on the number of VDAs in the environment and the number of registration requests flooding into the elected DDC.



Unless the VDAs get re-registered the connection brokering requests of the users shall not be catered. Once the VDAs get registered to the elected DDCs, the connection brokering for the users can be accommodated by any active DDCs.



NOTE:

On the elected DDC, The secondary broker (i.e. the Citrix High Availability service ) has no current VDA registration data, but as soon as a VDA communicates with it, a re-registration process is triggered. During that process, the secondary broker also gets current session information about that VDA.

While the secondary broker is handling connections, the principal broker ( Broker Service ) continues to monitor the connection to the Site database. Hence we continue to get events such as :



We will see event ID 3502 : The Citrix Broker service has reached its maximum registration limit of 10000 machines, if 10000 VDAs get registered with the elected broker .



NOTE: 
 
VDI limits:

1.    In a single-zone VDI deployment, up to 10,000 VDAs can be handled effectively during an outage.

2.    In a multi-zone VDI deployment, up to 10,000 VDAs in each zone can be handled effectively during an outage, to a maximum of 40,000 VDAs in the site. 

Reference : https://docs.citrix.com/en-us/xenapp-and-xendesktop/7-15-ltsr/manage-deployment/local-host-cache.html

Citrix Recommendations:

• Configure zones. Upto 40000 VDAs can be registered when Citrix High Availability Service takes over in a multi zone configuration.
• Avoid repeated / multiple restarts on the DDCs during the issue.
• Enable Citrix AOT logs in the environment, in order to check if LHC does not engage.
• To check whether Call Home is disabled, enter Get-CitrixCallHome. 

• If you see if it is enabled, you may also want to check if the schedule collection and upload is even configured. To confirm the schedule, enter Get-CitrixCallHomeSchedule