Enabling High Availability for VDI
High availability for VDI means that a VDI provider must never lose a connection with a VDI Agent. If the connection is lost, the guest VMs will become unavailable for user connections. High availability for VDI is accomplished by installing at least three RAS Publishing Agents and configuring a VDI provider to automatically select one of them. This way, if one of the Publishing Agents goes offline (and with it the built-in VDI Agent), the VDI provider will be automatically assigned to the VDI Agent running in the next available Publishing Agent.
To configure high availability for VDI, use the information and instructions below.
At least three Publishing Agents are required
Make sure you have at least three RAS Publishing Agents installed and running. You may also have additional Publishing Agents in standby mode, but you must have at least three agents in the active state for the high availability functionality to work. All Publishing Agents must be able to communicate with each other.
An odd number of agents is recommended
To properly control a possible split-brain situation, strictly more than half of all available Publishing Agents should be able to communicate with each other at any given time. Consider the following examples:
For the reasons explained above, you should always install an odd number of Publishing Agents. This way, one of the groups of agents will always be in the majority and will continue to handle all VDI providers. Please note that the general recommendation (regardless or the high availability functionality described here) is to have three RAS Publishing Agents running in a Site. For details, see Secondary Publishing Agents.
Please also note that Publishing Agents in standby mode don't participate in the high availability operations. These agents stay inactive until one of the active Publishing Agent goes completely offline. When that happens, an agent in standby mode is activated and takes place of the lost agent. From this point forward, it is considered a part of the high availability setup. When the lost agent is brought back online, everything goes back to what it was before.
Configuring a VDI provider for high availability
Use one of the following to configure a VDI provider for high availability:
If initially you have less than three RAS Publishing Agents in a Site, a VDI provider is assigned a specific preferred Publishing Agent. When at some point you increase the number of Publishing Agents to three or more and want to enable high availability for one or more VDI providers, you need to reconfigure each host so that the Preferred Publishing Agent property is set to Automatic.