## 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: Let's say there are three Publishing Agents in a Site. All of them can communicate with each other. If one of the agents suddenly loses a connection with the other two, the two agents will know that they are in the majority and will take over the VDI provider hosts that are currently managed by the first agent. Let's now say that there are four Publishing Agents. If one of them loses a connection to the remaining three, the same scenario will occur as in the example above. But if two agents simultaneously lose a connection to the other two, none of the two groups will be in the majority and therefore none will be able to make a decision who should take over the VDI provider hosts. In a situation like this, steps must be taken to prevent a split-brain scenario, which will happen if the agents continue to operate independently from each other. As a solution to this problem, all agents will simply abandon all VDI providers at the same time, so no data loss or any other problem can possibly happen. 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: For an existing VDI provider, open the Properties dialog, select the Agent Settings tab and in the Preferred Publishing Agent field, select Automatic. When adding a new VDI provider, on the second wizard page where you specify the host type and address, click the Advanced Settings link and then select Automatically in the Preferred Publishing Agent drop-down box. Note that the Automatic option is selected by default when there are three or more Publishing Agents available. 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.