Enabling high availability for VDI
High availability for VDI means that a Provider must never lose a connection with a Provider 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 Connection Brokers. This way, if one of the Connection Brokers goes offline (and with it the built-in Provider Agent), the Provider will be automatically assigned to the Provider Agent running in the next available Connection Broker.
To configure high availability for VDI, use the information and instructions below.
At least three Connection Brokers are required
Make sure you have at least three RAS Connection Brokers installed and running. When RAS Connection Brokers from your site are online, high availability is enabled automatically. You may also have additional Connection Brokers in standby mode, but you must have at least three agents in the active state for the high availability functionality to work. All Connection Brokers 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 Connection Brokers should be able to communicate with each other at any given time. Consider the following examples:
- Let's say there are three Connection Brokers 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 Provider hosts that are currently managed by the first agent.
- Let's now say that there are four Connection Brokers. 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 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 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 Connection Brokers. This way, one of the groups of agents will always be in the majority and will continue to handle all Providers. Please note that the general recommendation (regardless or the high availability functionality described here) is to have three RAS Connection Brokers running in a Site. For details, see Secondary Connection Brokers.
Please also note that Connection Brokers in standby mode don't participate in the high availability operations. These agents stay inactive until one of the active Connection Broker 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 Provider for high availability
Parallels RAS can maintain high availability in the following ways:
- Parallels RAS automatically selects a Connection Broker for the Provider. If this Connection Broker goes down, Parallels RAS moves the workload to one of the remaining Agents.
- You select a Connection Broker for the Provider manually. If this Connection Broker goes down, Parallels RAS moves the workload to one of the remaining Agents.
Use one of the following to configure a Provider for high availability:
- For an existing Provider, open the Properties dialog, select the Agent Settings tab and in the Preferred Connection Broker field, select Automatic or manually select your preferred Connection Broker.
- When adding a new Provider, on the second wizard page where you specify the host type and address, click the Advanced Settings link and in the Preferred Connection Broker drop-down list select Automatically or manually select your preferred Connection Broker. Note that the Automatic option is selected by default when there are three or more Connection Brokers available.