High availability overview

Background — reasons for a HA setup

In a standard ThinLinc setup, there is a single point of failure — the machine running the VSM server. If the VSM server is down, no new ThinLinc connections can be made, and reconnections to existing sessions can’t be established. Existing connections to VSM agent machines still running will however continue to work. A ThinLinc cluster of medium size with one machine running as VSM server and three VSM agent machines is illustrated in Fig. 3.

Fig. 3 A non-HA ThinLinc cluster setup

Here the incoming connections are handled by the VSM server which distributes the connections to the three VSM agent machines. If the VSM server goes down, no new connections can occur. The VSM server is a single point of failure in your ThinLinc setup.

Solution — elimination of single point of failure

In order to eliminate the single point of failure, we configure the VSM server in a HA configuration where two machines share the responsibility for keeping the service running. Note that ThinLinc’s HA functionality only handles the parts of your HA setup that keeps the ThinLinc session database synchronized between the two machines. Supplementary software is required, read more about this in Theory of operation.

When ThinLinc as well as your systems are configured this way, the two machines are in constant contact with each other, each checking if the other one is up and running. If one of the machines goes down for some reason, for example hardware failure, the other machine detects the failure and automatically takes over the service with only a short interruption for the users. No action is needed from the system administrator.

Theory of operation

Fig. 4 A ThinLinc HA cluster setup

In a HA setup, as illustrated in Fig. 4 two equal machines are used to keep the VSM server running. One of the machines is primary, the other one is secondary. The primary machine is normally handling VSM server requests, but if it fails, the secondary machine kicks in. When the primary machine comes online, it takes over again. That is, in normal operation, it’s always the primary machine that’s working, the secondary is in standby, receiving information from the primary about new and deleted sessions, maintaining its own copy of the session database.

Both machines have a unique hostname and a unique IP address, but there is also a third IP address that is active only on the node currently responsible for the VSM server service. This is usually referred to as a resource IP address, which the clients are connecting to. ThinLinc does not move this resource IP address between servers, supplementary software is required for this purpose.