When setting up Enterprise Terminal Server (ETS), you should also have Terminal Server Nodes ready to create your pool of Terminal Server Nodes. Enterprise Terminal Server will be used to access them. As a rule, subscriptions for ETS are purchased together with subscriptions for Terminal Server Nodes. However, you may have valid subscriptions of Terminal Server Nodes from a separate purchase and in that case you can purchase ETS subscriptions only.
Enterprise Terminal Server is the product which offers multi-node capabilities (load-balancing) and failover mechanisms to ensure access to sessions running on the Terminal Server Nodes will continue. When setting up Enterprise Terminal Server, some considerations of the number of Terminal Server Node Subscriptions needed are recommended.
Load-balancing functionalities of Enterprise Terminal Server means that Terminal Server Nodes must be added on. It is therefore recommended to install at least two external Terminal Server Nodes with an Enterprise Terminal Server.
By installing more Terminal Server Nodes, administrators can set up a load-balanced scenario using the multi-node capabilities of ETS. See this tutorial for more details:
Adding Terminal Server Nodes to Enterprise Terminal Server via the User Interface
A NoMachine failover cluster is a group of two servers that work together to maintain high availability of applications and services running on external Terminal Server Node hosts in a multi-node environment. This is an active/passive cluster where the primary server is at the beginning active and ready to accept users' connections, while the secondary server is passive, its task is to constantly monitor the primary server.
When the secondary server loses contact with the primary server, the cluster failover occurs: the secondary server takes the place of the primary server to permit continuity of services. Sessions running on the external remote nodes continue to stay running, management of server-node connections is passed from the primary to the secondary server, the virtual IP of the cluster is taken by the secondary server, an ARP notification is sent to local network devices to update their ARP cache entries (Ethernet MAC to IP address link associations).
As a rule, Enterprise Terminal Server and Terminal Server Node Subscriptions are purchased together because you would like to set up a scenario similar to the one illustrated here:
Setting up highly available remote access to virtual desktops load-balanced among multiple Linux hosts
Installing 2 x Enterprise Terminal Servers with no nodes is technically possible and if the primary server of the two were to go down, the secondary server would still be activated. However, running sessions on the node of a primary server which then crashes would mean all session content would be lost. Access to new sessions would be available on the secondary server.
It is therefore recommended to install:
2 x ETS with at least one external Terminal Server Node. This would mean access to sessions running on the single external node would continue.