The primary and backup data servers connect to each other via socket. If the primary data server stops, then the backup server takes over. If the primary then comes back online, then the primary takes over again and the backup returns to standby mode. The data client connections will move between the two servers accordingly.
In display server deployments, the primary display server and backup display server do not connect to each other. The rtvdisplay servlet is configured to connect first to the primary and, if that fails, it tries to connect to the backup. At any point, if the one it is connected to becomes unavailable, then it will try to connect to the other. You can configure whether to have the rtvdisplay server connect back to the primary server when it comes back online or stay connected to the backup server until it goes offline.
The HMTL UI client connects to the data server via an HA configured rtvquery servlet.
The primary and backup historian connect to each other via socket. If the primary historian stops, then the backup takes over. If the primary historian comes back online, then the primary takes over again and the backup returns to standby mode. Only the active historian writes to the database.
The historian is a data client of the data server and connects to it via a fault tolerant URL (socket only), which means that the data servers and historians can fail over separately or together.