Backup sessions in an XRF complex that uses USERVAR

In an XRF complex that uses USERVAR, the alternate IMS system opens backup sessions for all class-1 terminal sessions that are open on the active IMS system, thus reducing the time necessary to switch the terminals during a takeover.

Whenever a class-1 terminal user logs on to XRF IMS, the active IMS system records this new session in the log. The alternate IMS system reads this record and initializes a backup session for the terminal. Data does not flow on the backup session as long as it remains a backup session. When the user logs off from the active IMS, the alternate IMS system terminates the backup session.

At a takeover, the alternate IMS requests, through VTAM®, that NCP switch each of the class-1 terminals that are communicating with the active IMS from primary sessions to the preopened backup sessions. NCP responds to these requests. Transparency of takeover depends on what is happening on the terminal at the moment of takeover. The user might be unaware of the takeover or might be asked to reenter the last transaction.