Design considerations

The disk or diskette in the controller is the most reliable method and does not require involvement of the workstation operator if retransmission of a message is required. Either the permanent file or the transient files of the disk or diskette can be used. The REPLACE instruction should be used when writing to the disk or diskette file. REPLACE causes the data to be physically written to the disk or diskette and not just buffered in the controller's control storage. Disk or diskette storage assures sequence number retention across a power failure or a controller failure that can destroy the contents of controller's control storage. However, disk or diskette access and data transfer time can add significant overhead to each recoverable transaction.

If the controller's control storage is used to retain sequence numbers, a power loss or controller failure would destroy the contents of the controller's storage, making message resynchronization impossible. By not performing message resynchronization, recoverable input and output messages can be lost or duplicated.

Message sequence numbers can be stored on a workstation output component such as the display or the journal printer. However, this method requires workstation-operator intervention to retrieve the numbers during STSN processing.