Keeping half sessions synchronized

Sync-point responses (DR2) are used between ISC session partners to ensure that both partners' sync-point managers can commit or back out recoverable resources synchronously.

All messages sent or received on an IMS ISC session are defined as either recoverable or irrecoverable, depending on the message type. The session-response protocols are used to ensure that both ends of an ISC session mutually understand the recoverability attributes associated with each message. The response protocol used must be consistent with the IMS message type.

The ISC sync point can be explicit or implicit:

Explicit: The input requests an RQD2 response.
Implicit: The input requests the sending of an exception response (RQE2) and change-direction on output made available by the transaction, or the sending of an LUSTATUS in lieu of output data from the transaction).

To further increase integrity and recoverability, IMS can supplement the sync point facility by logging the information. Use of log write-ahead ensures that the sync point indication is recorded on the log. This makes sync point information available to IMS restart procedures before the sync-point response is actually sent or change-direction is replied to (implied sync point).

Before reading these topics, be sure to understand the definitions and relationships of the ISC sync point and associated commit and backout processes to the IMS application program, described in these paragraphs.