Sync points requested on input to IMS

For any type of input, IMS does not schedule the intended transaction until the complete input message is successfully received.

Sender-detected errors, errors resulting from processing of IMS input, and session failure prior to the receipt of the complete input message cause the entire message to be discarded or backed out. However, the input message cannot be canceled by ISC session failures or protocols after the complete message is received and made available for scheduling.

When an input message is backed out because of errors detected during IMS input processing or during synchronous transaction execution, the session partner is notified, either by means of session termination or by an exception response to the ISC input. These events occur for this backout:
  • Backout results in resetting the associated DFC and ATTACH states to those of the last sync point.
  • Backout during transaction execution results in backing out application updates and messages, except express messages made after the last application sync point.
  • The application sync point and ISC sync point are not necessarily the same.
  • Backout during input to IMS has no effect on other recoverable IMS resources, such as databases, because input messages are not available for scheduling or execution until they are received completely and without error.
  • The result of the backout is only the current input message, even if several consecutive input (irrecoverable) messages were received and executed or enqueued for scheduling after the last input sync point was requested from IMS.

The definition and relationship of successful ISC input sync points to IMS application sync points depend upon whether the ISC input was synchronous or asynchronous. For asynchronous input, the sync point reflects only that IMS is now responsible for message recovery. No implications exist relative to the scheduling or execution of transactions or to the availability of transaction output. After the message is successfully enqueued, the DFC and ATTACH sync point information is updated and the requested sync-point response is returned to the session partner.

Although some IMS exceptions for synchronous input exist, the sync point is intended to reflect the success of the IMS transaction execution and sync point. IMS updates the DFC and ATTACH sync point information as appropriate and commits all associated transaction resources (for example, DL/I databases) and the output transaction message reply when responding to the ISC attached input sync-point request.

These exceptions apply to sync points for synchronous input to IMS:
  • The ISC sync-point response is returned by IMS when a transaction-inserted response mode or conversational reply message (first message inserted to the I/O PCB or alternate response PCB) is made available for output using a transaction sync point. Additional transaction processing and sync points are not reflected in the ISC session. No ISC sync point-response occurs for transaction sync points prior to the transaction's inserting the reply message, as in a program-to-program switch. Nonfirst messages inserted to the I/O PCB or alternate response PCB, messages inserted to nonresponse PCBs, and IMS express messages intended for the ISC session are queued for future asynchronous delivery after successful delivery of the reply message.
  • Except during program-to-program switches to another conversational transaction, IMS generates a subsystem error message as the result of abnormally terminating a conversational transaction that attempts to cause a transaction sync point without first having inserted an output conversational reply message. The subsystem error message causes an exception response to be sent to the input sync-point request and the input message to be backed out.