Use of responses or brackets to acknowledge recoverable input

To facilitate message resynchronization, IMS allows an input update or a recoverable-inquiry transaction to optionally request a definite response.

IMS allows this request for a definite response if the user specifies the OPTIONS=OPTACK for the workstation.

With this operand specified, IMS can acknowledge input with the next output through use of the input and output bracket indicators.

If the OPTACK option is defined, performance can be improved if the workstation requests begin-bracket, change-direction, and exception DR1 or DR2 (rather than DR1 or DR2) on recoverable input to IMS. The next output from IMS acknowledges the input by indicating end-bracket only. The type of output messages that are sent from IMS depend on the type of recoverable input, the defined response mode, and the availability of output.

Contention can occur when a session is in a between-brackets state. In this case, IMS sends an unsolicited output message indicating both BB and EB or a BID command at the same time that the workstation is sending an input message indicating begin-bracket and change-direction. This output message does not acknowledge the input message and can be either accepted or rejected by the workstation. Rejecting an IMS nonrecoverable output message can result in losing the message. If contention occurs when IMS is sending a recoverable IMS output message (output requesting DR2 response), the workstation must send either an exception or a definite response before IMS can receive the input message.