Using the Expedited Message Handler

The IMS Fast Path Expedited Message Handler (EMH) can also use the shared queues in a shared-queues environment. IMS calls the Fast Path Input Edit/Routing exit routine to determine if an incoming message should be processed by Fast Path.

If Fast Path must process the message, either the local IMS system processes it, or IMS places it on the shared queues.

Fast Path can use EMHQ structures, which are defined in the same way as the MSGQ structures (in the DFSSQxxx member of PROCLIB), to share Fast Path transactions. However, an IMS system with Fast Path installed is not required to have an EMHQ structure or share Fast Path transactions. If you do not define an EMHQ structure, all Fast Path transactions are processed locally.

If an IMS system joins the z/OS® cross-system coupling facility group, abends, or shuts down, it notifies all sharing IMS systems by sending a message with the program name table. Sending this message makes all IMS systems aware of which IMS systems are servicing particular programs. Likewise, when a program is started or stopped on an IMS system, it notifies all sharing IMS systems, and each IMS system updates its program name table.

If no active region is available in the IMSplex to service a program, all IMS systems that have enqueued messages for that program issue message DFS2529I to the inputting terminals. These terminals are then unlocked. Any new input for the program is rejected with message DFS2533I.

Because Fast Path input messages are not recoverable, they are not written to the SRDS data set during CQS checkpoint. You can recover EMHQ structures; however, no Fast Path input messages are retained on the SRDS data set, and only Fast Path output messages can be recovered from the EMH queues. For each terminal awaiting output from an input message that has been lost during EMHQ structure rebuild, IMS sends message DFS2766I and unlocks the terminal. In addition, IMS writes a X'5936' log record for the lost message.

The EMHQ structures can be copied, and all messages are copied from the old structure to the new one, regardless of whether they are recoverable.

For EMHQ rebuild, it is possible to specify that the rebuilt EMHQ structure be accessible while the rebuild is in progress. Use the WAITRBLD= parameter in the DFSSQxxx PROCLIB member data set.