Requesting asynchronous transaction processing: SCHEDULER FM header

ATTACH is defined for synchronous scheduling and execution; SCHEDULER is defined for asynchronous scheduling and execution.

The SCHEDULER process can be attached to send messages to be processed asynchronously. That is, IMS perceives the messages as being processed without regard to scheduling or execution timing, whether output will result at all, and ignoring any synchronous relationship between the input and any resulting reply. Each half session between the logical units might indicate, by using the session bind parameters, whether the SCHEDULER process is supported and available to receive asynchronous messages. The IMS half session always indicates the SCHEDULER process is available. The other half session might indicate that the SCHEDULER process is or is not available to receive asynchronous input.

When the bind indicates that the other half session does not support the SCHEDULER process, IMS defaults to sending each output with the ATTACH FM header.

The SCHEDULER process is attached by indicating the SNA name (X'02') as the ATTACH DPN parameter and by concatenating the SCHEDULER FM header to the ATTACH FM header. During the time that the SCHEDULER is attached, each subsequent message to be scheduled asynchronously must be sent with at least the SCHEDULER FM header, but does not require another ATTACH FM header. Variable-length ATTACH parameters passed to the SCHEDULER are ignored on input and are not sent with output by IMS. The SCHEDULER FM header carries the name of the process to be scheduled (SCDDPN), as well as information necessary for scheduling and return-reply routing. SCDDPN is a required parameter for the SCHEDULER and must be supplied on IMS SCHEDULER input. SCDDPN on messages sent by IMS are set from a previous input SCDRDPN or optionally by MFS using the output message format descriptor. When no other value is available, SCDDPN is set to ISCEDT as a default.

The SCHEDULER process is always available for IMS input. IMS sends an ATTACH with the ATTDPN parameter set to X'02' (SCHEDULER) concatenated with a SCHEDULER FM header under each of the following conditions:

When the other half session supports the SCHEDULER model (as defined in the bind parameters).
When sending an asynchronous reply resulting from input entered previously on the session and scheduled using the SCHEDULER process.
When sending unsolicited asynchronous messages. Examples of such messages are an IMS message switch or an output reply resulting from an application not scheduled directly as a result of input on the session.

Within IMS, the SCHEDULER supports both single- and multiple-chain input and output messages using the ATTACH ATTIU parameter. IMS does not permit multiple input or output messages to be scheduled using the ATTACH multiple-chain indicator.

The SCHEDULER FM header can be present only once for each input or output IMS message. For output MFS demand-paged messages, the SCHEDULER FM header is present only on the first output SNA chain (first output MFS page). IMS does not send or receive the SCHEDULER SCHEDSTAT, PURGE, or PURGSTAT FM headers.