The ATTACH and SCHEDULER FM headers under MFS

All input and output messages include an implicit or explicit ATTACH FM header and, optionally, a SCHEDULER FM header.

MFS DPM provides format definition options to permit the insertion of input routing parameters (ATTRDPN/SCDRDPN, ATTRPRN/SCDRPRN, and ATTPRN/SCDPRN) into the input data stream. The input ATTDPN/SCDDPN is the MFS MID used to format the input message. MFS DPM provides output format definition options to permit the insertion of user-defined information into these headers.

The ATTACH FM header and SCHEDULER FM header, if required, are sent without data as the only element of the first transmission chain of demand-paged output. The first input paging request to the demand-paged output must contain an ATTACH (for DPN=QMODEL); subsequent input page requests for the same demand-paged output message can optionally contain an ATTACH (and must also indicate DPN=QMODEL). For all other MFS input and output messages the ATTACH (and SCHEDULER, if required) FM headers are sent with data as the first or only element in the first or only transmission chain of the message.

MFS uses the following ATTACH and SCHEDULER FM header parameters:
  • ATTDP and ATTIU. These ATTACH parameters indicate whether the message is demand-paged or whether it is single or multiple chain.
  • ATTDBA. This ATTACH parameter indicates the output data blocking algorithm or input data deblocking algorithm to be used by IMS. The data entity deblocked on input becomes the input data record to MFS. MFS then produces a standard IMS message consisting of pages and segments. On output, the MFS record becomes the data entity for output VLVB blocking. An exception is MFS DPM stream mode, which is sent as chain (ATTDBA) output.
  • ATTDPN/SCDDPN and ATTPRN/SCDPRN. The ATTDPN/SCDDPN parameter is used on input to activate MFS input formatting. The ATTPRN/SCDPRN can be inserted into the input data stream and, therefore, made available to the application. On output message replies from IMS, the ATTDPN/SCDDPN or ATTPRN/SCDPRN can be inserted into the ATTACH or SCHEDULER FM headers of the output message processed by MFS as specified by MFS format descriptions. These override the ATTRDPN/SCDRDPN and ATTRPRN/SCDRPRN that might have been wrapped from the source input message ATTACH or SCHEDULER FM headers.
  • ATTRDPN/SCDRDPN. The ATTRDPN/SCDRDPN can optionally be inserted into the input data stream and, therefore, made available to the application for input messages processed by MFS DPM. The return destination process name can be inserted into the output ATTACH or SCHEDULER FM header of the output message processed by MFS. The MFS-suggested return destination process to be associated with a reply to the output message is the MID name specified in the NXT= operand of the MOD.
  • ATTRPRN/SCDRPRN. The return primary resource name (RPRN) can be inserted into the ATTACH or SCHEDULER FM header of the output message processed by MFS. It can be specified using MFS definition. If returned to IMS with a reply to the message on which it was sent, it determines the destination — transaction or LTERM — that is to receive the input reply.
  • ATTDQN/SCDDQN. The destination queue name is sent by IMS on demand-paged output as a message identifier and should be returned on all requests associated with demand-paged messages.

When the first chain (ATTACH SCHEDULER) of an asynchronous demand-paged output message is successfully sent, the output queue (IMS LTERM) is automatically locked against input or output. This function is also automatic following a QSTATUS FM header sent by IMS because of a cursor error on a demand-paged request. The queue lock state is reset when the page request (ATTACH QMODEL, QGETN with QNAME) is received and the first page of data is sent without resending the ATTACH or SCHEDULER FM header. The queue lock condition is also reset across session failure and subsequent restart. During the time that IMS waits for this page request (QGETN), provided conversation or response mode is not used, IMS can send or receive messages for other LTERMs. During the time that the output LTERM is locked, no input is accepted from the ISC session that produces output to the locked queue because of the input/output component relationships defined for the locked queue using the IMS system definition NAME macro statement or an ETO logon descriptor. Input is accepted that produces output for other LTERMs.