Controlling demand-paged messages: QMODEL FM headers

Queue model (QMODEL) headers are sent and received to control demand-paged messages. All demand-paging requests (except RAP) must be made using the QMODEL-defined FM headers.

Restriction: MFS DPM-Bn does not support operator control table requests or input messages while sending demand-paged output.

The QMODEL headers that IMS supports are listed in the following table.

Table 1. IMS-supported QMODEL headers
QMODEL headers Sent by IMS1 Received by IMS2
QXFR X  
QGETN   X
QGET   X
QPURGE   X
QSTATUS X  
Notes:
  1. QMODEL headers are not sent between two IMS subsystems.
  2. A QMODEL FM header must precede a QMODEL page request for the first page. These FM headers can be preceded by the QMODEL ATTACH FM header with the DPN parameter containing the value X'03'.

QXFR is a QMODEL reply. All other headers are QMODEL requests. Each half session between the logical units might indicate, by using the session bind parameters, whether the QMODEL process is available to receive QMODEL requests. The bind parameters have no effect on QMODEL replies. IMS always indicates QMODEL available and is prepared to receive QMODEL (paging) requests to IMS demand-paged output. The other half session might or might not indicate QMODEL available. IMS ignores the other half-session bind indication for QMODEL requests, because only QMODEL replies can be sent by IMS.

Queue model uses LU 6.1 protocols to handle messages as follows:
  • Although SNA allows multiple messages to be active, IMS messages are serial. That is, only one message is active at any given time. When active message processing is completed, another message can be processed. Synchronous (ATTACH without SCHEDULER) output MFS DPM demand-paged messages are considered active immediately when the first chain (ATTACH) is sent. Demand-paged output sent with ATTACH SCHEDULER does not become active until the first page request is returned to IMS.
  • MFS demand-paged output provides for two types of message organization: linear (OPTIONS=DPAGE) and hierarchic (OPTIONS=PPAGE). For both types, pages can be retrieved sequentially (get next) or linearly (get by cursor). However, linear retrieval is permitted only if operator logical paging (OLP) is defined. Hierarchic retrieval is not supported.