Synchronous and asynchronous processing on ISC VTAM links

Message transmission can be synchronous or asynchronous between the terminal entering the message and the receiving subsystem. Messages can also be either synchronous or asynchronous with respect to the ISC session.

Note: This topic applies to messages that are transmitted on ISC VTAM links. For information that applies to ISC TCP/IP links, see ISC communication with CICS over TCP/IP.

From the point of view of the CICS application, using the CICS SEND/RECEIVE EXEC commands produces synchronous processing on the session, while the CICS START/RETRIEVE EXEC commands result in asynchronous processing. When CICS is the front-end subsystem, both processing types are supported. When IMS is the front-end subsystem, only asynchronous processing can occur, unless the special support for front-end/back-end system utilization provided by IMS Front-End Switch exit routine is used.

The following table shows how the CICS START/RETRIEVE and SEND/RECEIVE EXEC commands are supported from a CICS point of view.

Table 1. The SEND/RECEIVE and START/RETRIEVE commands
Message type CICS to IMS IMS to CICS
SEND/RECEIVE (synchronous) YES YES
START/RETRIEVE (asynchronous) YES YES

The following table shows the differences in the CICS application program interface when using SEND/RECEIVE or START/RETRIEVE.

Table 2. CICS API for SEND/RECEIVE and START/RECEIVE
Attribute SEND/RECEIVE START/RETRIEVE
Mode Synchronous with (SEND INVITE) or without (SEND LAST) reply Asynchronous
FMH ATTACH ATTACH SCHEDULER
Name length Supports 8-byte names Supports 4-byte names1
Change Direction (CD) Supported CICS front-end cannot send CD. IMS sends CD to CICS if COMPT=SINGLE2 or MULT2.
Multiple chains per message (IUTYPE) Supported2 Not supported
IMS component selection (DATASTREAM) Supported

Not supported for input.
Input component=1.
Output component=any.
RECORDFORMAT:3 Undefined (U) Supported4 Not supported
RECORDFORMAT:3 RU Supported4 Not supported
RECORDFORMAT:3 VLVB Supported Supported
RECORDFORMAT:3 CHAIN5 Supported Supported
FMH built by

Application program
(BUILD ATTACH)

System: DPN, PRN, RDPN,
  RPRN specified in
  EXEC START command6
Access to FMH Yes (EXTRACT Attach)

Any supplied parameters
available on EXEC RETRIEVE
command6
Relation to ISC session

Directly related
(ALLOCATED)

 Independent
IMS transaction types supported

Response mode
Conversation mode
Message switches7
Commands
Nonresponse,
  nonconversational7

Nonresponse mode
Nonconversation
Message switches
Commands8
Recoverable I/O Supported Supported
Nonrecoverable I/O Supported Supported
Multisegment input Supported Single segment only
Multisegment output Supported Single segment only
IMS edits Basic edit Basic edit
Available ISC Edit ISC Edit
MFS: Without paging Supported Supported
MFS: Autopaged input: Single chain Supported Supported
MFS: Autopaged input: Single chain: Multichain Supported Not supported
MFS: Autopaged output Supported Not supported
MFS: Demand-paged output Supported Not recommended
Notes:
  1. Names can be embedded in the message text. The DPN field can be converted from a 4-character to an 8-character name by using DEFINE TRANSACTION REMOTENAME (or DFHPCT TYPE=REMOTE).
  2. Only used to send or receive MFS autopaged input or output messages.
  3. The ATTACH parameters are defined in the CONNECTION definition. However, the application using START/RETRIEVE must understand and provide the correct data format within the application.
  4. Marked as reserved by CICS, but can be specified on the BUILD ATTACH.
  5. Chained output from IMS requires the use of MFS.
  6. The RDPN and RPRN are limited to 4-character names.
  7. Nonresponse, nonconversational, and message switch input use SEND LAST only. CICS acquires replies to nonresponse and nonconversational transactions using RECEIVE.
  8. /DIS, /RDIS, /FOR only.