VTAM support for sync point services

Note: Sync point services cannot be used within a full-duplex conversation.

To support the application program's implementation of sync point services, VTAM® includes the following functions:

Defining the system
- Enables the application program to indicate the synchronization level that is supported by the local LU. The application program does this by specifying the keyword SYNCLVL=CONFIRM or SYNCLVL=SYNCPT on the APPL definition statement.
- Enables the application program to specify the frequency of occurrence of the ATTN(LOSS) exit that is being driven. The application program does this by specifying the keyword ATNLOSS=LAST or ATNLOSS=ALL on the APPL definition statement.
  Note: The use of the ATNLOSS operand on the APPL definition statement is not restricted to sync point services.
Allocating conversations and activating sessions
- Establishes a sync point session if both LUs support the sync point services.
- Allocates sync point conversations on sync point sessions if the application program requests sync point in its allocation request.
- Because an FMH-5 for a conversation requires the Logical Unit of Work (LUW) identifier, the application should include the LUW in the FMH-5 presented to VTAM with the APPCCMD CONTROL=ALLOC macroinstruction.
Sending and receiving data
- Enables the application program to send PS headers, which are logical records with a length field of X'0001' used to convey sync point information.
Rejecting sessions
- Deactivates sessions by using session instance identifiers. Terminates any conversation associated with the session.
  Note: This function is not restricted to sync point services.
Returning the conversation state
- Returns the current conversation state at the completion of each conversation-related command. This information is contained in the CONSTATE field in many of the APPCCMD macroinstructions.
  Note: This function is not restricted to sync point processing.
Suspending sessions
- Suspends a subsequent conversation's outflow of normal flow requests and suspends the normal deactivation of a session currently in use by a sync point conversation.
  Note: If an application program is using persistent LU-LU session support during sync point processing and fails after APPCCMD CONTROL=SETSESS, QUALIFY=SUSPEND has been issued, but APPCCMD CONTROL=SETSESS, QUALIFY=RESUME has not been issued, VTAM unbinds the sync point sessions. In the same situation, VTAM also unbinds sync point sessions for which APPCCMD CONTROL=SETSESS, QUALIFY=SYNCBEG has been issued but neither APPCCMD CONTROL=SETSESS, QUALIFY=SYNCEND nor APPCCMD CONTROL=SETSESS, QUALIFY=RESUME has been issued at the time of the failure.
Resuming sessions
- Resumes a session that has been suspended.

Table 1 shows several ways in which you might use the APPCCMD CONTROL=SETSESS macroinstructions.

Table 1. Sync point processing
Macroinstruction	Description of Circumstances
APPCCMD CONTROL=SETSESS, QUALIFY=SUSPEND APPCCMD CONTROL=SETSESS, QUALIFY=RESUME	Use these two commands when you have a period of processing time in which the session might have to be unbound before VTAM stops the outbound flow.
APPCCMD CONTROL=SETSESS, QUALIFY=SYNCBEG APPCCMD CONTROL=SETSESS, QUALIFY=SUSPEND APPCCMD CONTROL=SETSESS, QUALIFY=RESUME	Use this combination of commands if you have a period of processing time before the outbound flow is to be stopped, if you are using persistent LU-LU session support, and if you want VTAM to unbind the session during that processing period. (In this situation, APPCCMD CONTROL=SETSESS, QUALIFY=SYNCEND could be used, but it would have to be used with APPCCMD CONTROL=SETSESS, QUALIFY=RESUME.)
APPCCMD CONTROL=SETSESS, QUALIFY=SYNCBEG APPCCMD CONTROL=SETSESS, QUALIFY=SYNCEND	Use these two commands when you do not ever want the outbound flow stopped, but you do want VTAM to unbind a session during a failure when persistence is enabled.