Symmetrical session shutdown for LU 6.1 (SBI and BIS)

SBI is a VTAM®-expedited flow command, and BIS is a normal flow command.

These commands control only the normal flow requests that initiate new brackets. They do not preclude replies that can flow within existing brackets. Either half session can send SBI at any time to request the receiving half session to suppress future initiation of a bracket. After the receiver of the SBI reaches a point between brackets that is acceptable for shutdown, the receiver returns the BIS command to the SBI initiator. The BIS sender then enters a no begin-bracket or NOBB state, during which that half session can receive bracket initiation requests and may send requests or replies in the normal flow when in-brackets, but cannot initiate a bracket on its own. The primary half session is responsible for detecting and resolving SBI race conditions by maintaining indications of which half session is in NOBB state and terminating the session if both half sessions reach this state.

A successful completion of a shutdown sequence refers to successful entry into the NOBB state by both half sessions— that is, normal session termination.

IMS sends the SBI command either when the IMS /QUIESCE command is entered or when the quiesce option is specified on the /CHECKPOINT command. When BIS is returned from the other subsystem, IMS continues with the QUIESCE or CHECKPOINT command process. If the /QUIESCE command was entered and a CHECKPOINT QUIESCE is not in progress, IMS sends BIS at the next transition to a between-brackets state. If a CHECKPOINT QUIESCE is in progress when IMS receives BIS, IMS continues to send output according to other /CHECKPOINT parameters (FREEZE, PURGE, and DUMPQ) and then sends BIS when the shutdown process is complete.

After receiving an SBI or BIS, IMS sends BIS at the next between-brackets state when a CHECKPOINT QUIESCE is not in progress, or at the end of the shutdown process when a CHECKPOINT QUIESCE is in progress.

When IMS is the primary half session, IMS checks after either sending or receiving BIS to ensure that both half sessions are in NOBB state. If both are, IMS resets all message counts and terminates the session. Otherwise, IMS continues with normal processing. When IMS is the secondary half session, IMS waits for the primary half session to terminate the session. If an LTERM subpool is allocated to the session, it is deallocated (and associated message counts are reset) prior to the session termination when both half sessions are in NOBB state. This deallocation/termination process allows a subsequent cold start of the session and frees any allocated subpools for use by other sessions. This process also ensures that no resynchronization or recovery is required when the session is restarted.

Because initiating a bracket is not allowed after the NOBB state is entered, a problem occurs when a single-bracket-chain (begin- and end-bracket) input message requires an immediate reply (such as a system or error message) that initiates a bracket. Rather than preclude end-bracket on these input messages while in NOBB state, IMS accepts and continues to process all input. When a reply occurs that would require IMS to initiate a bracket while in NOBB state, the session is terminated.

While IMS is in response mode, in conversational mode, or waiting for a Fast Path response, subpool deallocation cannot occur nor can BIS be sent, because no EB has flowed either on output or input (that is, IMS is not between-brackets).