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You should be aware of the following possible events related to
timing:
- XCF does not necessarily deliver messages in the order in which
they were sent.
- Message delivery occurs asynchronously. It is possible for a message
to be received by the target member using IXCMSGIX before XCF returns
control from IXCMSGOX to the member that sent the signal. If the
receiving member provides an acknowledgement signal back to the sender,
it is even possible that the acknowledgement signal will be received
by the sender before the sender receives control back from issuing
the IXCMSGOX invocation that sent the message.
- A target member could become inactive while its message user routine
is executing. In that case, the message user routine completes normally,
but XCF does not deliver any more messages for that member.
- A target member could become inactive after the SRB for its message
user routine is scheduled, but before the message user routine runs.
In that case, XCF discards the message because the member cannot receive
it, and does not deliver any more messages for that member. XCF does
not notify the sender that the message was discarded, but notifies
the sender's group user routine that the target member's state has
changed.
- A sending member's system might fail while the member is trying
to send a message, and the target member might not receive the message.
In this case, if the target member has a group user routine, the
target member would receive notification of the system failure. If
the target member was expecting a message, this notification might
explain why the message was not received.
- A target member's system might fail before XCF can deliver its
message. In this case, if the sender has a group user routine, the
sender would receive notification of the system failure. If the sender
was expecting an acknowledgment, this notification might explain why
no acknowledgment was received.
- XCF might deliver a message to a target member, but the member,
or its system, might fail before the member can take any action on
the message (if some action was required). If the sender has a group
user routine, the sender would receive notification of member or system
failure. If the sender was expecting some action to take place, this
notification might explain why the action did not occur.
- A loss of signaling connectivity between systems might occur.
The operator might be able to start or restart additional signaling
paths and reestablish connectivity. In this case, XCF delivers messages
normally, but with some delay.
- A new member might send a message that XCF could deliver before
the target member's group user routine receives notification that
the new member exists.
- A target member's system might be temporarily non-operational,
causing delivery of messages to be delayed until the system resumes
activity.
- A member invoking IXCMSGOX might get a return code indicating
that the target of the message was not found. This could occur before
the member's group user routine was notified that the target member
(or the target member's system) is gone.
- When a message user routine runs, the member that sent the message,
or the system on which the sender resides, might no longer be in the
sysplex. In that case, if the receiving member sends a response,
XCF indicates that the original sender (now the target) does not exist.
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