Timing Considerations

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.