Intercommunication

Message sending and receiving

Figure 28 shows the relationships between entities when messages are transmitted, and shows the flow of control.

Figure 28. Distributed queue management model

Notes:

There is one MCA per channel, depending on the platform. There may be one or more channel control functions for a given queue manager.
The implementation of MCAs and channel control functions is highly platform dependent; they may be programs or processes or threads, and they may be a single entity or many comprising several independent or linked parts.
All components marked with a star can use the MQI.

Channel parameters

An MCA receives its parameters in one of several ways:

If started by a command, the channel name is passed in a data area. The MCA then reads the channel definition directly to obtain its attributes.
For sender, and in some cases server channels, the MCA can be started automatically by the queue manager trigger. The channel name is retrieved from the trigger process definition, where applicable, and is passed to the MCA. The remaining processing is the same as that described above.
If started remotely by a sender, server, requester, or client-connection, the channel name is passed in the initial data from the partner message channel agent. The MCA reads the channel definition directly to obtain its attributes.

Certain attributes not defined in the channel definition are also negotiable:

Split messages: If one end does not support this, split messages will not be sent.
Conversion capability: If one end cannot perform the necessary code page conversion or numeric encoding conversion when needed, the other end must handle it. If neither end supports it, when needed, the channel cannot start.
Distribution list support: If one end does not support distribution lists, the partner MCA sets a flag in its transmission queue so that it will know to intercept messages intended for multiple destinations.

Channel status and sequence numbers

Message channel agent programs keep records of the current sequence number and logical unit of work number for each channel, and of the general status of the channel. Some platforms allow you to display this status information to help you control channels.