Message sending and receiving

The following figure shows the distributed queue management model, detailing the relationships between entities when messages are transmitted. It also shows the flow for control.

Figure 1. Distributed queue management model

Note:

There is one MCA per channel, depending on the platform. There might be one or more channel control functions for a particular queue manager.
The implementation of MCAs and channel control functions is highly platform-dependent. They can be programs or processes or threads, and they can 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 previously described. Server channels must only be set up to trigger if they are fully qualified, that is, they specify a CONNAME to connect to.
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 split messages then the split messages are not 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 knows 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.