Use sharing conversations in a client application

In an environment where sharing conversations is permitted, conversations can share an MQI channel instance.

Sharing conversations is controlled by two fields, both called SharingConversations, one of which is part of the channel definition (MQCD) structure and one of which is part of the channel exit parameter (MQCXP) structure. The SharingConversations field in the MQCD is an integer value, determining the maximum number of conversations that can share a channel instance associated with the channel. The SharingConversations field in the MQCXP is a boolean value, indicating whether the channel instance is currently shared.

In an environment where sharing conversations is not permitted, new client connections specifying identical MQCDs will not share a channel instance.

• Both the client-connection and server-connection ends of the channel instance are configured for sharing conversations, and these values are not overridden by channel exits.
• The client connection MQCD value (supplied on the client MQCONNX call or from the client channel definition table (CCDT)) exactly matches the client connection MQCD value supplied on the client MQCONNX call or from the CCDT when the existing channel instance was first established. Note that the original MQCD might have been subsequently altered by exits or by channel negotiation, but that the match is made against the value which was supplied to the client system before these changes were made.
• The sharing conversations limit on the server side is not exceeded.

If a new client application connection matches the criteria to run sharing a channel instance with other conversations, this decision is made before any exits are called on that conversation. Exits on such a conversation cannot alter the fact that it is sharing the channel instance with other conversations. If there are no existing channel instances matching the new channel definition, a new channel instance is connected.

Channel negotiation only occurs for the first conversation on a channel instance; the negotiated values for the channel instance are fixed at that stage and cannot be altered when subsequent conversations start. TLS authentication also only occurs for the first conversation.

If the MQCD SharingConversations value is altered during the initialization of any security, send or receive exits for the first conversation on the socket at either the client-connection or the server-connection end of the channel instance, the new value it has after all these exits are initialized is used to determine the sharing conversations value for the channel instance (the lowest value takes precedence).

If the negotiated value for sharing conversations is zero, the channel instance is never shared. Further exit programs that set this field to zero similarly run on their own channel instance.

If the negotiated value for sharing conversations is greater than zero then MQCXP SharingConversations is set to TRUE for subsequent calls to exits, indicating that other exit programs on this channel instance can be entered simultaneously with this one.

When you write a channel exit program, consider whether it will run on a channel instance that might involve sharing conversations. If the channel instance might involve sharing conversations, consider the effect on other instances of the channel exit of changing MQCD fields; all MQCD fields have common values across all the sharing conversations. After the channel instance is established, if exit programs try to alter MQCD fields they might encounter problems because other instances of exit programs running on the channel instance could be attempting to alter the same fields at the same time. If this situation could arise with your exit programs, we must serialize access to the MQCD in your exit code.

If we are working with a channel which is defined to share conversations, but we do not want sharing to occur on a particular channel instance, set the MQCD value of SharingConversations to 1 or 0 when you initialize a channel exit on the first conversation on the channel instance. See SharingConversations for an explanation of the values of SharingConversations.

Example

Sharing conversations is enabled.

You are using a client-connection channel definition which specifies an exit program.

The first time that this channel starts, the exit program alters some of the MQCD parameters when it is initialized. These are acted on by the channel, so the definition that the channel is running with is now different from the one that was originally supplied. The MQCXP SharingConversations parameter is set to TRUE.

The next time that the application connects using this channel, the conversation runs on the channel instance which was started previously, because it has the same original channel definition. The channel instance the application connects to the second time is the same instance as the first time it connected. Consequently, it uses the definitions that have been altered by the exit program. When the exit program is initialized for the second conversation, although it can alter MQCD fields, they are not acted on by the channel. These same characteristics apply to any subsequent conversations which share the channel instance.