Initial multicast concepts

IBM MQ Multicast can be easily integrated into existing systems and applications by using the Communication Information (COMMINFO) object. Two TOPIC object fields enable the quick configuration of existing TOPIC objects to support or ignore multicast traffic.

Objects needed for multicast

The following information is a brief overview of the two objects needed for IBM MQ Multicast:

COMMINFO object

The COMMINFO object contains the attributes associated with multicast transmission. For more information about the COMMINFO object parameters, see DEFINE COMMINFO.

The only COMMINFO field that MUST be set is the name of the COMMINFO object. This name is then used to identify the COMMINFO object to a topic. The GRPADDR field of the COMMINFO object must be checked to ensure that the value is a valid multicast group address.

TOPIC object

A topic is the subject of the information that is published in a publish/subscribe message, and a topic is defined by creating a TOPIC object. For more information about the TOPIC object parameters, see DEFINE TOPIC. Existing topics can be used with multicast by changing the values of the following TOPIC object parameters: COMMINFO and MCAST.

• COMMINFO This parameter specifies the name of the multicast communication information object.
• MCAST This parameter specifies whether multicast is allowable at this position in the topic tree. By default, MCAST is set to ASPARENT meaning that the multicast attribute of the topic is inherited from the parent. Setting MCAST to ENABLED allows multicast traffic at this node.

Multicast networks and topics

The following information is an overview of what happens to subscriptions with different types of subscription and topic definition. These examples all assume that the TOPIC object COMMINFO parameter is set to the name of a valid COMMINFO object:

Topic set to multicast enabled

If the topic string MCAST parameter is set to ENABLED, subscriptions from multicast capable clients are allowed and a multicast subscription is made unless:

• It is a durable subscription from a multicast capable client.
• It is a non-managed subscription from a multicast capable client.
• It is a subscription from a non-multicast capable client.

In these cases a non-multicast subscription is made and subscriptions are downgraded to normal publish/subscribe.

Topic set to multicast disabled

If the topic string MCAST parameter is set to DISABLED, a non-multicast subscription is always made and subscriptions are downgraded to normal publish/subscribe.

Topic set to multicast only

If the topic string MCAST parameter is set to ONLY, subscriptions from multicast capable clients are allowed and a multicast subscription is made unless:

• It is a durable subscription: Durable subscriptions are rejected with reason code 2436 (0984) (RC2436): MQRC_DURABILITY_NOT_ALLOWED
• It is a non-managed subscription: Non-managed subscriptions are rejected with reason code 2046 (07FE) (RC2046): MQRC_OPTIONS_ERROR
• It is a subscription from a non-multicast capable client: These subscriptions are rejected with reason code 2560 (0A00) (RC2560): MQRC_MULTICAST_ONLY
• It is a subscription from a locally bound application: These subscriptions are rejected with reason code 2560 (0A00) (RC2560): MQRC_MULTICAST_ONLY