Multicast and the MQI

Use this information to understand the major Message Queue Interface (MQI) concepts and how they relate to IBM MQ Multicast.

Multicast subscriptions are nondurable; because there are no physical queues involved, there is nowhere to store the offline messages that are created by durable subscriptions.

After an application has subscribed to a multicast topic, it is given back an object handle which it can consume or MQGET from, as if it were a handle to a queue. This means that only managed multicast subscriptions (subscriptions created with MQSO_MANAGED) are supported, that is; it is not possible to make a subscription and 'point' the messages at a queue. This means that messages must be consumed from the object handle returned on the subscription call. On the client, the messages are stored in a message buffer until they are consumed by the client; see MessageBuffer stanza of the client configuration file for more information. If the client does not keep up with the publishing rate, the messages are discarded as required, with the oldest messages discarded first.

It is normally an administration decision whether an application uses Multicast or not, specified by setting the MCAST attribute of a TOPIC object. If a publishing application must ensure that multicast is not used, it can use the MQOO_NO_MULTICAST option. Similarly, a subscribing application can ensure that multicast is not used by subscribing with the MQSO_NO_MULTICAST option.

IBM MQ Multicast supports the use of message selectors. A selector is used by an application to register its interest in only those messages with properties that satisfy the SQL92 query that the selection string represents. For more information about message selectors, see Selectors.

The following table lists all the major MQI concepts and how they relate to Multicast:

MQI Concept Action when tried using multicast Reason code
Put a zero length message Rejected 2005 (07D5) (RC2005): MQRC_BUFFER_LENGTH_ERROR
Grouping Rejected 2046 (07FE) (RC2046): MQRC_OPTIONS_ERROR
Segmentation Rejected 2443 (098B) (RC2443): MQRC_SEGMENTATION_NOT_ALLOWED
Distribution lists Rejected 2154 (086A) (RC2154): MQRC_RECS_PRESENT_ERROR
MQINQ Rejected for topics handles: MQINQ and MQSET of topics is not supported. 2038 (07F6) (RC2038): MQRC_NOT_OPEN_FOR_INQUIRE
MQINQ Accepted for managed handle. Only Current Depth can be inquired.

MQSET Rejected for all handles. 2040 (07F8) (RC2040): MQRC_NOT_OPEN_FOR_SET
Transactions (XA or not) Rejected 2072 (0818) (RC2072): MQRC_SYNCPOINT_NOT_AVAILABLE
Message browse Rejected 2036 (07F4) (RC2036): MQRC_NOT_OPEN_FOR_BROWSE
Lock messages Rejected 2046 (07FE) (RC2046): MQRC_OPTIONS_ERROR
Browse with mark Rejected 2036 (07F4) (RC2036): MQRC_NOT_OPEN_FOR_BROWSE
Pass context Rejected 2046 (07FE) (RC2046): MQRC_OPTIONS_ERROR
MQPUT1 Rejected. It is invalid to try and MQPUT1 to a Multicast only topic. 2560 (0A00) (RC2560): MQRC_MULTICAST_ONLY
Durable subscription Rejected if the topic is marked as "Multicast only", otherwise a non-Multicast subscription is made. 2436 (0984) (RC2436): MQRC_DURABILITY_NOT_ALLOWED
TopicString > 255 Rejected. If the topic string is greater than 255 characters, it is rejected in the client. 2425 (0979) (RC2425): MQRC_TOPIC_STRING_ERROR
Non-managed subscription made Rejected if the topic is marked as "Multicast only", otherwise a non-Multicast subscription is made. 2046 (07FE) (RC2046): MQRC_OPTIONS_ERROR
MQPMO_NOT_OWN_SUBS Rejected 2046 (07FE) (RC2046): MQRC_OPTIONS_ERROR
The following items expand on some of the MQI concepts from the previous table, and provides information on some of the MQI concepts that are not in the table:

    Message persistence
    For nondurable multicast subscribers, persistent messages from the publisher are delivered in an unrecoverable fashion.

    Message truncation
    Message truncation is supported, which means that it is possible for an application to:
    1. Issue an MQGET.
    2. Get MQRC_TRUNCATED_MSG_FAILED.
    3. Allocate a larger buffer.
    4. Reissue the MQGET to retrieve the message.

    Subscription expiry
    Subscription expiry is not supported. Any attempt to set an expiry is ignored.

Parent topic: Multicast programming