Example 1: MQ Publication consumer

The MQ Publication consumer is an IBM MQ message consumer that does not subscribe to topics itself.

To create the subscription and publication queue for this example run the following commands, or define the objects using IBM MQ Explorer.

DEFINE QLOCAL(STOCKTICKER) REPLACE;
DEFINE SUB(IBMSTOCKPRICESUB) DEST(STOCKTICKER) TOPICOBJ(IBMSTOCKPRICE) REPLACE;

The IBMSTOCKPRICESUB subscription references the IBMSTOCK topic object created for the publisher example and the local queue STOCKTICKER. The topic object IBMSTOCK defines the topic string that is used in the subscription, NYSE/IBM/PRICE. Note that the topic object and the queue used to receive publications need to be defined before the subscription is created.

There are a number of valuable facets to the MQ publication consumer pattern:

1. Multiprocessing: sharing out of the work of reading publications. The publications all go onto the single queue associated with the subscription topic. Multiple consumers can open the queue using MQOO_INPUT_SHARED.
2. Centrally managed subscriptions. Applications do not construct their own subscription topics or subscriptions; the administrator is responsible for where publications are sent.
3. Subscription concentration: multiple different subscriptions can be sent to a single queue.
4. Subscription durability: the queue receives all publications whether or not consumers are active.
5. Migration and coexistence: the consumer code works equally well for a point-to-point and a publish/subscribe scenario.

The subscription creates a relationship between the topic string NYSE/IBM/PRICE and the queue STOCKTICKER. Publications, including any currently retained publication, are forwarded to STOCKTICKER from the moment the subscription is created.

An administratively created subscription can be managed or unmanaged. A managed subscription takes effect as soon as it has been created, just like an unmanaged subscription. Not all the pattern facets are available to a managed subscription. See Example 3: Unmanaged MQ subscriber

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <cmqc.h>
int main(int argc, char **argv)
{
MQCHAR	 publicationBuffer[101];
MQCHAR48 subscriptionQueueDefault = "STOCKTICKER";
MQCHAR48 qmName = "";            /* Use default queue manager  */

MQHCONN Hconn = MQHC_UNUSABLE_HCONN;    /* connection handle      */
MQHOBJ  Hobj = MQHO_NONE;         /* object handle sub queue   */
MQLONG  CompCode = MQCC_OK;        /* completion code       */
MQLONG  Reason = MQRC_NONE;        /* reason code         */
MQLONG  messlen = 0;
MQOD   od = {MQOD_DEFAULT};        /* Unmanaged subscription queue */
MQMD   md = {MQMD_DEFAULT};        /* Message Descriptor      */
MQGMO  gmo = {MQGMO_DEFAULT};       /* Get message options     */
char *  publication=publicationBuffer;
char *  subscriptionQueue = subscriptionQueueDefault;

switch(argc){           /* Replace defaults with args if provided */
default:
subscriptionQueue = argv[1]
case(1):
printf("Optional parameter: subscriptionQueue\n");
}

do {
MQCONN(qmName, &Hconn, &CompCode, &Reason);
if (CompCode != MQCC_OK) break;
strncpy(od.ObjectName, subscriptionQueue, MQ_Q_NAME_LENGTH);
MQOPEN(Hconn, &od, MQOO_INPUT_AS_Q_DEF | MQOO_FAIL_IF_QUIESCING, &Hobj, &CompCode, &Reason);
if (CompCode != MQCC_OK) break;
gmo.Options = MQGMO_WAIT | MQGMO_NO_SYNCPOINT | MQGMO_CONVERT;
gmo.WaitInterval = 10000;
printf("Waiting %d seconds for publications from %s\n", gmo.WaitInterval/1000, subscriptionQueue);
do {
memcpy(md.MsgId, MQMI_NONE, sizeof(md.MsgId));
memcpy(md.CorrelId, MQCI_NONE, sizeof(md.CorrelId));
md.Encoding    = MQENC_NATIVE;
md.CodedCharSetId = MQCCSI_Q_MGR;
memset(publication, 0, sizeof(publicationBuffer));
MQGET(Hconn, Hobj, &md, &gmo, sizeof(publicationBuffer)-1, publication, &messlen,
&CompCode, &Reason);
if (Reason == MQRC_NONE)
printf("Received publication \"%s\"\n", publication);
}
while (CompCode == MQCC_OK);
if (CompCode != MQCC_OK && Reason != MQRC_NO_MSG_AVAILABLE) break;
MQCLOSE(Hconn, &Hobj, MQCO_NONE, &CompCode, &Reason);
if (CompCode != MQCC_OK) break;
MQDISC(&Hconn, &CompCode, &Reason);
} while (0);
printf("Completion code %d and Return code %d\n", CompCode, Reason);
}

X:\Subscribe1\Debug>Subscribe1
Optional parameter: subscriptionQueue
Waiting 10 seconds for publications from STOCKTICKER
Received publication "129"
Completion code 0 and Return code 0

There are a couple of standard IBM MQ C language programming tips to be aware of:

memset(publication, 0, sizeof(publicationBuffer));

Ensure the message has a trailing null for easy formatting using printf. The publisher example includes the trailing null in the message buffer passed to MQPUT by adding 1 to strlen(publication). Setting MQCHAR buffers to null is good programming style for IBM MQ C programs that use the buffers to store strings, ensuring a null follows an array of characters that does not completely fill the buffer.

MQGET(Hconn, Hobj, &md, &gmo, sizeof(publicationBuffer)-1, publication, &messlen, &CompCode, &Reason);

Reserve one null at the end of the message buffer to ensure the returned message has trailing null in case if (messlen == strlen(publication)); is true. This tip complements the preceding one, and ensures that there is at least one null in publicationBuffer that is not overwritten by the contents of publication.

Parent topic: Writing subscriber applications

Related concepts

• Example 2: Managed MQ subscriber
• Example 3: Unmanaged MQ subscriber
• Writing publisher applications