{"id":1531,"date":"2024-01-04T11:21:37","date_gmt":"2024-01-04T11:21:37","guid":{"rendered":"https:\/\/marketaylor.synology.me\/?p=1531"},"modified":"2025-07-20T07:17:56","modified_gmt":"2025-07-20T06:17:56","slug":"mqi-callback","status":"publish","type":"post","link":"https:\/\/marketaylor.synology.me\/?p=1531","title":{"rendered":"Unexpected behaviour in MQI Callback"},"content":{"rendered":"\n

While investigating a reported issue in the MQI NodeJS bindings<\/a>, I had to dig into the C library’s MQI callback functions. I found some behaviour that I didn’t expect. And some slight differences depending on whether you run an application with local or client bindings. [This difference turned out to be a defect in the MQ client, fixed in MQ 9.4.3 – discussed further below.]<\/p>\n\n\n\n\n\n\n\n

MQI Callbacks<\/h3>\n\n\n\n

The C MQI can drive callback functions when messages arrive on queues, giving an asynchronous approach for your applications. You don’t need to sit in a blocking MQGET(WAIT) or polling loop to retrieve the messages. The MQCB verb sets up the callback; you give it a function address and the MQ libraries call that function when needed.<\/p>\n\n\n\n

When you register the callback function through MQCB, one of the options is to choose which type of callbacks to receive. The MQCBD->CallbackType<\/code>field can be set to either MQCBT_EVENT_HANDLER<\/code> or MQCBT_MESSAGE_CONSUMER<\/code>. If you look at the amqsghac.c<\/em> sample program, you will see how it uses Events to report on the reconnection attempts made automatically by the client library.<\/p>\n\n\n\n

The callback function itself has a number of parameters. For this article, the interesting parameter is the MQCBC (CallbackContext) structure. Within that structure, 3 relevant fields are the Reason<\/code>, CallType<\/code> and Hobj<\/code>.<\/p>\n\n\n\n

The unexpected behaviour<\/h3>\n\n\n\n

I had a program that registered a Message Consumer callback. But I was surprised to find that sometimes the CallType<\/code> was set to MQCBCT_EVENT<\/code>. Why was I being given an Event, when I had not registered to receive one? <\/p>\n\n\n\n

It turns out that this is correct, and is even documented<\/a> as such. But I wasn’t expecting it. There is a subtle distinction between the CallType<\/code> and CallbackType<\/code> fields in their respective MQI structures.<\/p>\n\n\n\n

With a synchronous MQGET, if you end the queue manager, the verb is given an Reason code like MQRC_QMGR_QUIESCING<\/code>. But with the asynchronous operation, the equivalent notification is given by an Event instead of being tied to, say, a MSG_NOT_REMOVED<\/code> call type. Basically, failed MQGET responses are split into two types – those that are considered events, and those not.<\/p>\n\n\n\n

So a Message Consumer callback function must be prepared to deal with the Event CallType. <\/p>\n\n\n\n

Once I’d worked out what was going on, it was fine. It just took a slight reset of how I was thinking of the processing.<\/p>\n\n\n\n

Clients and Local bindings<\/h3>\n\n\n\n

I did find one slight difference between local bindings and client connection. The difference should not be important, and most properly-written programs ought not to care. But I thought it worth noting anyway.<\/p>\n\n\n\n

When the Message Consumer callback runs in local bindings mode, the associated Hobj<\/code> is set to the value that was specified during the corresponding MQCB even when the callback happens for a connection-wide Event such as the queue manager being shutdown. <\/p>\n\n\n\n

But in client mode, the Hobj<\/code> is set to 0. <\/p>\n\n\n\n

Note: This behaviour turned out to be a bug in the client layer; it was fixed in MQ 9.4.3 so the Hobj<\/code> is now the same as the local bindings mode. But you may still need to be aware of the bug if you are working with older versions of MQ.<\/strong><\/p>\n\n\n\n

Given that the queue manager is being shutdown, you would usually not be expected or able to make any additional “cleanup” calls such as using the Hobj<\/code> to MQCLOSE the queue. So the application should always ignore the Hobj<\/code>. Depending on how the queue manager is ending, you might have the opportunity to commit\/backout any in-flight transactions; those don’t depend on the object handle. But again, this Hobj<\/code> difference was slightly confusing.<\/p>\n\n\n\n

Conclusion<\/h3>\n\n\n\n

Seeing an event when I wasn’t expecting one, and seeing different object handle values than I was expecting, made me think at first that there might be a bug in one of the MQI libraries. It turns out that there isn’t. <\/p>\n\n\n\n

It mattered to me because of the packages I have released that wrap the C MQI functions for use in other environments. But I thought I’d write this up to help or warn anyone else who might be doing something similar.<\/p>\n\n\n\n

Updates:<\/h3>\n\n\n\n

2025\/07\/20: Reference to bug fix in MQ client code<\/p>\n

This post was last updated on July 20th, 2025 at 07:17 am<\/p>","protected":false},"excerpt":{"rendered":"

While investigating a reported issue in the MQI NodeJS bindings, I had to dig into the C library’s MQI callback functions. I found some behaviour that I didn’t expect. And some slight differences depending on whether you run an application with local or client bindings. [This difference turned out to be a defect in the … Continue reading “Unexpected behaviour in MQI Callback”<\/span><\/a><\/p>\n","protected":false},"author":1,"featured_media":0,"comment_status":"open","ping_status":"open","sticky":false,"template":"","format":"standard","meta":{"_jetpack_memberships_contains_paid_content":false,"footnotes":""},"categories":[5],"tags":[],"class_list":["post-1531","post","type-post","status-publish","format-standard","hentry","category-mq"],"yoast_head":"\nUnexpected behaviour in MQI Callback - Mark Taylor's Blog<\/title>\n<meta name=\"description\" content=\"The MQI Callback feature had behaviour that I didn't expect. 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