Universal Messaging Developer Guide : Enterprise APIs : Enterprise Developer's Guide for C++ : Message Queues : Asynchronous Transactional Queue Consuming
Asynchronous Transactional Queue Consuming
Asynchronous transactional queue consumers consume events from a callback on an interface that all asynchronous consumers must implement. We call this interface an nEventListener. The listener interface defines one method called 'go' which when called will pass events to the consumer as they are delivered from the Universal Messaging Realm Server.
Transactional queue consumers have the ability to notify the server when events have been consumed (committed) or when they have been discarded (rolled back). This ensures that the server does not remove events from the queue unless notified by the consumer with a commit or rollback.
An example of a transactional asynchronous queue reader is shown below:


class myAsyncTxQueueReader : public nEventListener {

private:
nQueueAsyncTransactionalReader *reader = null;
nQueue *myQueue = null;

public:
myAsyncTxQueueReader(){
// construct your session and queue objects here
// begin consuming events from the queue
nQueueReaderContext *ctx = new
nQueueReaderContext(this, 10);
reader = myQueue->createAsyncTransactionalReader(ctx);
}

void go(nConsumeEvent *event) {
printf("Consumed event %d",event->getEventID());
reader->commit();
}

int main(int argc, char** argv) {
new myAsyncTxQueueReader();
return 0;
}
}

As previously mentioned, the big difference between a transactional asynchronous reader and a standard asynchronous queue reader is that once events are consumed by the reader, the consumers need to commit the events consumed. Events will only be removed from the queue once the commit has been called.
Developers can also call the .rollback() method on a transactional reader that will notify the server that any events delivered to the reader that have not been committed, will be rolled back and redelivered to other queue consumers. Transactional queue readers can also commit or rollback any specific event by passing the event id of the event into the commit or rollback calls. For example, if a reader consumes 10 events, with event id's 0 to 9, you can commit event 4, which will only commit events 0 to 4 and rollback events 5 to 9.
Asynchronous queue consumers can also be created using a selector, which defines a set of event properties (see Event Dictionaries) and their values that a subscriber is interested in. For example if events are being published with the following event properties:

nEventProperties *props =new nEventProperties();
props->put("BONDNAME","bond1");
If you then provide a message selector string in the form of:

std::string selector = "BONDNAME='bond1'";
And pass this string into the constructor for the nQueueReaderContext object shown in the example code, then your consumer will only consume messages that contain the correct value for the event property BONDNAME.
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