Synchronous Transactional Queue Consumption
Synchronous queue consumers consume events by calling pop() on the Universal Messaging queue reader object. Each pop call made on the queue reader will synchronously retrieve the next event from the queue.
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 a rollback.
An example of a transactional synchronous queue reader is shown below:
public class mySyncTxQueueReader {
nQueueSyncTransactionReader reader = null;
nQueue myQueue = null;
public mySyncTxQueueReader() {
// construct your session and queue objects here
// construct the transactional queue reader
nQueueReaderContext ctx = new nQueueReaderContext(this, 10);
reader = myQueue.createTransactionalReader(ctx);
}
public void start() {
while (true) {
// pop events from the queue
nConsumeEvent event = reader.pop();
go(event);
// commit each event consumed
reader.commit(event.getEventID());
}
}
public void go(nConsumeEvent event) {
Console.WriteLine("Consumed event "+event.getEventID());
}
public static void Main(String[] args) {
mySyncTxQueueReadersqr = new mySyncTxQueueReader();
sqr.start();
}
}
As previously mentioned, the big difference between a transactional synchronous reader and a standard synchronous 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 roll back 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 IDs 0 to 9, you can commit event 4, which will only commit events 0 to 4 and roll back events 5 to 9.
Note:
When you use nQueueSyncTransactionReader's commit or rollback APIs to acknowledge or roll back received events, the APIs commit or roll back all received events, including events received by other synchronous transactional readers for this queue in the same session. To avoid this behavior when consuming from a queue with multiple transactional readers, please create each reader with its own dedicated session.
Subscription with a Filtering Selector
Synchronous queue consumers can also be created using a selector, which allows the subscription to be filtered based on event properties and their values.
For example, assume some events are being published with the following event properties:
nEventProperties props = new nEventProperties();
props.put("BONDNAME", "bond1");
A developer can create a message selector string such as:
String selector = "BONDNAME='bond1'";
Passing this string into the constructor for the nQueueReaderContext object shown in the example code will ensure that the subscriber will only consume messages that contain the correct value for the event property BONDNAME.