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Synchronous
queue consumers consume events by calling pop() on the
Nirvana 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 rollback.
An example of a transactional synchronous queue reader
is shown below:
public class mySyncTxQueueReader {
nQueueSyncTransactionReader reader = null;
nQueue myQueue = null;
public mySyncTxQueueReader() throws Exception
{
// 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() throws Exception {
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) {
System.out.println("Consumed event "+event.getEventID());
}
public static void main(String[] args) {
try {
mySyncTxQueueReadersqr = new mySyncTxQueueReader();
sqr.start();
} catch (Exception e) {
e.printStackTrace();
}
}
}
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 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.
Sychronous queue consumers can also be created using
a selector, which defines a set of event
properties and their values that a consumer is interested
in. For example if events are being published with the
following event properties:
nEventProperteis props =new nEventProperties();
props.put(“BONDNAME”,”bond1”);
If you then provide a message selector string in the
form of:
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.
An example of a synchronous queue consumer can be found
here.
For more information on Nirvana message queues, please
see the API documentation.
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