Ajax/amq: Lost messages in MessageListenerServlet if more than maxMessages waiting

[gminet <gm...@easynet.be>]

Hi,

To test our ajax/amq application, we diminished the maximumMessages value in
MessageListenerServlet to some ridiculous value: 2 instead of 100.

We immediately began to lose every third message whenever there were more
that 2 messages waiting in the queue. I think we found the bug in
MessageListenerServlet:

In the following extract of doMessages() :

 // Look for any available messages
                message = consumer.receiveNoWait();
                while (message != null && messages < maximumMessages) {
                    String id = (String) consumerIdMap.get(consumer);
                    writer.print("<response id='");
                    writer.print(id);
                    writer.print("'>");
                    writeMessageResponse(writer, message);
                    writer.println("</response>");
                    messages++;
                    message = consumer.receiveNoWait();
                }

If we reach the messages > maximumMessages condition, the last one that has
been removed from the queue (message = consumer.receiveNoWait()) wont ever
be sent to the client;

If just added the following code as a quick fix after the while loop:

 if (message != null) {
                   String id = (String) consumerIdMap.get(consumer);
                    writer.print("<response id='");
                    writer.print(id);
                    writer.print("'>");
                    writeMessageResponse(writer, message);
                    writer.println("</response>");
                    messages++;
                }

Hope it helps

Regards
Gaetan
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Re: Ajax/amq: Lost messages in MessageListenerServlet if more than maxMessages waiting

[James Strachan <ja...@gmail.com>]

On 5/20/07, gminet <gm...@easynet.be> wrote:
>
> Hi,
>
> To test our ajax/amq application, we diminished the maximumMessages value in
> MessageListenerServlet to some ridiculous value: 2 instead of 100.
>
> We immediately began to lose every third message whenever there were more
> that 2 messages waiting in the queue. I think we found the bug in
> MessageListenerServlet:
>
> In the following extract of doMessages() :
>
>  // Look for any available messages
>                 message = consumer.receiveNoWait();
>                 while (message != null && messages < maximumMessages) {
>                     String id = (String) consumerIdMap.get(consumer);
>                     writer.print("<response id='");
>                     writer.print(id);
>                     writer.print("'>");
>                     writeMessageResponse(writer, message);
>                     writer.println("</response>");
>                     messages++;
>                     message = consumer.receiveNoWait();
>                 }
>
> If we reach the messages > maximumMessages condition, the last one that has
> been removed from the queue (message = consumer.receiveNoWait()) wont ever
> be sent to the client;
>
> If just added the following code as a quick fix after the while loop:
>
>  if (message != null) {
>                    String id = (String) consumerIdMap.get(consumer);
>                     writer.print("<response id='");
>                     writer.print(id);
>                     writer.print("'>");
>                     writeMessageResponse(writer, message);
>                     writer.println("</response>");
>                     messages++;
>                 }
>
> Hope it helps
>
> Regards
> Gaetan

Many thanks for the fix; patch applied on trunk!

I made a slight change to the code to avoid having to duplicate code;
I just split the boolean expression and moved the check for null
messages inside the while loop

-- 
James
-------
http://macstrac.blogspot.com/