Hello,<div><br></div><div>I am evaluating using rabbitMQ as a component in our backend system. I am planning to run it with a clustered configuration with queues mirrored on at least one node beside the one the queue is declared on. Each publisher opens channels with publisher confirms and the code implements a blocking call that waits for the ack to be received back before it returns control to the app. There will be a lot of publisher processes (order of magnitude in the tens of thousands) and each publisher will have a fairly low message per second rate (let's say 1 message per second).</div>
<div><br></div><div>I am running a very small experiment to evaluate the latency of the publish call and I am seeing that rabbitMQ holds the ack for roughly 25 milliseconds before sending it to the publisher.�To clarify what I am seeing, below is a wireshark capture that I took on the rabbit node for a single published message (gaps in the packet number are due to other traffic live on the machine).</div>
<div><br></div><div><font face="courier new, monospace">No. � � Time. � � � � � Source � � � � Src_port Destination � Dest_port Proto � Length �Info</font></div><div><font face="courier new, monospace">174<span class="" style="white-space:pre"> </span>3.070464<span class="" style="white-space:pre"> </span><span class="">publisher_ip</span><span class="" style="white-space:pre"> </span>28086<span class="" style="white-space:pre"> </span></font><span style="font-family:'courier new',monospace">rabbitmq_ip</span><span class="" style="font-family:'courier new',monospace;white-space:pre"> </span><span style="font-family:'courier new',monospace">amqp</span><span class="" style="font-family:'courier new',monospace;white-space:pre"> </span><span style="font-family:'courier new',monospace">AMQP</span><span class="" style="font-family:'courier new',monospace;white-space:pre"> </span><span style="font-family:'courier new',monospace">104</span><span class="" style="font-family:'courier new',monospace;white-space:pre"> </span><span style="font-family:'courier new',monospace">Basic.Publish�</span></div>
<div><font face="courier new, monospace">175<span class="" style="white-space:pre"> </span>3.070483<span class="" style="white-space:pre"> </span></font><span style="font-family:'courier new',monospace">publisher_ip</span><font face="courier new, monospace"><span class="" style="white-space:pre"> </span>28086<span class="" style="white-space:pre"> </span></font><span style="font-family:'courier new',monospace">rabbitmq_ip</span><font face="courier new, monospace"><span class="" style="white-space:pre"> </span>amqp<span class="" style="white-space:pre"> </span>AMQP<span class="" style="white-space:pre"> </span>90<span class="" style="white-space:pre"> </span>Content-Header�<br>
</font></div><div><font face="courier new, monospace">177<span class="" style="white-space:pre"> </span>3.070674<span class="" style="white-space:pre"> </span></font><span style="font-family:'courier new',monospace">publisher_ip</span><span class="" style="font-family:'courier new',monospace;white-space:pre"> </span><span style="font-family:'courier new',monospace">28086</span><span class="" style="font-family:'courier new',monospace;white-space:pre"> </span><span style="font-family:'courier new',monospace">rabbitmq_ip</span><span class="" style="font-family:'courier new',monospace;white-space:pre"> </span><span style="font-family:'courier new',monospace">amqp</span><span class="" style="font-family:'courier new',monospace;white-space:pre"> </span><span style="font-family:'courier new',monospace">AMQP</span><span class="" style="font-family:'courier new',monospace;white-space:pre"> </span><span style="font-family:'courier new',monospace">77</span><span class="" style="font-family:'courier new',monospace;white-space:pre"> </span><span style="font-family:'courier new',monospace">Content-Body�</span></div>
<div><font face="courier new, monospace">200<span class="" style="white-space:pre"> </span>3.072493<span class="" style="white-space:pre"> </span></font><span style="font-family:'courier new',monospace">rabbitmq_ip</span><span class="" style="font-family:'courier new',monospace;white-space:pre"> </span><span style="font-family:'courier new',monospace">amqp</span><span class="" style="font-family:'courier new',monospace;white-space:pre"> </span><span class="" style="font-family:'courier new',monospace">consumer_ip</span><span class="" style="font-family:'courier new',monospace;white-space:pre"> </span><span style="font-family:'courier new',monospace">28085</span><span class="" style="font-family:'courier new',monospace;white-space:pre"> </span><span style="font-family:'courier new',monospace">AMQP</span><span class="" style="font-family:'courier new',monospace;white-space:pre"> </span><span style="font-family:'courier new',monospace">149</span><span class="" style="font-family:'courier new',monospace;white-space:pre"> </span><span style="font-family:'courier new',monospace">Basic.Deliver Content-Header Content-Body�</span></div>
<div><font face="courier new, monospace">202<span class="" style="white-space:pre"> </span>3.073846<span class="" style="white-space:pre"> </span></font><span style="font-family:'courier new',monospace">consumer_ip</span><font face="courier new, monospace"><span class="" style="white-space:pre"> </span>28085<span class="" style="white-space:pre"> </span></font><span style="font-family:'courier new',monospace">rabbitmq_ip</span><span class="" style="font-family:'courier new',monospace;white-space:pre"> </span><span style="font-family:'courier new',monospace">amqp</span><span class="" style="font-family:'courier new',monospace;white-space:pre"> </span><span style="font-family:'courier new',monospace">AMQP</span><span class="" style="font-family:'courier new',monospace;white-space:pre"> </span><span style="font-family:'courier new',monospace">89</span><span class="" style="font-family:'courier new',monospace;white-space:pre"> </span><span style="font-family:'courier new',monospace">Basic.Ack�</span></div>
<div><font face="courier new, monospace">207<span class="" style="white-space:pre"> </span>3.098541<span class="" style="white-space:pre"> </span></font><span style="font-family:'courier new',monospace">rabbitmq_ip</span><span class="" style="font-family:'courier new',monospace;white-space:pre"> </span><span style="font-family:'courier new',monospace">amqp</span><span class="" style="font-family:'courier new',monospace;white-space:pre"> </span><span class="" style="font-family:'courier new',monospace">publisher_ip</span><span class="" style="font-family:'courier new',monospace;white-space:pre"> </span><span style="font-family:'courier new',monospace">28086</span><span class="" style="font-family:'courier new',monospace;white-space:pre"> </span><span style="font-family:'courier new',monospace">AMQP</span><span class="" style="font-family:'courier new',monospace;white-space:pre"> </span><span style="font-family:'courier new',monospace">89</span><span class="" style="font-family:'courier new',monospace;white-space:pre"> </span><span style="font-family:'courier new',monospace">Basic.Ack�</span></div>
<div><font face="arial, helvetica, sans-serif"><br></font></div><div><font face="arial, helvetica, sans-serif">I did multiple experiments and this behavior is very consistent. All messages are delivered reasonably fast apart of the very last one that seem to be held in rabbitMQ for 25 milliseconds before it's delivered on the network. Why is this happening? Is RabbitMQ trying to aggregate multiple messages together before delivering them on the network? Is there a way to speed up this behavior?</font></div>
<div><font face="arial, helvetica, sans-serif"><br></font></div><div><font face="arial, helvetica, sans-serif">Cheers, Pierpaolo</font></div>