<p>How To deploy in kubernetes a message consumer for kafka, amqp or any other message broker which scales up and down ? My hypothesis is that the consumer runs a loop which pulls messages.</p>

<p>I d like kubernetes To create more pods when many messages arrive in the broker queue and remove some pods when too few messages arrive in the queue.</p>

<p>Which component has the initiative of the ending of the pods? The pod itself because it can't fetch a message from the queue? Or kubernetes because the pod doesnt consume cpu?</p>

<p>If any pod ends when the queue is empty, i m afraid that pods Will keep born and die as long as the queue is empty.</p>

<p>The Kubernetes Horizontal Pod Autoscaler has support for <a href="https://kubernetes.io/docs/tasks/run-application/horizontal-pod-autoscale/#support-for-custom-metrics" rel="noreferrer">custom and external metrics</a>. With more traditional messaging brokers like AMQP (1 queue / many competing consumers) you should be able to easily scale the consumer based on queue depth (such as <em>If queue depth is >= 10000 msg, scale up. If queue depth is &lt;= 1000 msg scale down</em>). You could also do it based on your the average client throughput (such as <em>if average throughput is >= 5000 msg/s, scale up</em>) or average latency. The Horizontal Pod Autoscaler would do the scale up and scale down for you. It will observer the metrics and decide when a pod should be shutdown or started. The consumer application is not aware of this - it doesn't need any special support for this. But you will need to get these metrics and expose them so that Kubernetes can consume them which is currently not completely trivial.</p>

<p>With Kafka, this will be a bit harder since Kafka implements competing consumers very differently from more traditional messaging brokers like AMQP. Kafka topics are split into partitions. And each partition can have only one consumer from a single consumer group. So whatever autoscaling you do, it will not be able to handle situations such as:</p>

<ul>
<li>Small number of partitions for given topic (you will never have more active consumers than the number of partitions) </li>
<li>Asymmetric partition load (some partitions being very busy while other are empty)</li>
</ul>

<p>Kafka also doesn't have anything like queue depth. But you can for example use the information about the consumer lag (which shows how much is the consumer behind the producer for given partition) to do the scaling. </p>
