Irresponsible concurrentConsumers property of the Apache Camel JMS endpoint

Question

I use concurrentConsumers=20 property for JMS endpoint in my route, but I have got different number of consumers.

My route is:

<?xml version="1.0" encoding="UTF-8"?>
<blueprint xmlns="http://www.osgi.org/xmlns/blueprint/v1.0.0">

    <bean id="artemisConnectionFactory" class="org.apache.activemq.artemis.jms.client.ActiveMQJMSConnectionFactory">
        <argument index="0" value="tcp://localhost:61616"/>
        <argument index="1" value="karaf"/>
        <argument index="2" value="karaf"/>
    </bean>

    <bean id="jms" class="org.apache.camel.component.jms.JmsComponent">
        <property name="connectionFactory" ref="artemisConnectionFactory"/>
    </bean>

    <camelContext id="test-jms-context" xmlns="http://camel.apache.org/schema/blueprint">
        <route id="messages-consumer-route" autoStartup="true">
            <from uri="jms:queue:concurrent1?concurrentConsumers=20"/>
            <log loggingLevel="INFO" message="start"/>
            <delay>
                <constant>10000</constant>
            </delay>
            <log loggingLevel="INFO" message="finish"/>
        </route>
    </camelContext>
</blueprint>

But on route/context/bundle restart I have got a random number of consuming threads: from 1 to 20 (concurrent consumers value).

I've tested on Camel 3.4.5 and 3.21.0.SNAPSHOT with both ActiveMQ Artemis 2.19.0 and 2.28.0.

Why number of threads not matches exactly to concurrentConsumers property?

Answer 1

I imagine that Camel will only create as many consumers as absolutely necessary to consume the messages. This will almost certainly be impacted by the flow control on the consumers. I recommend specifying consumerWindowSize=0 on your connection URL, e.g.:

tcp://localhost:61616?consumerWindowSize=0

This will ensure that one consumer doesn't prefetch a lot of messages and starve other consumers. That would increase the chances that more consumers are created.