Why header value isn't inserted into Apache Camel's `to` url parameter of the route in Spring DSL?

Question

We have a route that will accept the kafka.KEY and use that as mqtt url parameter to send the data to the right topic.

<routes
    xmlns="http://camel.apache.org/schema/spring">
    <route id="KafkaToMQTT">
        <from uri="kafka://mqtt?brokers=localhost:9092"/>
        <to uri="micrometer:timer:camel.proxy.kafka.mqtt.stream?action=start"/>
        <log message="Headers ${header.kafka.KEY}"/>
        <to uri="mqtt:mqtt?host=tcp://localhost:1883&amp;publishTopicName=try${header.kafka.KEY}"/>
        <to uri="log://camel.proxy?groupInterval=3&amp;level=INFO"/>
        <to uri="micrometer:timer:camel.proxy.kafka.mqtt.stream?action=stop"/>      
    </route>
</routes>

In the log messages I see the ${header.kafka.KEY} correctly, while in the mqtt I'm getting the topic as literally try${header.kafka.KEY}

What is the reason for that, how to make the header to be used there?

Answer 1

To avoid that the right element instead of to should be used, that is toD.

toD concatenates the url correctly, so the right route XML is:

<routes
    xmlns="http://camel.apache.org/schema/spring">
    <route id="KafkaToMQTT">
        <from uri="kafka://mqtt?brokers=localhost:9092"/>
        <to uri="micrometer:timer:camel.proxy.kafka.mqtt.stream?action=start"/>
        <log message="Headers ${header.kafka.KEY}"/>
        <toD uri="mqtt:mqtt?host=tcp://localhost:1883&amp;publishTopicName=ESP_02/try${header.kafka.KEY}"/>
        <to uri="log://camel.proxy?groupInterval=3&amp;level=INFO"/>
        <to uri="micrometer:timer:camel.proxy.kafka.mqtt.stream?action=stop"/>      
    </route>
</routes>