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https://issues.apache.org/jira/browse/CAMEL-14233?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16986801#comment-16986801
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Claus Ibsen commented on CAMEL-14233:
-------------------------------------

Yeah ideally you should use OVERRIDE header if you set it manually. But the 
code likely still need to evaluate the header per message in that iterator, 
instead of once outside

> camel-kafka - topic overriding not possible when using aggregation
> ------------------------------------------------------------------
>
>                 Key: CAMEL-14233
>                 URL: https://issues.apache.org/jira/browse/CAMEL-14233
>             Project: Camel
>          Issue Type: Bug
>          Components: camel-kafka
>    Affects Versions: 2.24.2
>            Reporter: Rafał Gała
>            Priority: Minor
>             Fix For: 3.1.0
>
>
> When exchange aggregation is used, using GroupedExchangeAggregationStrategy 
> for example:
>  
> {code:java}
> from(..)
> .process(some processor here that sets KafkaConstants.TOPIC header here)
> .aggregate(new GroupedExchangeAggregationStrategy ())
> .to(kafka:...)
> {code}
> it is not possible to override topic per exchange by using 
> KafkaConstants.TOPIC header, because in *createRecord* of KafkaProducer 
> class, the topic is chosen from header of aggregating Exchange which may not 
> be set because it may have been set only on Exchanges that were aggregated. 
> When creating ProducerRecord from Iterable, the topic should be chosen from 
> header of each Exchange separately:
> {code:java}
> protected Iterator<ProducerRecord> createRecorder(Exchange exchange) throws 
> Exception {
>     String topic = endpoint.getConfiguration().getTopic();
>     if (!endpoint.getConfiguration().isBridgeEndpoint()) {
>         String headerTopic = exchange.getIn().getHeader(KafkaConstants.TOPIC, 
> String.class);
>         boolean allowHeader = true;
>         // when we do not bridge then detect if we try to send back to 
> ourselves
>         // which we most likely do not want to do
>         if (headerTopic != null && 
> endpoint.getConfiguration().isCircularTopicDetection()) {
>             Endpoint from = exchange.getFromEndpoint();
>             if (from instanceof KafkaEndpoint) {
>                 String fromTopic = ((KafkaEndpoint) 
> from).getConfiguration().getTopic();
>                 allowHeader = !headerTopic.equals(fromTopic);
>                 if (!allowHeader) {
>                     log.debug("Circular topic detected from message header."
>                             + " Cannot send to same topic as the message 
> comes from: {}"
>                             + ". Will use endpoint configured topic: {}", 
> from, topic);
>                 }
>             }
>         }
>         if (allowHeader && headerTopic != null) {
>             topic = headerTopic;
>         }
>     }
>     if (topic == null) {
>         // if topic property was not received from configuration or header 
> parameters take it from the remaining URI
>         topic = URISupport.extractRemainderPath(new 
> URI(endpoint.getEndpointUri()), true);
>     }
> ...
>     Object msg = exchange.getIn().getBody();
>     // is the message body a list or something that contains multiple values
>     Iterator<Object> iterator = null;
>     if (msg instanceof Iterable) {
>         iterator = ((Iterable<Object>) msg).iterator();
>     } else if (msg instanceof Iterator) {
>         iterator = (Iterator<Object>) msg;
>     }
>     if (iterator != null) {
>         final Iterator<Object> msgList = iterator;
> {code}
> The msgTopic variable below should be set from KafkaConstants.TOPIC header of 
> next exchange from collection
> {code}
>         final String msgTopic = topic;
>         return new Iterator<ProducerRecord>() {
>             @Override
>             public boolean hasNext() {
>                 return msgList.hasNext();
>             }
>             @Override
>             public ProducerRecord next() {
>                 // must convert each entry of the iterator into the value 
> according to the serializer
>                 Object next = msgList.next();
>                 Object value = tryConvertToSerializedType(exchange, next, 
> endpoint.getConfiguration().getSerializerClass());
>                 if (hasPartitionKey && hasMessageKey) {
>                     return new ProducerRecord(msgTopic, partitionKey, null, 
> key, value, propagatedHeaders);
>                 } else if (hasMessageKey) {
>                     return new ProducerRecord(msgTopic, null, null, key, 
> value, propagatedHeaders);
>                 } else {
>                     return new ProducerRecord(msgTopic, null, null, null, 
> value, propagatedHeaders);
>                 }
>             }
>             @Override
>             public void remove() {
>                 msgList.remove();
>             }
>         };
>     }
> ...
> }
> {code}
>  



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