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How exactly is JMSReplyTo handled by Apache Camel? When does camel implicitly utilises the destination?

Using spring-camel, I have built a route that consumes from a JMS topic (with JMSReplyTo expected to be set for each input message), splits the message into smaller chunks, sends them to a REST processsor, then aggregates the answers and should produce an output message to the destination pointed by JMSReplyTo. Unfortunately, camel implicitly utilises the JMSReplyTo destination in one of the intermediate steps (producing an unmarshalled POJO).
We have a functional requirement to adapt JMSReplyTo in order to provide a request-reply messaging service.
I am able to read the JMSReplyTo header before ending the route and I am explicitly converting it to CamelJmsDestinationName, which successfully overrides the destination for JMS component and produces the message on the output topic. I am not sure if this is the best approach and the problem is that camel still utilises the JMSReplyTo on its own.
My RouteBuilder configuration is as follows:
from("jms:topic:T.INPUT")
.process(requestProcessor)
.unmarshal().json(JsonLibrary.Jackson, MyRequest.class)
.split(messageSplitter)
.process(restProcessor)
.aggregate(messagesAggregator)
.unmarshal().json(JsonLibrary.Jackson, BulkResponses.class)
.process(responseProcessor)
.to("jms:topic:recipientTopic");
T.INPUT is the name of the input topic, while recipientTopic is just a placeholder that will be replaced by CamelJmsDestinationName.
I'm not keen on using CamelJmsDestinationName and a sort of a mocked up topic name in route configuration so I'm open to find a better solution. It would be great if camel utilised the JMSReplyTo automatically to produce the output message to the output topic.
Currently, the problem is that camel produces an intermediate output on the JMSReplyTo topic BUT the output is an unmarshalled MyRequest object, which results in an exception saying "ClassNotFoundException: (package name).MyRequest", which is obvious since this is only a class used in my internal processing - I don't want to produce this to the output topic. It seems like Camel does implicitly use the JMSReplyTo destination between requestProcessor and messageSplitter processing... Why? What am I doing wrong? What are the best practices?

Use "disableReplyTo=true" in Endpoint. Camel will not try to use any reply option.
Refer: https://camel.apache.org/jms.html for more details

I have found the answer... this is absurdly easy but I haven't seen it anywhere in the documentation.
You just need to call .stop() to mark the route as completed, and Camel will reply the body you configured in the last step to the destination configured in ${header.JMSReplyTo}. It's that simple.
So you can do:
from("jms:my-queue")
.unmarshall().json(JsonLibrary.Jsonb, InboundMessage.class)
.bean(SomeProcessingBean.class)
....
.log(LoggingLevel.INFO, "Sending reply to: " + simple("${header.JMSReplyTo}").getExpression().toString())
.marshall().json(JsonLibrary.Jsonb, ReplyMessage.class)
.stop();
And you will receive reply.
I wonder why no one has found this before... Nothing when I search the doc or here.... I must be dumb, or the doc is incomplete...but I am not dumb, so.

Camel Exchange on different routes in the same route builder

My REST application will post data to a queue (Q1) on rabbitMQ. There's another separate application that will read from Q1, process the data and post the result back to Q2. My application will read the data from Q2 and return the result. Many clients will use these 2 queues so I generate a UUID and set it in the header so that I can listen on Q2 (the response topic). I will then query each incoming message and match the incoming UUID in the header to the one I generated when I posted to Q1.
from("direct:test")
.choice().when(isValid)
.bean(FOOProcessor.class, "setFooQuery")
.to(FOO_REQUEST_QUEUE).log(LoggingLevel.INFO, "body=${in.body}")
.otherwise()
.setBody(constant("error"))
.setHeader(Exchange.HTTP_RESPONSE_CODE, constant(400)).log(LoggingLevel.INFO, "body=${in.body}")
.to("direct:error");
from(FOO_RESPONSE_QUEUE)
.unmarshal(new JacksonDataFormat(JsonNode.class))
.bean(FooProcessor.class, "setFooResponse")
.to("direct:end");
from("direct:error").log(LoggingLevel.DEBUG, "end");
from("direct:end").log(LoggingLevel.DEBUG, "end");
The trouble is the 2 "from" statements - they create to separate Camel exchanges/contexts and I can't get the original UUID. Any suggestions?

I solved this by using a processor that had a route builder embedded in it (with its own producer and consumer).
The processor provided a reference to the main exchange from this
process(final Exchange exchange)

apache camel - add message alert to deadletter queue

String queueA = "rabbitmq://host:5672/queue-a.exchange?queue=queue-a.exchange..etc
from(queueA)
.routeId("idForQueueA")
.onException(Exception.class)
.maximumRedeliveries(0)
// .processRef("sendEmailAlert") * not sure this belongs here*
.to(deadLetterQueueA)
.useOriginalMessage()
.end()
.processRef("dataProcessing")
.processRef("dataExporting")
.end();
Explaining the code above:
Messages are taken from queueA. Upon various processes being successful the message is consumed. If it fails its added to the dead letter queue "deadLetterQueueA". This all works ok.
My question is
When messages arrive in the deadletter queue I want to add alerts so we know to do something about it... How could I to add an email alert when a message arrives in the dead letter queue. I dont want to lose the original message if the alert fails - nor do I want the alert to consume the message.
My thoughts are.. I would need to split the message on an exception so its sent to two different queues? One for the alert which then sends out an email alert and then consumes itself. Then one for the dead letter queue that just sites there? However I'm not sure how to do this?

You can split a message to go to multiple endpoints using a multicast (details here):
.useOriginalMessage().multicast().to(deadLetterQueueA, "smtp://username#host:port?options")
This uses the camel mail component endpoints described here. Alternatively, you can continue processing the message after the to. So something like:
.useOriginalMessage()
.to(deadLetterQueueA)
.transform().simple("Hi <name>, there has been an error on the object ${body.toString}")
.to("smtp://username#host:port?options")
If you had multiple recipients, you could use a recipients list
public class EmailListBean {
#RecipientList
public String[] emails() {
return new String[] {"smtp://joe#host:port?options",
"smtp://fred#host:port?options"};
}
}
.useOriginalMessage()
.to(deadLetterQueueA)
.transform().simple("...")
.bean(EmailListBean.class)
Be careful of using JMS queues to store messages while waiting for a human to action them. I don't know what sort of message traffic you're getting. I'm assuming if you want to send an email for every failure, it's not a lot. But I would normally be wary of this sort of thing, and chose to use logging or database persistence to store the results of errors, and only use a JMS error queue to notify other processes or consumers of the error or to schedule a re-try.

There are two ways you can do this , but based on your message volume you might not want to send email on every failed message.
You can use the solution provided by AndyN , or you can use the Advisory Topics ActiveMQ.Advisory.MessageDLQd.Queue.* , whenever a message gets in to the DLQ the enqueue count of the topic will increase by 1 . By monitoring the Queue Depth you might now be able to send a mail to based on the number of the errors that ocurred.
If you want to do it at the producer end. You can use any one of the solutions provided by AndyN

Camel aggregation strategy

I am parsing a CSV file, splitting it and routing it through multiple processors in camel. There are two endpoints , one having erroneous data while other has validated data.
I need suggestion in aggregating the data.
Let's say the CSV file has 10 records out of which 6 reached one endpoint while 4 reached to another. How can I know if all 10 has completed from the file at each endpoint and move ahead of aggregator.
I need to create two files one with valid data and other with corrupt data from a single file.

Lets look at what the splitter returns.
According to the documentation on Camel 2.2. or older the splitter will by default return the last split message using your example this would probably be the last line to complete its processor so it might not be line 10 (using your example).
On Camel 2.3 and newer the splitter will by default return the original input message i.e. all 10 lines. This is the default behavior and you dont need to code anything for this to work. When the splitter is finished by default it will pass this message along to the next end point.
So if I was using the following DSL on Camel 2.3 or newer:
<camelContext trace="false" id="blueprintContext" xmlns="http://camel.apache.org/schema/blueprint">
<route id="splitExample">
<from uri="timer://myTimer?period=2000"/>
<setBody>
<simple>A\nB\nC</simple>
</setBody>
<log message="The message body before the splitter contains ${body}"/>
<split>
<tokenize token="\n"></tokenize>
<log message="Split line ${body}"/>
</split>
<log message="The message body after the splitter contains ${body}"/>
</route>
</camelContext>
The following would appear in the log:
INFO The message body before the splitter contains
A
B
C
INFO Split line A
INFO Split line B
INFO Split line C
INFO The message body after the splitter contains
A
B
C
As you can see camel by default combines the messages back into one after the splitter returns. To override this behavior you would need to implement your own aggregator. To do so create a class lets call it MyAggregationStrategy and make the class implement AggregationStrategy. I used the example in the apache documentation from here. example we will aggregate incoming bids and want to aggregate the highest bid.
private static class MyAggregationStrategy implements AggregationStrategy {
public Exchange aggregate(Exchange oldExchange, Exchange newExchange)
{
if (oldExchange == null)
{
// the first time we only have the new exchange so it wins the first round
return newExchange;
}
int oldPrice = oldExchange.getIn().getBody(Integer.class);
int newPrice = newExchange.getIn().getBody(Integer.class);
// return the "winner" that has the highest price
return newPrice > oldPrice ? newExchange : oldExchange;
}
}
After you have done this you then tell the splitter to use your aggregator by doing the following:
Spring/XML DSL:
<split strategyRef="MyAggregationStrategy ">
In Java:
from("direct:start")
// aggregated by header id and use our own strategy how to aggregate
.aggregate(new MyAggregationStrategy())
Hopefully this gives you enough insight about how the splitter works. In your case I would probably set a header value for each line indicating if it was successful or failed then I would use my customer aggregator to create a new message with failed and success grouped into two lists as the message body. One list with the failed and one list with the completed line items.
This new aggregated message can then be sent to a processor or another endpoint for further processing. For example you can then take the failed list and send that to a route which produces a file. The seda component can help a lot here.

Camel pattern for multiple sources specific messages aggregation and redirect to destination

I have one problem, and don't know how to solve it using camel. I searched for related EIP in camel documentation, but without results.
Now I have simple route:
<route id="routeId">
<from uri="Source1"/>
<from uri="Source2"/>
<to uri="Destination"/>
</route>
Both sources sends JMS messages to Destination and at some point when Source finish its job it send specific end message, with some flag. What I need to do is to collect or count those end messages and send single end message to destination when I receive end messages from both sources. Only when i receive two end messages (imagine that its just simple message with some header flag) then i should send single one to destination.
Sorry if problem explanation isn't clear enough.
Thanks in advance.

the Camel aggregator and filter patterns can be used for this scenario...
use a filter to detect "end" messages and route them through an aggregator
use a custom aggregation strategy to build up the single end message with a count
use a custom completion predicate to trigger the completion message
something like this...
from("source1").to("direct:aggregateRoute");
from("source2").to("direct:aggregateRoute");
from("direct:aggregateRoute")
.filter(header("isEndMessage").isEqualTo("true"))
.aggregate(constant(true), new MyAggregationStrategy())
.completionPredicate(new MyCompletionStrategy())
.to("destination");

If you just want to pick from multiple inputs and does not want to perform any modification on the incoming message,
you can do something like this:
from("URI1", "URI2", "URI3").to("DestinationUri");
for more info check this link it helped me