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Camel consumer route with input from another consumer route

I have a kafka consumer route from where I get some data.
from("Kafka:foo?brokers=localhost:9092")
Once I receive data from the consumer, use that data in the topic name for a paho mqtt consumer.
from("paho:#?brokerUrl=tcp://localhost:1883")
I'm not able to figure out how to set the dynamic header CamelMqttTopic, from first consumer, as both seems independent flows. I'm using camel with Spring framework. Excuse me if my basic camel understanding is flawed.

You can override the MQTT topic using the CamelPahoOverrideTopic message header with a value being the Kafka topic accessed through the kafka.TOPIC message header:
from("kafka:foo?brokers=localhost:9092")
.setHeader(PahoConstants.CAMEL_PAHO_OVERRIDE_TOPIC, simple("${headers[kafka.TOPIC]"))
.to("paho:#?brokerUrl=tcp://localhost:1883");

How can I return response from RabbitMQ producer to RestController?

I have got two Spring Boot application. First one is REST application. The REST one is communicating with second application through RabbitMQ message queue. I'm sending a request to method with the #GetMapping("/") and this method producing a message to example-queue. A method with #RabbitListener(queues = {"example-queue"}) taking the message and create a object at database. Now, how can I send my response (saved object) to the method with #GetMapping("/")? I need a response from consumer to ResponseEntity.ok();. How can I do that? Thank you.

Just see if you can make the interaction with RabbitMQ consumer as a request-reply pattern.
The #RabbitListener can just return your object and be marked with a #SendTo. This way the framework will look into a replyTo property of the request message.
On the producer side you can just use an AmqpTemplate.convertSendAndReceive().
See more in docs: https://docs.spring.io/spring-amqp/docs/current/reference/html/#request-reply

How can a particular thread be notified in Spring Boot

I have a REST service defined in Spring Boot, which exposes a synchronous REST API, to be invoked by a user from a Web based UI. The service is expected to provide a real time response.
The service talks to an external endpoint in an asynchronous fashion. That is:
- A single one - way outbound message for the request
- A single one - way inbound message for the response
How can I combine the two messages to provide an impression of a synchronous behavior? I am thinking of a few approaches like:
The Rest Service posts a request to the endpoint. Once the endpoint responds, the response is added to a ConcurrentHashMap. The Rest Service queries the HashMap every few milliseconds and once it finds the right response it exits with a valid HTTP reason code.
It is akin to polling and I am thinking if we can avoid that.
The Rest Service posts a request to the endpoint. Once the endpoint responds, the waiting thread in the Rest Service is notified. However the waiting thread should conclude only if the right response is received (i.e. matching correlation Ids etc.)
Is this possible?
I realize that this is similar to a JMS Queue Request Response scenario, where each JMS queue request opens up a listener on the response queue with a message selector criteria.
However in this case I have to implement this using HTTP.
Any thoughts on this are welcome. I am fully convinced I am missing something very basic, but am not sure what.
Thanks a lot!

STOMP Websocket synchronous communication using Spring

I have a requirement were some of the STOMP websocket connections needs to be handled synchronously.
Meaning I have a client (spring) subscribed to a topic ("/topic").
I have a server (spring) that has defined the broker ("/topic") also defined handlers ("/app/hello").
Now is it possible for the client to send a request to /app/hello and then wait for a response before sending the next request to /app/hello.
How do I return value on my sever (STOMP spec says about RECEIPT frames but I don't think this is something that can be manually controlled).
How do I wait for the value on my client after a send.

To connect a Java client to a websocket endpoint you can use the tyrus reference implementation of JSR356 - Websockets For Java.
Basically you will need to implement a client endpoint (javax.websocket.Endpoint) and a message handler (javax.websocket.MessageHandler). In the endpoint you register the message handler with the current session on open:
public class ClientEndpoint extends Endpoint {
...
#Override
public void onOpen(final Session aSession, final EndpointConfig aConfig) {
aSession.addMessageHandler(yourMessageHandler);
}
}
To connect to the server endpoint you can use the ClientManager:
final ClientManager clientManager = ClientManager.createClient();
clientManager.connectToServer(clientEndpoint, config, uriToServerEndpoint);
The message handler's onMessage method will be invoked, if the server endpoint sends something to the topic.
Depending on your needs you can either choose to implement the mentioned interfaces or use the corresponding annotations.
UPDATE:
The STOMP website lists several implementations of the STOMP protocol. For Java there are Gozirra and Stampy. I have no experience with these frameworks but the examples are pretty straight forward.

Asynchronous request-reply with Spring Boot and RabbitMQ

We want to implement the following scenario:
A producer service sends some input params to another service asking for the details based on these params.
A producer wants to specify the queue where it will be listening for the reply.
Moreover, a producer wants to provide some metadata so that it can correlate the params it sent with a result it got.
Please advice how to do this properly.

See the AsyncRabbitTemplate.
It uses the correlationId and replyTo properties to convey that information to the service that handles the request.