Create queue runtime in Grails with RabbitMQ plugin - spring

Question

I have a system where external systems can subscribe to events generated by my system. The system is written in Grails 2, using the RabbitMQ plugin for internal messaging. The events to external systems are communicated via HTTP.
I would like to create a queue for each subscriber to prevent that a slow subscriber endpoint slows down messages to an other subscriber. Subscriptions can occur runtime, that's why defining the queues in the application config is not desirable.
How can I create a queue with a topic binding runtime with the Grails RabbitMQ plugin?
As reading messages from RabbitMQ queues is directly coupled to services, a side-problem to creating the queue runtime could be to have multiple instances of that Grails service. Any ideas?

Answer 1

I don't have a ready solution for You but if you follow the code in the RabbitmqGrailsPlugin Descriptor especially the doWithSpring section
You should be able to recreate the steps necessary to initialize a new Queue and associated Listener dynamically at runtime.
It all comes down then to pass the needed parameters, register necessary spring beans and start the listeners.
To answer your second question I think you can come up with some naming convention and create a new queue handler for each queue. An example how to create spring beans dynamically can be found here: dynamically declare beans
Just a short example how I would quickly register a Queue it requires much more wiring etc...
def createQ(queueName) {
def queuesConfig = {
"${queueName}"(durable: true, autoDelete: false,)
}
def queueBuilder = new RabbitQueueBuilder()
queuesConfig.delegate = queueBuilder
queuesConfig.resolveStrategy = Closure.DELEGATE_FIRST
queuesConfig()
queueBuilder.queues?.each { queue ->
if (log.debugEnabled) {
log.debug "Registering queue '${queue.name}'"
}
BeanDefinitionBuilder builder = BeanDefinitionBuilder.rootBeanDefinition(Queue.class);
builder.addConstructorArgValue(queue.name)
builder.addConstructorArgValue(Boolean.valueOf(queue.durable))
builder.addConstructorArgValue(Boolean.valueOf(queue.exclusive))
builder.addConstructorArgValue(Boolean.valueOf(queue.autoDelete))
builder.addConstructorArgValue(queue.arguments)
DefaultListableBeanFactory factory = (DefaultListableBeanFactory) grailsApplication.mainContext.getBeanFactory();
factory.registerBeanDefinition("grails.rabbit.queue.${queue.name}", builder.getBeanDefinition());
}
}

Answer 2

I ended up using Spring AMQP which is used by the Grails RabbitMQ plugin. Removed some methods/arguments as they are not relevant to the sample:
class MyUpdater {
void handleMessage(Object message) {
String content = new String(message)
// do whatever you need with the message
}
}
import org.springframework.amqp.core.BindingBuilder
import org.springframework.amqp.core.Queue
import org.springframework.amqp.core.TopicExchange
import org.springframework.amqp.rabbit.core.RabbitAdmin
import org.springframework.amqp.rabbit.listener.SimpleMessageListenerContainer
import org.springframework.amqp.rabbit.listener.adapter.MessageListenerAdapter
import org.springframework.amqp.support.converter.SimpleMessageConverter
import org.springframework.amqp.rabbit.connection.ConnectionFactory
class ListenerInitiator {
// autowired
ConnectionFactory rabbitMQConnectionFactory
protected void initiateListener() {
RabbitAdmin admin = new RabbitAdmin(rabbitMQConnectionFactory)
// normally passed to this method, moved to local vars for simplicity
String queueName = "myQueueName"
String routingKey = "#"
String exchange = "myExchange"
Queue queue = new Queue(queueName)
admin.declareQueue(queue)
TopicExchange exchange = new TopicExchange(exchange)
admin.declareExchange(exchange)
admin.declareBinding( BindingBuilder.bind(queue).to(exchange).with(routingKey) )
// normally passed to this method, moved to local var for simplicity
MyUpdater listener = new MyUpdater()
SimpleMessageListenerContainer container =
new SimpleMessageListenerContainer(rabbitMQConnectionFactory)
MessageListenerAdapter adapter = new MessageListenerAdapter(listener)
adapter.setMessageConverter(new SimpleMessageConverter())
container.setMessageListener(adapter)
container.setQueueNames(queueName)
container.start()
}