How to configure exclusive consumer with Grails and JMS / ActiveMQ? - grails

Question

I have a Grails app that subscribes to a given ActiveMQ topic using the JMS plugin. How can I make the TestService class an exclusive consumer? Details of exclusive consumer here
The use case is that I am running the consumer on AWS EC2 and the ActiveMQ feed has a durability of 5 mins and it takes longer than this to replace the instance if it dies. I can't afford to lose messages and message order must be preserved, hence I wish to use multiple instances, where the first instance to connect will be the one that the broker sends every message, and the others are sat in reserve. In the event of the first instance dying, the AMQ broker will send the messages to one of the other instances.
Also, what criteria are used by JMS to determine when an exclusive consumer has died or gone away?
// resources.groovy
beans = {
jmsConnectionFactory(org.apache.activemq.ActiveMQConnectionFactory) {
brokerURL top://example.com:1234
userName = 'user'
password = 'password'
}
}
class TestService {
static exposes = ["jms"]
static destination = "SOME_TOPIC_NAME"
static isTopic = true
def onMessage(msg) {
// handle message
// explicitly return null to prevent unwanted replyTo attempt
return null
}
}

Answer 1

First of all, your example uses topics, that won't work; you want queues:
class TestService {
static expose = ["jms"]
static destination = "MYQUEUE"
...
}
Configuring exclusive consumers in ActiveMQ is straightforward:
queue = new ActiveMQQueue("MYQUEUE?consumer.exclusive=true");
..but may be tricky with the Grails plugin; you can try these:
class TestService {
static expose = ["jms"]
static destination = "MYQUEUE?consumer.exclusive=true"
def onMessage(msg){ ...}
}
class TestService {
static expose = ["jms"]
#Queue(
name = "MYQUEUE?consumer.exclusive=true"
)
def handleMessage(msg){ ...}
}
Regarding your question on how the broker determines if a consumer dies, I'm not sure how it's done exactly in ActiveMQ, but in most JMS implementations, TCP failures trigger an exception on the connection; the peer (the broker in this case) handles the exception and fails over to the next available consumer.
Hope that helps