Messaging Integration

Grails needs to provide a universal messaging system not just something specific to e-mail, although this is clearly important. Rather than introducing two concepts that are essentially the same (email and messaging) they need to be unified into a single concept to avoid duplication of effort.

Asynchronous messaging essentially defines the concept of a topic and subscribers and publishers to a topic.

Publishers

A publisher is simple a groovy class that follows the convention *Publisher. By default the first part of the name defines the topic it is publishing to:

class ExamplePublisher {
      
	def sendX() {
		return [modelObject:"blah"]
	}
}

Here we're publishing to an "example" topic and the sendX() method creates a model that will be used in the sending. In the case of e-mail the sendX method will delegate the model to a views/email/x.gsp view to create the e-mail content itself. Alternatively in JMS this model will be passed in the messaging context to the subscriber.

If a publisher needs to send a message to another topic it can do:

def sendXToTopicName() {
         return [mnodelObject:"blah"]
     }

The topic that a publisher publishes to be default can also be changed using the "topic" property:

def topic = "another"

Subscribers

A subscriber is an object the receives messaging notifications. They listen to a "topic" in the same way publishers publish to a topic. For example:

class ExampleSubscriber {

    def onMessage(ctx) {

    }
}

It gets passed a context object which in the case of JMS may be the model or in the case of a POP3 or IMAP e-mail listener it could be the email itself.

Topics

Topics can vary in what they do and achieve. A topic could define an email server to send and listen for new e-mails to and from. Thus allowing the publish and subscribe of email. Alternatively it could be configured as a JMS topic for messaging. Topics will be configured in the ApplicationConfig.groovy file as follows:

class ApplicationConfig {
	def topics =  {
              example(type:EMAIL) {
                    server = "localhost"
                    port = 3434
                    // etc
              }
        }
}

Messaging Integration