AJAX and Messaging. The AJAX Pull Architecture Dilemma

With more and more AJAX applications popping up, I am seeing this and solving this problem often. It is well known in the thick client world that to keep thick clients over the network in the same state you have to use a Push Architecture with a messaging architecture in the back end. Let's take a look at the most common example on an application that needs to keep state synchronized across all clients, the Dashboard.

The Dashboard is usually a table (sometime something much more complicated) that contains information that is updated in real time by other clients and other services. For example, imagine a trucking company that has a table that contains all their trucks, who is driving their truck, what the truck is carrying, where the truck is going is going and the truck estimated time of arrival to its destination. This information is vital to the dispatchers to allow them to know what truck will be available when and where. Then they can make the most efficient use of the trucks on the road.

The trick is to keep this tables synchronized across all the clients without having the clients poll the database every x seconds to see if there are any changes. Such strategy simple does not scale. What you do in this case is you set up a messaging server in the back end like JBoss Messaging or TIBCO, create a topic in the Messaging server that all the clients subscribe to and then every change made to the table by clients, or maybe a GPS service, is published to the topic and all clients get a new message and update their table. This is a more efficient model.

How do you achieve a similar push architecture with AJAX? It is easy to create a dynamic AJAX table that updates itself asynchronously without ever resubmitting the page. You can do it with straight Javascript and HTML, or you can use a library like Google's GWT or DWR. The problem is that the AJAX page will eat up your server resources. By constantly polling for changes, you will have constant queries on your database when they are not necessary. But, how do you implement messaging with AJAX?

If you want to make your life easier, use Apache ActiveMQ as the JMS provider.
ActiveMQ provides Javascript remoting objects for the AJAX pages to listen for messages asynchronously. If you want to use another framework or JMS provider, you will need to create a POJO or servlet on the server side that writes and listens to the JMS topic, and the remoting Javascript objects need to listen to events from that POJO or servlet. JBoss is working on a similar feature for their JMS product. TIBCO also has AJAX libraries and support.

With all the messaging support for messaging and AJAX in the market, there is no excuse to use Pull Architectures in AJAX clients anymore.

Email me if you need more details.

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