Wrangling up XMPP

Question

Wikipedia defines XMPP as:

...an open-standard communications protocol for message-oriented middleware based on XML.

xmpp.org defines XMPP as:

The Extensible Messaging and Presence Protocol (XMPP) is an open XML technology for real-time communication, which powers a wide range of applications.

Although I'm sure both these definitions are very accurate, they don't tell me a thing about what I - a Java developer - can actually do with XMPP!

For instance, I've heard XMPP can be used with message-oriented middleare (MOM). How so? Can XMPP somehow integrate with my Apache Camel routes, my ESB or some SOA implementation to deliver a better/faster/more robust business tier? If so, how?!?!

A good, King's-English explanation of XMPP, along with some practical examples (preferable MOM-centric) would be greatly appreciated. Thanks in advance!

Answer 1

XMPP can be used for a wide range of messaging based applications. Basically, it provides core services which can be used to build XML based messaging applications. Its based on a decentralized client-server architecture and utilizes long-lived TCP connections for communicating...

core services include...

• channel encryption, authentication, presence, contact lists, one-to-one messaging, multi-party messaging, notifications
• service discovery, capabilities advertisement, structured data formats, workflow management, peer-to-peer media sessions

textbook use cases...

• instant messaging (using presence, contact lists, one-to-one messaging)
• group chat, gaming, systems control, geolocation, middleware/cloud computing, data syndication
• bots (weather, database interface, system monitoring)

messaging modes/patterns...

• point-to-point messaging is used to send to a specific receiver
• multi-user messaging is used to message to a group of receivers
• publish/subscribe support is used when there are large volume of events and systems are interested in differing subsets of events. Publishers put events into topics and subscribers indicate which topics they are interested in. This decouples the publisher/subscriber and allows for scalable real-time messaging. For more information, see this article: http://www.isode.com/whitepapers/xmpp-pubsub.html

deployment methods...

• XMPP user - connects as a normal user and responds to requests addressed to the user
• XMPP Server plugins - deployed as part of the server plugin architecture
• XMPP Components - service external to an XMPP server that connects and behaves like a plugin

Java Integration

• Smack API - A pure Java library, it can be embedded into your applications to create anything from a full XMPP client to simple XMPP integrations such as sending notification messages and presence-enabling devices.
• Camel XMPP - A Camel component that allows integration with Smack API in Camel routes

To your specific question "can it be used in SOA/middleware?"....

• yes, it can be used to wire together applications via XML messaging and XMPP APIs
• whether its the best technology choice depends heavily upon requirements
• one good use case, interactive system monitoring/management...here are some other examples

Also, XMPP integration with Camel is trivial. See this camel-xmpp unit test for a basic example of interfacing with a Google Talk server. Also, Camel's framework allows you to build an application and easily swap out different messaging technologies (JMS, STOMP, mina, etc).

Answer 2

I can start combining information from al kinds of sources found on the internet using Google (keywords: XMPP Java MoM examples), rewrite (or even copy) the definition of XMPP, but of course I will not do so. There is just too much. I also do not have examples available for you.

Below I will list the links I found most interesting, so you can start reading and get more knowledge on the subject.

1) http://www.xmpp.org/ This is probably the best starting point. Browse through the menu left to right and top to bottom. That is what I did. The site lists servers, clients and libraries, so you should be able to find the desired examples this way.

2) http://www.ibm.com/developerworks/webservices/library/x-xmppintro/index.html Clear article, which also mentions MoM. No Java examples, but Ruby.

3) http://fyi.oreilly.com/2009/05/what-can-you-do-with-xmpp.html Maybe you should just get the book? No examples in the article.

4) http://kirkwylie.blogspot.com/2008/07/real-mom-is-hard-lets-use-xmpp.html Interesting article where the last line basically says: Why use XMPP if you can use AMQP or JMS?

I hope this helps you I finding what you need.

Answer 3

XMPP is an open and extensible standard for real time communications.

XMPP comes with a core that is defined in its rfc, which describes the basic protocol for doing instant messaging and exchanging presence information. However where XMPP really shines is in its extensibility: XMPP defines the building blocks (presence, message and iq stanzas) to create protocols of communication. These typically come as extensions. A list of currently available standard extensions can be found here. The most important of these are typically available for all the popular XMPP servers. It is exactly this extensibility that makes XMPP appropriate as message-oriented middleware.

Let me take as an example Publish-Subscribe which is a typical pattern for middleware and becomes a necessity as soon as you depart from the scenario with a few entities where simple messaging is adequate. PubSub is used in situations where entities, or producers, produce information that is to be consumed by other entities, the consumers. Typically, the information is written to nodes, to which consumers are subscribed. Being subscribed they receive notifications when an item is added/updated/deleted. An incredible amount of use-cases can be elegantly covered by PubSub, from queuing long-running jobs and having workers handle them, to micro-blogging. XMPP has a very robust and widely available extension to handle PubSub in a standard way, described in XEP-0060 and providing out of the box a workflow for handling publishing, subscriptions, notifications and security. Having a look at the use-cases in the XEP will give you an idea for the simplicity of the whole thing.

Now, while most use-cases are covered by using (or abusing) existing standard extensions, eventually you might need the little extra custom protocol that is not covered elsewhere. Using your language of choice you can write an XMPP component defining your own protocol. You then connect the component to the XMPP server you are running and by using simple namespacing let the server know what kind of messages you can handle and let the server advertise your protocol capabilities to clients connecting to it. There is no end to how simple or complex you can make this. Lack of better example but maybe good enough for illustration, here is a component I wrote that leverages XMPP to do real-time collaborative editing in the Plone CMS (similar to Google docs). While the details can get complicated I think having a look at the "Protocol Specication" on that page will give you an idea.

Finally, concerning Java specific libraries as @boday mentions, there are libraries around that make it easy to start with as well as Apache Camel integration (although it only does simple messaging as far as I can see). Keep in mind though that the investment in understanding how XMPP works and being able to go beyond using existing libraries is really worth it and can lead to extremely powerful and yet simple integration.