Introduction:

PubSub is a XMPP extension which allows publishing and subscribing to events. This is useful when you instantly want to notify many clients about something interesting happening on your server.

Quoting the authors of PubSub specs:

The protocol enables XMPP entities to create nodes (topics) at a pubsub service and publish information at those nodes; an event notification (with or without payload) is then broadcasted to all entities that have subscribed to the node. Pubsub therefore adheres to the classic Observer design pattern and can serve as the foundation for a wide variety of applications, including news feeds, content syndication, rich presence, geolocation, workflow systems, network management systems, and any other application that requires event notifications.

To understand this better, think of newspapers as publishers and the people who subscribe to the newspapers as subscribers. Getting your daily newspaper is similar to checking your RSS feeds because you only get information at regular intervals. (Of course you could keep buying newspapers every 'x' hours, i.e. keep refreshing your RSS reader so often). This is as inefficient as it sounds. For instance, you never know if a newspaper actually has new stuff, you need to buy and read it before you realize there's nothing new. And there will be always be a delay (unless you are really lucky) between the time something happens and you read it in the your 'n'th newspaper of the day.

PubSub solves this problem by having a system where subscribers subscribe to nodes and the publisher notifies its subscribers when something happens. Nodes are like TV channels, so you only subscribe to the channels you are interested in.

How it can be useful to you:

In addition to the points mentioned in the quote, pubSub can be used in your project to send instant notifications, maintain the same state across multiple sessions, or monitor live activity.

Introducing django-pubsub:

django-pubsub allows you write PubSub enabled applications easily.

This demo using django-pubsub shows how to use the module. The stack used in the demo is:

fcgi

nginx

ejabberd with mod_http_bind and mod_pubsub

You can register a model with the pubsub app and all registered models will automatically send out notification (with the payload) on save to the default node.

Then you can use the PubSub client provided in media to subscribe and receive events from the registered models.

The default payload includes an XML serialized instance of the model. You can use jquery to extract required fields from the payload.

You can also directly use pubsub.publish to publish any item to any node. An example is provided in the demo included with the app.

How it works:

Since XMPP and HTTP speak different languages, you need some kind of a bridge to connect these two. This can be achieved using BOSH.

The BOSH bridge is provided by ejabberd's mod_http_bind module. (You can also use Punjab for this purpose)

If you are using mod_http_bind, you will also need to setup appropriate url forwarding using nginx because mod_http_bind listens on the port 5280 and will not be able to communicate with the client on port 8080.

Strophe will use the BOSH url to talk to ejabberd. Our PubSub client which is written using Strophe will then be able to subscribe and receive notifications from the server.

On the server side, the pubsub app uses xmpppy to send out the correct Iq stanzas on save or publish . The client will receive these stanzas on subscribing to the appropriate node.

Conclusion:

If your app needs real-time updates, push notifications, and scalable infrastructure, you can make use of XMPP and Strophejs.

References:

ejabberd's blog post on related topic:

http://www.process-one.net/en/blogs/article/introducing_the_xmpp_application_server

a similar project using comet, orbit and twisted instead of xmpp:

http://www.clemesha.org/blog/realtime-web-apps-python-django-orbited-twisted

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Thank you for reading the Agiliq blog. This article was written by Javed on Dec 22, 2010 in xmpp . You can subscribe ⚛ to our blog. We love building amazing apps for web and mobile for our clients. If you are looking for development help, contact us today ✉.