PubSub in Zotonic

In a previous article I introduced the principle of resource sharing: websites acting as both publishers of their own resources and subscribers to resources published by others.

This pubsub is done over XMPP: the open and extensible protocol for the realtime exchange of messages. The resources move over the XMPP wires represented in the Atom format for content syndication.

In Zotonic, pubsub is implemented in the mod_pubsub module. This module, an Erlang gen_server, connects as a client to an XMPP server. It maintains this connection all the time as long as Zotonic is running. mod_pubsub implements both the publishing as well as the subscribing side of the resource sharing mechanism.

Setting up PubSub

Setting up mod_pubsub is quite easy: since it is implemented as an XMPP client, the only thing you need is a dedicated XMPP account at a server which is configured with pubsub support.

Since the XMPP server needs publish/subscribe to be enabled, it's the easiest to run your own server. Servers known to work are ejabberd (at least version 2.0.1) and openfire. Make sure that you do enable pubsub support in the server, with support for instant node creation and with the access controls set so that anyone may create pubsub nodes. With ejabberd, this is the default setting.

Configuration keys in Zotonic are the following:

• mod_pubsub.jid: the jid of your dedicated account.
• mod_pubsub.password: its password.
• mod_pubsub.pubsub_domain: the pubsub domain: this is the JID of the pubsub service: for instance, pubsub.yourserver.com.

Please remember: Only activate the publish/subscribe module after setting the config keys!

Once you've activated the module (click "activate" next to the "Publish/Subscribe" line in the Modules overview), you might not notice anything: pubsub should now "just work" :) If you see an error popup in the admin, there might be something wrong with your credentials (wrong JID or password).

In the navigation bar at the left side of the screen you now see a "publish/subscribe" item. Clicking this brings you to the page where you see an overview of all the subscriptions this site has. The button at the top lets you subscribe to a page on another site. Try it! Of course, the site at the other end should support resource sharing as well: you can confirm this by viewing the source of a page at that site, verifying that the link rel=xmpp.feed meta tag is present. You can for instance try subscribing to the URL of this article.

A word of warning

A note: the resource sharing is still in development in Zotonic: the "happy path" works pretty good, but error reporting is still lacking: if something goes wrong while publishing or subscribing, you probably wouldn't notice it. It will fail silently without any notification in the Zotonic admin. If you suspect that this is happening: try starting Zotonic in debug mode to see all output. If you notice crashes when you save a page, you can at least inspect the output.

I'm working currently on a mechanism to have better error reporting, notification and logging integrated in the admin. Expect to hear more on this topic soon.

Happy pubsubbing!!
Arjan