XEP-0021: Jabber Event Notification Service (ENS)

2.1 Event¶

An event can be defined as a change to the value of one or more properties of a resource.

In the ENS, an event type is referred to by a JID (Jabber IDentifier), including the JID resource. For example, consider a hypothetical publisher that is linked to an IMAP message store. It might notify the ENS of the fact the a message has arrived, deleted, or filed, using the following JIDs:

• rob@imap.cataclysm.cx/NewMessage
• joe@imap.cataclysm.cx/DeletedMessage
• jim@imap.cataclysm.cx/FiledMessage

Alternatively, an end-user client that wanted to notify the ENS when its avatar changes might do so using a JID like "rob@cataclysm.cx/avatar"

2.2 Subscriber¶

A subscriber is a Jabber entity that receives notifications about events. Usually, a subscriber will be an end-user client, but it may be any Jabber entity.

As the name suggests, a subscriber can subscribe and unsubscribe to various events via the ENS. When it subscribes, the publisher responsible for the event it is subscribing to will be asked by the ENS to authorise the subscription request. To facilitate this, the subscriber may provide an XML fragment containing information that the publisher can use to authorise it. The use of this fragment is application specific.

Once subscribed to an event, the subscriber will receive any notifications that the publisher sends about that event.

2.3 Publisher¶

A publisher is the Jabber entity responsible for actually sending event notifications to the ENS. A notification contains the event type JID of the event that occured, and an optional "payload" XML fragment, that is passed untouched by the ENS to the subscriber. The contents of this payload is application-specific and can be used to provide detailed information about the event to the subscriber. For example, in the case of the NewMessage event above, the payload might contain the contents of the To:, From: and Subject: headers of the message.

Additionally, the publisher is responsible for deciding who may subscribe to events it publishes. When the ENS receives a subscription request, it will ask the publisher to decide whether or not the subscriber may subscribe to a particular event. This authorisation request may also contain an XML fragment from the subscriber containing information that may be used for authorisation.

3.1 Subscription request¶

To subscribe to a particular event, the subscriber sends a packet like of the following form to the ENS:

<iq id='sub1' type='set' from='subscriber-jid' to='ens-jid'>
  <subscribe xmlns='http://xml.cataclysm.cx/jabber/ens/' jid='event-jid'/>
</iq>
    

<iq id='sub2' type='set' from='subscriber-jid' to='ens-jid'>
  <subscribe xmlns='http://xml.cataclysm.cx/jabber/ens/' jid='event-jid'>
    <auth-info xmlns='jabber:iq:auth'>password</auth-info>
  </subscribe>
</iq>
      

<iq id='sub2' type='set' from='subscriber-jid' to='ens-jid'>
  <subscribe xmlns='http://xml.cataclysm.cx/jabber/ens/' jid='event-jid'>
    <reliable/>
  </subscribe>
</iq>
      

3.2 Subscription response¶

Once subscribed, the ENS will return a packet of the following form to the subscriber:

<iq id='sub1' type='result' from='ens-jid' to='subscriber-jid'>
  <subscribed xmlns='http://xml.cataclysm.cx/jabber/ens/' jid='event-jid'/>
</iq>
    

If an error occured during subscription (such as the publisher not allowing the subscriber to subscribe), an error packet will be returned to the subscriber:

<iq id='sub1' type='error' from='ens-jid' to='subscriber-jid'>
  <subscribe xmlns='http://xml.cataclysm.cx/jabber/ens/' jid='event-jid'/>
  <error code='401'>Unauthorized</error>
</iq>
    

The actual error fragment in the packet is a direct copy of the one returned by the publisher when it fails the authorisation request from the ENS. If the publisher does not provide one, error code 503 (Service Unavailable) will be returned instead. If the publisher does not respond to the authorisation request (after an implementation-specific timeout), error code (Remote Server Timeout) will be returned.

3.3 Unsubscription request¶

To unsubscribe from a particular event, the subscriber sends a packet like of the following form to the ENS:

<iq id='unsub1' type='set' from='subscriber-jid' to='ens-jid'>
  <unsubscribe xmlns='http://xml.cataclysm.cx/jabber/ens/' jid='event-jid'/>
</iq>
    

3.4 Unsubscription response¶

Once unsubscribed, the ENS will return a packet of the following form to the subscriber:

<iq id='unsub1' type='result' from='ens-jid' to='subscriber-jid'>
  <unsubscribed xmlns='http://xml.cataclysm.cx/jabber/ens/' jid='event-jid'/>
</iq>
    

No further notifications for the event will be received.

3.5 Publish¶

When a publisher publishes a notification to the ENS, the ENS will forward the notification to any subscribers for that event. A notification sent to a subscriber takes the following form:

<iq id='enspub1' type='set' from='ens-jid' to='subscriber-jid'>
  <publish xmlns='http://xml.cataclysm.cx/jabber/ens/' jid='event-jid'/>
</iq>
    

A notification may also contain a (application-specific) "payload" XML fragment:

<iq id='enspub2' type='set' from='ens-jid' to='subscriber-jid'>
  <publish xmlns='http://xml.cataclysm.cx/jabber/ens/' jid='event-jid'>
    <xml-payload/>
  </publish>
</iq>
    

Section 4.1 has more information about the payload.

If the subscriber responds to the notification with an error, the subscriber will be automatically unsubscribed from the event by the ENS.

3.6 Publish response (reliable subscriptions)¶

If the <reliable/> option was specified when the subscriber subscribed to the event, then the subscriber is expected to acknowledge a notification with a packet of the following form:

<iq id='enspub1' type='result' from='subscriber-jid' to='ens-jid'>
  <published xmlns='http://xml.cataclysm.cx/jabber/ens/'/>
</iq>
    

If the subscriber does not respond, or responds with an error, the notification will be resent by the ENS after a (implementation-specific) timeout.

4.1 Publish¶

To publish a notification, the publisher sends a packet of the following form to the ENS:

<iq id='pub1' type='set' from='event-jid' to='ens-jid'>
  <publish xmlns='http://xml.cataclysm.cx/jabber/ens/'/>
</iq>
    

A notification may also contain a (application-specific) "payload" XML fragment:

<iq id='pub1' type='set' from='event-jid' to='ens-jid'>
  <publish xmlns='http://xml.cataclysm.cx/jabber/ens/'>
    <xml-payload/>
  </publish>
</iq>
    

The payload can be any well-formed XML data. Everything inside the <publish/> tag will be forwarded untouched to the subscriber.

4.2 Publish response¶

Once the ENS has dispatched the notification to subscribers, it will return a packet of the following form to the publisher:

<iq id='pub1' type='result' from='ens-jid' to='event-jid'>
  <published xmlns='http://xml.cataclysm.cx/jabber/ens/'/>
</iq>
    

4.3 Authorisation request¶

A publisher is required to approve subcription requests. When a subscriber attempts to subscribe to an event, the publisher will receive a packet of the following form from the ENS:

<iq id='ensauth1' type='get' from='ens-jid' to='event-jid'>
  <authorise xmlns='http://xml.cataclysm.cx/jabber/ens/' jid='subscriber-jid'/>
</iq>
    

The subscriber may include an <auth-info/> XML fragment containing some (application-specific) information that the publisher can use to authorise it:

<iq id='ensauth1' type='get' from='ens-jid' to='event-jid'>
  <authorise xmlns='http://xml.cataclysm.cx/jabber/ens/' jid='subscriber-jid'>
    <auth-info xmlns='jabber:iq:auth'>password</auth-info>
  </authorise>
</iq>
    

4.4 Authorisation response¶

To signal to the ENS that a subscriber should be allowed to subscribe, the publisher should return a packet of the following form:

<iq id='ensauth1' type='result' from='event-jid' to='ens-jid'>
  <authorised xmlns='http://xml.cataclysm.cx/jabber/ens/' jid='subscriber-jid'/>
</iq>
    

To deny the subscription, the publisher should return an error packet, containing the appropriate <error/> fragment:

<iq id='ensauth1' type='error' from='event-jid' to='ens-jid'>
  <authorise xmlns='http://xml.cataclysm.cx/jabber/ens/' jid='subscriber-jid'/>
  <error code='401'>Unauthorized</error>
</iq>
    

The <error/> fragment will be copied untouched into the error response sent back to the subscriber. See section 3.2 for more details.

6.1 ENS discovery¶

The ENS, as described, works well. However, no provisions are made for a subscriber to find out which ENS a publisher is publishing to. It does a subscriber no good to subscribe to an event on the wrong ENS - the subscription would succeed (assuming the publisher allows it), but no notifications would ever be received.

There are several potential solutions, each with their problems:

• Leave it to the subscriber to find the appropriate ENS outside of the ENS framework itself. This might be via a browse to the publisher, or maybe just entering it into their client's configuration. Obviously, this is very application-specific.
• Force the publisher to publish to multiple ENSs, as necessary. This would require additions to the protocol (to tell the publisher of new ENSs), and would require the publisher to maintain a list of JIDs it needs to publish to. This solution is pointless - if the publisher is going to publish to multiple JIDs and maintain its own list, it might as well publish direct to subscribers.
• Have some sort of ENS-to-ENS protocol, and have ENSs proxy publishes for other ENSs. This does not fix the problem, it just moves it away from the subscriber and into the ENS. An ENS will still need to find out which ENS the publisher is publishing to.
• Integrate ENS into the session manager. This leaves us with a glorified presence system, and makes the ENS basically unusable by non-session-manager-based server components.

This problem may be outside of the scope of this specification.

6.2 How reliable should <reliable/> be?¶

Currently, if a subscriber obtains a reliable subscription, and then disappears from the network (as an end-user client might), the ENS will continue to send notifications to it (ignoring errors) until it unsubscribes. If the subscriber never comes back, then ENS will send notifications forever (in theory).

At what point should even <reliable/> have its limits? Should we unsubscribe a reliable subscriber who bounces (ie. the Jabber server failed to deliver) more than, say, 10 publishes? Or maybe they should be unsubscribed if they do not respond (as in section 3.6) to anything for, say, 10 minutes?

<reliable/> is an interesting idea, but it may be that it is too problematic for its own good.

6.3 Longevity of subscriptions¶

The topic of reliable subscriptions raises the question as to how long a subscription lasts. Should a subscription last forever (like <presence/> does), even across restarts of the server?

If end-user clients are to be subscribers, then under this scheme the ENS would have to subscribe to presence, so as to know when the client has disconnected. Since presence is a function of the session manager, this could have the effect of making the ENS less generic that we may like.

6.4 Replacing <presence/> with the ENS?¶

As I see it, basic presence will always be a function of the session manager, for the following reasons:

• No ENS discovery problems
• Subscriptions are maintained across sessions
• In very widespread use (ie. everywhere)

I think the places where the ENS can boom will be in applications like avatars, and genuine event-based systems (like the kind described above - "you have new mail", "someone has scheduled a meeting with you in your online calendar", etc).

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