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Server-sent Events and Error notification

From: John Fallows <john.fallows@kaazing.com>
Date: Fri, 6 Feb 2009 20:30:25 -0800
Message-ID: <c5b3a7130902062030l445bd00o8a064cfaed95648@mail.gmail.com>
To: "public-html-comments@w3.org" <public-html-comments@w3.org>

When building HTML 5 applications that leverage one or more SSE
streams, it may be important to change the application behavior (such
as updating the user interface) when an SSE stream aborts retry, due
to some network or server error condition.

At present this does not seem possible, see the following scenario as
an example:

1. application adds message listener to an event-source
2. application adds http://www.sse.org/stream to an event source
3. application updates user interface to indicate live connection
(possibly dormant - no data yet)
4. network or server error causes SSE stream to complete
5. SSE implementation automatically retries connection (after waiting
"retry" milliseconds)
6. network or server failure causes SSE to abort retry, no longer connected
7. application continues to display user interface assuming everything
still OK  (incorrect)

So, in an effort to address this problem, the following proposal makes
a minor enhancement to the existing SSE specification, such that error
conditions can be detected by the application and handled
appropriately.

Reuse the onerror concept from media elements to deliver a
ProgressEvent named "error" to the error event listeners on the
event-source element, and add an equivalent onerror attribute to the
event-source element for completeness.

Now, repeating the same scenario, with the enhanced event-source
proposal, we get the following:

1. application adds message listener to an event-source
2. application adds error listener to an event-source
3. application adds http://www.sse.org/stream to an event source
4. application updates user interface to indicate live connection
(possibly dormant - no data yet)
5. network or server error causes SSE stream to complete
6. SSE implementation automatically retries connection (after waiting
"retry" milliseconds)
7. network or server failure causes SSE to abort retry, no longer connected
8. SSE implementation delivers "error" event to event-source error listeners
9. application updates user interface to indicate disconnected stream  (correct)

The error event would likely need to be delivered for any usecase
where the SSE implementation ceases to retry connection, and the
ProgressEvent would need to include a source URL that matches the now
broken stream URL previously added to the event-source by the
application because a single event-source element can have multiple
event streams.

Adding the source URL to ProgressEvent would have the added benefit of
allowing the same JavaScript function to be directly reused on more
than one media element.

Kind Regards,
John Fallows.
-- 
>|< Kaazing Corporation >|<
John Fallows | CTO | +1.650.960.8148
888 Villa St, Ste 410 | Mountain View, CA 94041, USA
Received on Saturday, 7 February 2009 04:31:02 GMT

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