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Re: WebDAV PUSH based on RFC 6202

From: Peter Saint-Andre <stpeter@stpeter.im>
Date: Wed, 09 Apr 2014 10:57:31 -0700
Message-ID: <53458A0B.9080309@stpeter.im>
To: Ken Murchison <murch@andrew.cmu.edu>, WebDAV <w3c-dist-auth@w3.org>
[resend to list]

Well, I mention this Internet-Draft mostly to show that people have been 
thinking about this problem for at least 10 years. You could use XMPP 
for such notifications, although it might depend on what kinds of 
endpoints you're trying to support (e.g., do they or can they do XMPP). 
Other options are technologies like pubsubhubbub. There are also lots of 
proprietary technologies for such notifications, although of course we 
don't want anything proprietary. However IMHO long polling is not the 
best approach these days - we wrote RFC 6202 mostly to say that long 
polling is a bad idea and that it's better to use WebSockets instead 
(RFC 6455). But WebSockets only gives you "TCP for the Web" and you'd 
need to define some kind of application over WebSocket in order to get 
anything done (examples are SIP over WebSocket, XMPP over WebSocket, etc.).

Peter

On 4/9/14, 9:44 AM, Ken Murchison wrote:
> I wasn't aware of this draft.  Do you feel that this approach is
> superior, and if so, would be consider resurrecting the draft?
>
>
> On 04/09/2014 12:36 PM, Peter Saint-Andre wrote:
>> <troll>
>> http://tools.ietf.org/id/draft-hildebrand-webdav-notify-00.txt
>> </troll>
>>
>> On 4/9/14, 9:28 AM, Ken Murchison wrote:
>>> All,
>>>
>>> The Calendaring and Scheduling Consortium (CalConnect) is looking at
>>> ways to have a server "push" changes made to a calendar/addressbook
>>> collection out to a client.  There are already a few proprietary
>>> mechanisms in place for doing so, but we would like to come up with
>>> something standard that would be relatively simple to implement for both
>>> clients and servers and would be applicable to any DAV collection.
>>>
>>> One idea that we are toying with is to leverage the existing
>>> DAV:sync-collection REPORT <http://datatracker.ietf.org/doc/rfc6578/>
>>> and HTTP long-polling <http://datatracker.ietf.org/doc/rfc6202/>.  I
>>> spent a few hours coding up a prototype version of HTTP long-polling and
>>> HTTP streaming for DAV:sync-collection REPORTs in my server which I
>>> describe below.  We (CalConnect) are considering using this approach or
>>> something similar as a starting point and we are interested in any/all
>>> feedback from the larger DAV community, including:
>>>
>>>   * Is this approach sane, is there a better way, or is any type of push
>>>     via HTTP a hopeless endeavor?
>>>   * Will this approach (or anything similar) break in the face of
>>>     intermediaries?
>>>   * Will existing HTTP/DAV stacks be able to handle long-polling and/or
>>>     streaming?
>>>   * Should the server advertise its ability to long-poll and/or stream
>>>     for the client to discover or simply leave it up to the client try
>>>     one or both and see what the server does, as is the case in my
>>>     implementation below?
>>>
>>>
>>> Long-polling:
>>>
>>> For long-polling, I leveraged the HTTP Prefer header
>>> <http://tools.ietf.org/html/draft-snell-http-prefer> and its 'wait'
>>> preference as a way for the client to tell the server that it wants to
>>> long poll.  If the client doesn't specify a DAV:sync-token (initial
>>> sync) or if there have been changes since the specified token, then the
>>> server will respond immediately.  Otherwise, it will only respond if a
>>> change is detected or when the timeout expires, whichever comes first.
>>> In the case of a delayed response, I issue a 100 (Continue) provisional
>>> response with a Preference-Applied header to notify the client that the
>>> server is indeed long-polling as requested.  This provisional response
>>> may or may not be necessary.  In my implementation I wait 1 sec less
>>> than specified to account for processing time so I don't go over what
>>> the client expects.
>>>
>>> Streaming:
>>>
>>> The client can request streaming behavior by simply including an Accept
>>> header with the 'multipart/mixed' media type (I chose this subtype for
>>> lack of something better - we could use the existing x-mixed-replace or
>>> create our own).  The client can also specify a timeout for the
>>> streaming using the same 'wait' preference as used for long-polling.  In
>>> the absence of a client-requested timeout, the server will continue to
>>> add body parts until the client disconnects or the server hits some
>>> internal timeout.  Because a multipart response allows for an epilogue
>>> following the final delimiter, a client can't just rely on the
>>> delimiters to detect the end of the response.  Therefore, the server
>>> MUST use either chunked TE or close the connection following the
>>> multipart response.  In my example below, I close-delimit the response
>>> for better readability.  FWIW, I think the same holds true for
>>> multipart/byteranges responses.  In my implementation I include a
>>> Content-Length header in the body-part headers so the client can detect
>>> the end of the XML body without looking for the trailing delimiter
>>> (mainly because the trailing delimiter doesn't appear until the next
>>> body part).
>>>
>>> Examples:
>>>
>>> Here is an example of long-polling with 3 requests (I added superfluous
>>> Date headers to show the timing).  The first request returns immediately
>>> due to a pre-existing change, the second returns upon detecting a
>>> subsequent change some 95 sec later, and the third times out after 3
>>> min.
>>>
>>> REPORT /dav/calendars/user/ken/Default/ HTTP/1.1
>>> Host: localhost
>>> Date: Wed, 30 Oct 2013 18:11:11 GMT
>>> Content-Type: application/xml
>>> Content-Length: 260
>>> Prefer: return=minimal, wait=180
>>>
>>> <?xml version="1.0" encoding="UTF-8"?>
>>> <C:sync-collection xmlns:D="DAV:"
>>> xmlns:C="urn:ietf:params:xml:ns:caldav">
>>> <D:sync-token>http://cyrusimap.org/ns/sync/1368011844-354</D:sync-token>
>>>    <D:sync-level>1</D:sync-level>
>>>    <D:prop/>
>>> </C:sync-collection>
>>>
>>> HTTP/1.1 207 Multi-Status
>>> Date: Wed, 30 Oct 2013 18:11:11 GMT
>>> Vary: Accept-Encoding, Brief, Prefer
>>> Preference-Applied: return=minimal, wait=180
>>> Content-Type: application/xml; charset=utf-8
>>> Content-Length: 421
>>>
>>> <?xml version="1.0" encoding="utf-8"?>
>>> <D:multistatus xmlns:D="DAV:" xmlns:C="urn:ietf:params:xml:ns:caldav">
>>>    <D:response>
>>> <D:href>/dav/calendars/user/ken/Default/4E4B3490-6F01-41B9-AA5B-FE2CD6A30632.ics</D:href>
>>>
>>>      <D:propstat>
>>>        <D:prop/>
>>>        <D:status>HTTP/1.1 200 OK</D:status>
>>>      </D:propstat>
>>>    </D:response>
>>> <D:sync-token>http://cyrusimap.org/ns/sync/1368011844-355</D:sync-token>
>>> </D:multistatus>
>>>
>>>
>>>
>>> REPORT /dav/calendars/user/ken/Default/ HTTP/1.1
>>> Host: localhost
>>> Date: Wed, 30 Oct 2013 18:11:12 GMT
>>> Content-Type: application/xml
>>> Content-Length: 260
>>> Prefer: return=minimal, wait=180
>>>
>>> <?xml version="1.0" encoding="UTF-8"?>
>>> <C:sync-collection xmlns:D="DAV:"
>>> xmlns:C="urn:ietf:params:xml:ns:caldav">
>>> <D:sync-token>http://cyrusimap.org/ns/sync/1368011844-355</D:sync-token>
>>>    <D:sync-level>1</D:sync-level>
>>>    <D:prop/>
>>> </C:sync-collection>
>>>
>>> HTTP/1.1 100 Continue
>>> Date: Wed, 30 Oct 2013 18:11:12 GMT
>>> Preference-Applied: wait=180
>>>
>>> HTTP/1.1 207 Multi-Status
>>> Date: Wed, 30 Oct 2013 18:12:47 GMT
>>> Vary: Accept-Encoding, Brief, Prefer
>>> Preference-Applied: return=minimal, wait=180
>>> Content-Type: application/xml; charset=utf-8
>>> Content-Length: 375
>>>
>>> <?xml version="1.0" encoding="utf-8"?>
>>> <D:multistatus xmlns:D="DAV:" xmlns:C="urn:ietf:params:xml:ns:caldav">
>>>    <D:response>
>>> <D:href>/dav/calendars/user/ken/Default/4E4B3490-6F01-41B9-AA5B-FE2CD6A30632.ics</D:href>
>>>
>>>      <D:status>HTTP/1.1 404 Not Found</D:status>
>>>    </D:response>
>>> <D:sync-token>http://cyrusimap.org/ns/sync/1368011844-356</D:sync-token>
>>> </D:multistatus>
>>>
>>>
>>>
>>> REPORT /dav/calendars/user/ken/Default/ HTTP/1.1
>>> Host: localhost
>>> Date: Wed, 30 Oct 2013 18:12:48 GMT
>>> Content-Type: application/xml
>>> Content-Length: 260
>>> Prefer: return=minimal, wait=180
>>>
>>> <?xml version="1.0" encoding="UTF-8"?>
>>> <C:sync-collection xmlns:D="DAV:"
>>> xmlns:C="urn:ietf:params:xml:ns:caldav">
>>> <D:sync-token>http://cyrusimap.org/ns/sync/1368011844-356</D:sync-token>
>>>    <D:sync-level>1</D:sync-level>
>>>    <D:prop/>
>>> </C:sync-collection>
>>>
>>> HTTP/1.1 100 Continue
>>> Date: Wed, 30 Oct 2013 18:12:48 GMT
>>> Preference-Applied: wait=180
>>>
>>> HTTP/1.1 207 Multi-Status
>>> Date: Wed, 30 Oct 2013 18:15:47 GMT
>>> Vary: Accept-Encoding, Brief, Prefer
>>> Preference-Applied: return=minimal, wait=180
>>> Content-Type: application/xml; charset=utf-8
>>> Content-Length: 202
>>>
>>> <?xml version="1.0" encoding="utf-8"?>
>>> <D:multistatus xmlns:D="DAV:" xmlns:C="urn:ietf:params:xml:ns:caldav">
>>> <D:sync-token>http://cyrusimap.org/ns/sync/1368011844-356</D:sync-token>
>>> </D:multistatus>
>>>
>>>
>>>
>>> Here is the same sequence of events utilizing streaming with a timeout:
>>>
>>> REPORT /dav/calendars/user/ken/Default/ HTTP/1.1
>>> Host: localhost
>>> Date: Wed, 30 Oct 2013 18:11:06 GMT
>>> Content-Type: application/xml
>>> Content-Length: 260
>>> Prefer: return=minimal, wait=180
>>> Accept: multipart/mixed
>>>
>>> <?xml version="1.0" encoding="UTF-8"?>
>>> <C:sync-collection xmlns:D="DAV:"
>>> xmlns:C="urn:ietf:params:xml:ns:caldav">
>>> <D:sync-token>http://cyrusimap.org/ns/sync/1368011844-354</D:sync-token>
>>>    <D:sync-level>1</D:sync-level>
>>>    <D:prop/>
>>> </C:sync-collection>
>>>
>>> HTTP/1.1 207 Multi-Status
>>> Connection: close
>>> Date: Wed, 30 Oct 2013 18:11:06 GMT
>>> Vary: Accept-Encoding, Brief, Prefer
>>> Preference-Applied: return=minimal, wait=180
>>> Content-Type: multipart/mixed;
>>> boundary="localhost-29378-1383156666-1025603243"
>>>
>>> This is a message with multiple parts in MIME format.
>>>
>>> --localhost-29378-1383156666-1025603243
>>> Date: Wed, 30 Oct 2013 18:11:06 GMT
>>> Content-Type: application/xml; charset=utf-8
>>> Content-Length: 421
>>>
>>> <?xml version="1.0" encoding="utf-8"?>
>>> <D:multistatus xmlns:D="DAV:" xmlns:C="urn:ietf:params:xml:ns:caldav">
>>>    <D:response>
>>> <D:href>/dav/calendars/user/ken/Default/4E4B3490-6F01-41B9-AA5B-FE2CD6A30632.ics</D:href>
>>>
>>>      <D:propstat>
>>>        <D:prop/>
>>>        <D:status>HTTP/1.1 200 OK</D:status>
>>>      </D:propstat>
>>>    </D:response>
>>> <D:sync-token>http://cyrusimap.org/ns/sync/1368011844-355</D:sync-token>
>>> </D:multistatus>
>>>
>>> --localhost-29378-1383156666-1025603243
>>> Date: Wed, 30 Oct 2013 18:12:47 GMT
>>> Content-Type: application/xml; charset=utf-8
>>> Content-Length: 375
>>>
>>> <?xml version="1.0" encoding="utf-8"?>
>>> <D:multistatus xmlns:D="DAV:" xmlns:C="urn:ietf:params:xml:ns:caldav">
>>>    <D:response>
>>> <D:href>/dav/calendars/user/ken/Default/4E4B3490-6F01-41B9-AA5B-FE2CD6A30632.ics</D:href>
>>>
>>>      <D:status>HTTP/1.1 404 Not Found</D:status>
>>>    </D:response>
>>> <D:sync-token>http://cyrusimap.org/ns/sync/1368011844-356</D:sync-token>
>>> </D:multistatus>
>>>
>>> --localhost-29378-1383156666-1025603243
>>> Date: Wed, 30 Oct 2013 18:14:05 GMT
>>> Content-Type: application/xml; charset=utf-8
>>> Content-Length: 202
>>>
>>> <?xml version="1.0" encoding="utf-8"?>
>>> <D:multistatus xmlns:D="DAV:" xmlns:C="urn:ietf:params:xml:ns:caldav">
>>> <D:sync-token>http://cyrusimap.org/ns/sync/1368011844-356</D:sync-token>
>>> </D:multistatus>
>>>
>>> --localhost-29378-1383156666-1025603243--
>>>
>>> End of MIME multipart body.
>>>
>>> --
>>> Kenneth Murchison
>>> Principal Systems Software Engineer
>>> Carnegie Mellon University
>>>
>>
>
>
Received on Wednesday, 9 April 2014 17:57:56 UTC

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