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Re: WebDAV PUSH based on RFC 6202

From: Peter Saint-Andre <stpeter@stpeter.im>
Date: Wed, 09 Apr 2014 09:36:44 -0700
Message-ID: <5345771C.9090909@stpeter.im>
To: Ken Murchison <murch@andrew.cmu.edu>, WebDAV <w3c-dist-auth@w3.org>
<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 16:37:12 UTC

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