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RE: How do I set getlastmodified using PROPPATCH ?

From: Jim Whitehead <ejw@ics.uci.edu>
Date: Thu, 27 Jul 2000 09:32:57 -0700
To: John Glavin <jglavin@riverfrontsoftware.com>, w3c-dist-auth@w3.org

> I have converted my FTP application webdrive (www.webdrive.com) to support
> DAV and I need to be able to set the "getlastmodified" property using
> PROPPATCH so that utilities like powerdesk can syncrhonize files from a
> Windows PC to a DAV server.

Well, the intent of the getlastmodified property is to give the time of the
most recent state change of the resource. Thus, getlastmodified will
definitely be affected by a PUT, and may be affected by a PROPPATCH.  Since
the value of the property is determined by the server, it is a "live"
property.  Though I haven't looked into it, I suspect that most DAV servers
do not allow you to update the getlastmodified property using PROPPATCH.

However, all is not lost for synchronization. Since clock times are
notoriously unreliable over the network, HTTP provides a facility called
entity tags (Etags). An Etag is a unique identifier for a particular state
of a resource.  The uniquenes of an Etag is scoped to an individual
resource.  The basic idea is that when a resource changes, the Etag changes
too.  HTTP caches often use Etags due to this property.

You could use Etags for your synchronization activity in the following way.
When resources are originally downloaded, persistently save the Etags and
the local time of download in a file (.etags, or similar).  When it's time
to synchronize, you can check the local etag vs the remote etag to see if
the resource has changed on the remote server.  You can check the local
modification date vs. the download time to see if the file has changed on
the local machine.

> I have done testing with several servers like (Driveway.com, Zope.org,
> Sharemation.com)and notice that they all return the "getlastmodified"
> but sometimes the namespace is different from server to server.

The getlastmodified property is defined to be in the "DAV:" namespace.  This
namespace is mapped to a specific prefix letter within a specific XML
message.  In RFC 2518, you'll see plenty of examples where there is the
attribute: xmlns:d='DAV:'  This means, map the prefix letter "d" to the
namespace "DAV:".  Precise semantics and rules are given in
http://www.w3.org/TR/REC-xml-names/   So, while Driveway, Zope, Sharemation,
etc. all use different *prefix* letters, they all use the same *namespace*,

> When parsing this xml response I always look for ":getlastmodified" to
> determine the time for the file.

This will work most of the time, but would not handle the case where two
namespaces were in use, say "DAV:" and "http://www.me.org/foo/", each mapped
to a different prefix letter, say "d" and "f", and getlastmodified exists in
both namespaces.  Your parsing algorithm would treat them the same, though
they have different semantics.

> Do I have to specify the namespace in xml when doing a PROPPATCH ?


> Is this namespace different for each server ?

For properties defined in RFC 2518, no.

> Is there a generic way to specify the getlastmodified time to set ?

No. It is set as a side-effect of PUT, and possibly PROPPATCH.

- Jim
Received on Thursday, 27 July 2000 12:37:58 UTC

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