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RE: Web browsers should preserve the file system Last-Modified time of downloaded files

From: Larry Masinter <masinter@adobe.com>
Date: Wed, 9 Nov 2011 13:03:05 -0800
To: Scott Penrose <scottp@dd.com.au>, Karl Dubost <karld@opera.com>
CC: Brolin Empey <brolin@brolin.be>, "www-tag@w3.org List" <www-tag@w3.org>
Message-ID: <C68CB012D9182D408CED7B884F441D4D0611DABBBF@nambxv01a.corp.adobe.com>
I think the time skew is the determining factor for me... 

When two systems are on the internet communicating, they *might* be in sync but often are not. Maybe their clocks are only seconds apart. Maybe the latency between the two systems is small, but I think it is actually unusual for two systems to be in perfect sync. Whether or not it is unusual, it *does* happen.

Then, the "last-modified" time on the server is server-relative.  The client writes a file in its local file system, you might even get a file with "last modified" in the future.

-----Original Message-----
From: Scott Penrose [mailto:scottp@dd.com.au] 
Sent: Tuesday, November 08, 2011 7:00 PM
To: Karl Dubost
Cc: Brolin Empey; www-tag@w3.org List
Subject: Re: Web browsers should preserve the file system Last-Modified time of downloaded files

> Le 4 nov. 2011 à 19:18, Brolin Empey a écrit :
>> The Web browser should preserve the file system last modified time by default because this time cannot easily be recovered after it is discarded.

I have been thinking about this over the last week and which way it should happen.
It is interesting to note that many older backup systems use last modified as a method to work out if files need incremental backup.

The problem is that once you download a file, they are not linked. The file on your system does not match the remote system. So what does modified mean? 

Lets put 3 systems into play.

* Server1 - file.png, last modified 2010-01-01
* Server 2 - file.png downloaded, last modified 2010-01-01
* Client 1 - Download file from Server 2, use if mod since

Scenario 1 - Server 2 modifies the PNG (maybe a resize). Last modified should change to that day (2011-10-01), Client can download.

Scenario 2 - Server 1 had updated the logo in 2011-06-01; now Server 2 downloads from Server 1, preserving date modified as 2011-06-01; Client now checks if file has changed since last of 2011-10-01 - it has changed, but now client won't get it. On the other hand, if Server 2 had set last modified to today 2011-11-09 - it would not be an issue.

Scenario 3 - A client downloads a file to Download folder, has a look at recent files. User would expect to see newest files downloaded with todays date.

I don't think HTTP Clients should preserve modified date time. But there are exceptions - e.g. WebDAV / SVN over HTTP.

Received on Wednesday, 9 November 2011 21:03:50 UTC

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