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RE: Multi-server HTTP

From: Ford, Alan <alan.ford@roke.co.uk>
Date: Fri, 28 Aug 2009 12:38:05 +0100
Message-ID: <2181C5F19DD0254692452BFF3EAF1D6808590692@rsys005a.comm.ad.roke.co.uk>
To: "Henrik Nordstrom" <henrik@henriknordstrom.net>
Cc: "Robert Siemer" <Robert.Siemer-http@backsla.sh>, "Mark Nottingham" <mnot@mnot.net>, <ietf-http-wg@w3.org>, "Mark Handley" <m.handley@cs.ucl.ac.uk>
Hi Henrik, all,

Thanks for the clarification, so it seems we could in theory define the
ETag for this specification to ensure it matches across servers. That
would remove the need for all except the Mirrors: header, and possibly
Multiserver-Version (so that the server knows it's talking to a
multiserver-capable client and thus the ETag is defined this way). If we
didn't mind a small delay, we could probably do away with that too and
say the client could infer capability by getting a Mirrors: header back
from a HEAD request first, and then deciding what to do (assuming the
connection can be kept alive).

Which brings me onto another thing about Mirrors: header. One of our
longer-term goals with this would be to somehow provide wildcarded lists
of mirrors, so that a client could immediately run off and fetch bits of
a website from many mirrors, potentially speeding up loading time
considerably, and providing an alternative method of load balancing.

However, I'm struggling to see a neat way of doing this reliably, since
we couldn't get checksums for every file on the first handshake (or if
all content was static we might be able to, but it's a big overhead).
Does anybody have any ideas as to a neat way of doing this? Best I can
think of so far is some sort of version number/(pseudo)hash of the
entire directory structure! 

Regards,
Alan

> -----Original Message-----
> From: Henrik Nordstrom [mailto:henrik@henriknordstrom.net]
> Sent: 26 August 2009 21:30
> To: Ford, Alan
> Cc: Robert Siemer; Mark Nottingham; ietf-http-wg@w3.org; Mark Handley
> Subject: RE: Multi-server HTTP
> 
> tis 2009-08-25 klockan 10:24 +0100 skrev Ford, Alan:
> > Well that would certainly be a great solution for many use cases. If
> > there was a distributed set of virtual hosts of a given server, I
can
> > see that working quite well. Plus, this would solve the ETag issue
that
> > I mention (assuming I have understood correctly - nobody has yet
> > corrected me!), since in this case the client /is/ requesting the
same
> > resource from each IP address.
> 
> ETag is per URI, but it is entirely fine for a specification like this
> to require that the participating servers use the same ETag for the
same
> object version, for example based on an hash of the object data. How
> servers compose ETag is outside of HTTP specification and a property
of
> the server implementation, only requirement HTTP places is uniqueness
> among versions or variants of the same URI. The base HTTP
specifications
> places no direct requirements on how ETag from different URIs relate
to
> each other, but do hint that for objects having multiple URIs where
> those URIs are equal it's expected the ETag would also be the same.
> 
> However, many server implementations available today do not easily
allow
> this on the wide scale you require without additions on the server, as
> they base ETag on other metadata that may differ between mirrors of
the
> same object such as local file timestamp, filesystem inode numbers
etc,
> not really taking the actual content of the object into consideration.
> 
> Regards
> Henrik


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Received on Friday, 28 August 2009 11:39:31 GMT

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