C[omp]ute

From: "andrew cooke" <andrew@...>

Date: Fri, 9 Dec 2005 09:12:24 -0300 (CLST)

And here's the reason why voi:// is used...

--------------

On 2005 Dec 8 , at 21.42, Doug Tody wrote:

> A given URI may resolve into multiple URLs pointing to multiple
> instances.

That's the difference!  I had completely forgotten about the one-to-
many resolution.

I'm working this through out loud here, Doug, for my benefit rather
than yours, as I imagine you've been through this already, and
because it might be useful (to me if noone else) to have the whole
argument in one place.

The underlying reason is that the resources in question are biggish.
This breaks the assumptions of the best practice/architecture
analysis in two independent ways:

1. The resources are replicated, and large enough that the client's
location on the network matters.

2. The size means that HTTP is probably not the best transport
mechanism, but instead GridFTP, or BitTorrent, or something else.

In both cases, the client can't be expected to make a good decision
about which source to use (because that will depend on details of the
national and intercontinental network, which will moreover change in
time), nor which protocol to use (which will also depend on network
environment and time).  A local resolver can be expected to know
these things, either by discovery or configuration.

The assumption that's broken is the single, almost hidden, assumption
that the transport issue is solved -- `use HTTP'.  Even if that were
sorted out, and everyone decided that GridFTP (say) was the single
best transport, the analysis also assumes that there is a single
source -- a single DNS host -- for the resource; the replication in
(1) means that we're not assuming that.  That can also be got around,
by having a DNS name be handled by multiple geographically dispersed
IP addresses (Google is well known to do this), but this is
technically complicated and therefore fragile, and also centralised.

Even if they acknowledge the first HTTP point, the response to this
second point on the part of the TAG (the W3C Technical Architecture
Group, authors of the Web Architecture document) would be to point at
the replication implicit in (1).  One of the good features of HTTP is
that it is stateless, which means that it is very friendly to caches
and proxies, so you _can_ have a simple single source, and just rely
on caches to speed things up -- don't try to outsmart the network!
But the sizes undermine that argument, too: few places have the
resources to cache lots of multi-GB files, and if regional or
national centres were set up which could handle that, it would
require configuration cleverness to use them.  Thus the replication
is essentially a type of preemptive caching.

On the other hand: I suppose there is still one case for using HTTP
with a (nominally) single source, along with a smart local proxy,
which spots when you're requesting a resource/source it knows about,
and satisfies those requests using (transparently) a separate network
of replicas and protocols.  That way, the client gets all the
simplicity, predictability and API advantages of using HTTP naively
(because that would work fine over a local network).  The proxy is
effectively acting as a resolver, but the client is interacting with
it using an extremely simple and possibly built-in protocol/API, and
so doesn't have to care.  Is there milage in that?

...but I think I'm going on at too much length now, so I'll shut up!

All the best,

Norman

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