Hi,
Yes, that's the basic idea - the handle, and any static links that
use it, stays the same through attribute changes of the identified
object, such as changes in location. So the links continue to work
as the location, or any other attribute changes. Of course someone
has to maintain that mapping, much like someone has to maintain the
mapping between IP address and domain name (although the handles
are intended to be more finely grained than that).
A couple of additions to your summary -
hdl.handle.net is not a handle server -- it is a proxy server
(actually a set of proxy servers) that takes an HTTP GET and turns
it into a handle resolution. So its a client to the complete set of
handle servers in the world. And it talks to the set of Global
handle servers to find out which local handle server(s) it can talk
to to resolve every handle starting with a certain prefix.
And handles can map to things other than URLs. Every handle is a
bundle of type/value pairs (or you can think of it as a distributed
database of handle/type/value tuples). A lot of those types are
URLs and some systems, like the set of hdl.handle.net proxy
servers, are set up to basically operate on URLs, but you can use
any type you like. Some types are administrative in nature (when a
handle client talks to global it looks for a type of 'service
information' that is a piece of structured data telling it all it
needs, e.g., IPs, ports, public keys, to talk to the one or more
handle servers that know about that prefix), and some are
application oriented, such as producing menus associated with each
link. Not too much has been done with that capability so far,
outside of running the handle system itself, but this is starting
to pick up, e.g., see the D-Lib article on the China Digital Museum
Project which uses the handle system to track multiple locations.
(http://dx.doi.org//10.1045/july2006-tansley)
Larry
On Mar 22, 2007, at 8:04 PM, Michael Judd wrote:
Hi Tim,
I'm no handle expert but here goes...
Handles are just updatable mappings to URLs.
They allow you to publish URLs to documents that, as long as you
maintain the mapping, will always resolve.
So in your example, originally you might have given the URL http://
hdl.handle.net/98765/todo to someone so that they could see the
document at http://shield.mydomain.org/todo.txt.
(Any handle server should be able to resolve any handle, but since
handle servers themselves are suseptable to hostname changes etc.
you should use the cnri handle server (hdl.handle.net) when giving
out handle URLs as it is guaranteed not to change.)
Back to your example, after your hostname change you would update
98765/todo to point to http://dagger.mydomain.org/todo.txt so that
everyone who went to http://hdl.handle.net/98765/todo would
continue to see the correct document.
Originally you may have had another document at http://
whatever.mydomain.org/whatever.txt that you created the handle
http://hdl.handle.net/98765/whatever for. This handle will still
be valid as long as http://whatever.mydomain.org/whatever.txt
points to the document.
Handles with the same prefix (98765) don't have to resolve to the
same server. The prefix is just used to locate the actual handle
server.
Using DNS cnames and aliases you can keep URLs that simply move
servers or change hostnames resolving. But what happens if an
organizations domain name changes? Or what happens if the
repository software the documents exist in changes, and all the
relative links change? This is where the value of handles and
other persistant URL schemes come.
That's how I see it anyway. :)
Cheers!
Regards,
Michael Judd
Nathan Campus, Griffith University.
Brisbane 4111. Australia.
m.judd@griffith.edu.au
07 3735 3801
Tim Donnelly <tim@coalliance.org>
Sent by: handle-info-admin@cnri.reston.va.us
23/03/2007 07:39 AM
Please respond to
tim@coalliance.org
To
handle-info@cnri.reston.va.us
cc
Subject
[Handle-info] Please help me understand the value of Handle
I am confused on the way Handles work. Let me explain by way of
an example.
I have a prefix, for the sake of argument 98765. I have created a
handle 98765/todo that points to the URL http://
shield.mydomain.org/todo.txt. If I look at the handle in the
Admin tool it shows a data value of the URL.
But, what happens if at some point in the future, the name of the
host changes from shield.coalliance.org to dagger.mydomain.org?
That handle is now broken, correct?
I thought the whole point of Handles was to create a way to be
able to find information regardless of changes such as I described.
The way I envisioned the Handle service working is more like this:
98765 would actually resolve to http://shield.mydomain.org,
therefore handles I create would only include the final
destination at that location (todo.txt in my example above). So,
the Handle 98765/todo would equal http://shield.mydomain.org/
todo.txt. Then, if I later changed shield to dagger, I would have
to update my siteinfo with the Global Registry, telling it that
all 98765 prefixes now belong to dagger.mydomain.org.
The way it appears to actually work to me is:
The 98765 prefix is simply used by the Global Registry to tell the
proxy server which local handle server it should look to to
resolve. So, the proxy server looks up 98765 and knows to send
the request to my local server at IP 192.168.0.1. The local
server then says "todo" is equal to the URL and redirects the
users browser to that URL hard coded into the handle.
I have tested this by creating a Handle with the URL of todo.txt,
no domain info included. When I attempt to resolve it I get an
error that it cannot be found.
I'm sure this isn't making much sense but I am having a hard time
wrapping my brain around these concepts.
Thanks in advance.
Tim Donnelly
Systems/Network Administrator
Colorado Alliance of Research Libraries
(303)759-3399 x106
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