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"Z39.50 Server List" DTD

From: Ray Denenberg <rden@loc.gov>
Date: Tue, 20 Jun 2000 10:45:15 -0400
Message-ID: <394F837B.4D6A515@rs8.loc.gov>
To: ZIG List <z3950iw@lists.ufl.edu>, "ZIG List (at W3C)" <www-zig@w3.org>
Mark Hinnesbusch and I have worked up a simple DTD, in response to the
suggestion at the San Antonio meeting for a distributed Z39.50 directory of
servers. (Sorry, I really should have posted this a month or two ago.)

Review of the discussion in San Antonio:  We need a mechanism for a client to
dynamically discover Z39.50 servers. We briefly considered "broadcasting" and
"robots", but these ideas were passed over in favor of a  simpler mechanism, a
"distributed Z-directory".  The approach is to define a structure that a Z39.50
server may include within the Init response -- an otherInfo item that points the
client to one or more servers. The client may then initiate associations with
these servers, possibly for the sole purpose of discovering more servers, and so
on. The information to be supplied for each server would be: a name, IP address,
port, and a URL pointing to information about the  server.

We decided that the "structure" will be defined in XML, not ASN.1,  primarily
because these directory infoItems will find their way to applications that don't
directly support Z39.50 (though may communicate with applications that do).

As far as I know this is the first XML structure defined for use by Z39.50, and
so there are some details to be worked out. (One of the details, though
relatively very minor, is the form that the ASN.1 EXTERNAL will take, which is
why I initiated that thread. However, that discussion can proceed
independently.)

The important question -- and I think the answer is clear, but we need to get it
nailed down--  is: what object identifier should be used for the syntax of this
structure when it occurs in an Init Response?  Clearly (it seems to me) "XML" is
not sufficient (i.e. 1.2.840.10003.5.109.10), particulary for an object occuring
within an Init Response, because  the client will want some identifier of its
semantics, so it can throw it away without opening it, if it doesn't recognize
the oid.  So I think the answer is to register the DTD with an object
identifier, which identifies the semantics, and also implicitly identifies the
syntax as XML. This is not to say that the Document Type Declaration would not
also occur, redundantly, within the xml object.

The draft DTD follows. Please comment on this.

<!--
This DTD defines a "server list", that a Z39.50 server may include,
solicited or unsolicited, within the Init response. This is intended for
deployment of a distributed Z39.50 directory, allowing  Z39.50 clients to
dynamically discover Z39.50 servers. A server list may be included,  in
an otherInfo item,  pointing the client to one or more servers. The
client may ignore this information, or may use the information
as it sees fit, for example it may initiate associations with one or
more of these servers, possibly for the purpose of  discovering
additional servers, and so on. The information to be supplied for each
server  is  a single Z39.50s URL (element ZURLS), zero or more URLs (type
unspecified) that might provide information about the server (element
URL), and a single (optional) name for the server (element Name).
-->
<ELEMENT Server-list (Server+)>
<ELEMENT Server (ZURLS, URL*, Name?)>
<ELEMENT ZURLS (#PCDATA)>
<ELEMENT URL (#PCDATA)>
<ELEMENT Name  (#PCDATA)>





--
Ray Denenberg
Library of Congress
rden@loc.gov
202-707-5795
Received on Tuesday, 20 June 2000 10:45:35 GMT

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