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24 <title>Metaproxy - User's Guide and Reference</title>
27 <firstname>Adam</firstname><surname>Dickmeiss</surname>
30 <firstname>Marc</firstname><surname>Cromme</surname>
33 <firstname>Mike</firstname><surname>Taylor</surname>
36 <releaseinfo>&version;</releaseinfo>
38 <year>2005-2007</year>
39 <holder>Index Data ApS</holder>
43 This manual is part of Metaproxy version &version;.
46 Metaproxy is a universal router, proxy and encapsulated
47 metasearcher for information retrieval protocols. It accepts,
48 processes, interprets and redirects requests from IR clients using
49 standard protocols such as the binary
50 <ulink url="&url.z39.50;">ANSI/NISO Z39.50</ulink>
51 and the information search and retireval
52 web services <ulink url="&url.sru;">SRU</ulink>
53 and <ulink url="&url.srw;">SRW</ulink>, as
54 well as functioning as a limited
55 <ulink url="&url.http;">HTTP</ulink> server.
58 Metaproxy is configured by an XML file which
59 specifies how the software should function in terms of routes that
60 the request packets can take through the proxy, each step on a
61 route being an instantiation of a filter. Filters come in many
62 types, one for each operation: accepting Z39.50 packets, logging,
63 query transformation, multiplexing, etc. Further filter-types can
64 be added as loadable modules to extend Metaproxy functionality,
68 Metaproxy is <emphasis>not</emphasis> open-source software, but
69 may be freely downloaded, unpacked, inspected, built and run for
70 evaluation purposes. Deployment requires a separate, commercial,
76 <imagedata fileref="common/id.png" format="PNG"/>
79 <imagedata fileref="common/id.eps" format="EPS"/>
86 <chapter id="introduction">
87 <title>Introduction</title>
91 <ulink url="&url.metaproxy;">Metaproxy</ulink>
92 is a stand alone program that acts as a universal router, proxy and
93 encapsulated metasearcher for information retrieval protocols such
94 as <ulink url="&url.z39.50;">Z39.50</ulink>, and in the future
95 <ulink url="&url.sru;">SRU</ulink> and <ulink url="&url.srw;">SRW</ulink>.
96 To clients, it acts as a server of these protocols: it can be searched,
97 records can be retrieved from it, etc.
98 To servers, it acts as a client: it searches in them,
99 retrieves records from them, etc. it satisfies its clients'
100 requests by transforming them, multiplexing them, forwarding them
101 on to zero or more servers, merging the results, transforming
102 them, and delivering them back to the client. In addition, it
103 acts as a simple <ulink url="&url.http;">HTTP</ulink> server; support
104 for further protocols can be added in a modular fashion, through the
105 creation of new filters.
110 Fish, bananas, cold pyjamas,
111 Mutton, beef and trout!
112 - attributed to Cole Porter.
115 Metaproxy is a more capable alternative to
116 <ulink url="&url.yazproxy;">YAZ Proxy</ulink>,
117 being more powerful, flexible, configurable and extensible. Among
118 its many advantages over the older, more pedestrian work are
119 support for multiplexing (encapsulated metasearching), routing by
120 database name, authentication and authorization and serving local
121 files via HTTP. Equally significant, its modular architecture
122 facilitites the creation of pluggable modules implementing further
126 This manual will describe how to install Metaproxy
127 before giving an overview of its architecture, then discussing the
128 key concept of a filter in some depth and giving an overview of
129 the various filter types, then discussing the configuration file
130 format. After this come several optional chapters which may be
131 freely skipped: a detailed discussion of virtual databases and
132 multi-database searching, some notes on writing extensions
133 (additional filter types) and a high-level description of the
134 source code. Finally comes the reference guide, which contains
135 instructions for invoking the <command>metaproxy</command>
136 program, and detailed information on each type of filter,
141 <chapter id="license">
142 <title>The Metaproxy License</title>
143 <orderedlist numeration="arabic">
146 You are allowed to download this software for evaluation purposes.
147 You can unpack it, build it, run it, see how it works and how it fits
148 your needs, all at zero cost.
153 You may NOT deploy the software. For the purposes of this license,
154 deployment means running it for any purpose other than evaluation,
155 whether or not you or anyone else makes a profit from doing so. If
156 you wish to deploy the software, you must first contact Index Data and
157 arrange to purchase a DEPLOYMENT LICENCE. If you are unsure
158 whether or not your proposed use of the software constitutes
159 deployment, email us at <literal>info@indexdata.com</literal>
165 You may modify your copy of the software (fix bugs, add features)
166 if you need to. We encourage you to send your changes back to us for
167 integration into the master copy, but you are not obliged to do so. You
168 may NOT pass your changes on to any other party.
173 There is NO WARRANTY for this software, to the extent permitted by
174 applicable law. We provide the software ``as is'' without warranty of
175 any kind, either expressed or implied, including, but not limited to, the
176 implied warranties of MERCHANTABILITY and FITNESS FOR A
177 PARTICULAR PURPOSE. The entire risk as to the quality and
178 performance of the software is with you. Should the software prove
179 defective, you assume the cost of all necessary servicing, repair or
180 correction. In no event unless required by applicable law will we be
181 liable to you for damages, arising out of the use of the software,
182 including but not limited to loss of data or data being rendered
188 All rights to the software are reserved by Index Data except where
189 this license explicitly says otherwise.
195 <chapter id="installation">
196 <title>Installation</title>
198 Metaproxy depends on the following tools/libraries:
200 <varlistentry><term><ulink url="&url.yazplusplus;">YAZ++</ulink></term>
203 This is a C++ library based on <ulink url="&url.yaz;">YAZ</ulink>.
207 <varlistentry><term><ulink url="&url.libxslt;">Libxslt</ulink></term>
209 <para>This is an XSLT processor - based on
210 <ulink url="&url.libxml2;">Libxml2</ulink>. Both Libxml2 and
211 Libxslt must be installed with the development components
212 (header files, etc.) as well as the run-time libraries.
216 <varlistentry><term><ulink url="&url.boost;">Boost</ulink></term>
219 The popular C++ library. Initial versions of Metaproxy
220 was built with 1.33.0. Version 1.33.1 works too.
227 In order to compile Metaproxy a modern C++ compiler is
228 required. Boost, in particular, requires the C++ compiler
229 to facilitate the newest features. Refer to Boost
230 <ulink url="&url.boost.compilers.status;">Compiler Status</ulink>
231 for more information.
234 We have successfully built Metaproxy using the compilers
235 <ulink url="&url.gcc;">GCC</ulink> version 4.0 and
236 <ulink url="&url.vstudio;">Microsoft Visual Studio</ulink> 2003/2005.
239 <section id="installation.unix">
240 <title>Installation on Unix (from Source)</title>
242 Here is a quick step-by-step guide on how to compile all the
243 tools that Metaproxy uses. Only few systems have none of the required
244 tools binary packages. If, for example, Libxml2/libxslt are already
245 installed as development packages use those (and omit compilation).
252 gunzip -c libxml2-version.tar.gz|tar xf -
260 gunzip -c libxslt-version.tar.gz|tar xf -
271 gunzip -c yaz-version.tar.gz|tar xf -
279 gunzip -c yazpp-version.tar.gz|tar xf -
290 gunzip -c boost-version.tar.gz|tar xf -
301 gunzip -c metaproxy-version.tar.gz|tar xf -
310 <section id="installation.debian">
311 <title>Installation on Debian GNU/Linux</title>
313 All dependencies for Metaproxy are available as
314 <ulink url="&url.debian;">Debian</ulink>
315 packages for the sarge (stable in 2005) and etch (testing in 2005)
319 The procedures for Debian based systems, such as
320 <ulink url="&url.ubuntu;">Ubuntu</ulink> is probably similar
323 There is currently no official Debian package for YAZ++.
324 And the Debian package for YAZ is probably too old.
325 Update the <filename>/etc/apt/sources.list</filename>
326 to include the Index Data repository.
327 See YAZ' <ulink url="&url.yaz.download.debian;">Download Debian</ulink>
328 for more information.
331 apt-get install libxslt1-dev
332 apt-get install libyazpp-dev
333 apt-get install libboost-dev
334 apt-get install libboost-thread-dev
335 apt-get install libboost-date-time-dev
336 apt-get install libboost-program-options-dev
337 apt-get install libboost-test-dev
340 With these packages installed, the usual configure + make
341 procedure can be used for Metaproxy as outlined in
342 <xref linkend="installation.unix"/>.
346 <section id="installation.windows">
347 <title>Installation on Windows</title>
349 Metaproxy can be compiled with Microsoft
350 <ulink url="&url.vstudio;">Visual Studio</ulink>.
351 Version 2003 (C 7.1) and 2005 (C 8.0) is known to work.
353 <section id="installation.windows.boost">
356 Get Boost from its <ulink url="&url.boost;">home page</ulink>.
357 You also need Boost Jam (an alternative to make).
358 That's also available from the Boost home page.
359 The files to be downloaded are called something like:
360 <filename>boost_1_33-1.exe</filename>
362 <filename>boost-jam-3.1.12-1-ntx86.zip</filename>.
363 Unpack Boost Jam first. Put <filename>bjam.exe</filename>
364 in your system path. Make a command prompt and ensure
365 it can be found automatically. If not check the PATH.
366 The Boost .exe is a self-extracting exe with
367 complete source for Boost. Compile that source with
368 Boost Jam (An alternative to Make).
369 The compilation takes a while.
370 For Visual Studio 2003, use
372 bjam "-sTOOLS=vc-7_1"
374 Here <literal>vc-7_1</literal> refers to a "Toolset" (compiler system).
375 For Visual Studio 2005, use
377 bjam "-sTOOLS=vc-8_0"
379 To install the libraries in a common place, use
381 bjam "-sTOOLS=vc-7_1" install
383 (or vc-8_0 for VS 2005).
386 By default, the Boost build process installs the resulting
387 libraries + header files in
388 <literal>\boost\lib</literal>, <literal>\boost\include</literal>.
391 For more information about installing Boost refer to the
392 <ulink url="&url.boost.getting.started;">getting started</ulink>
397 <section id="installation.windows.libxslt">
398 <title>Libxslt</title>
400 <ulink url="&url.libxslt;">Libxslt</ulink> can be downloaded
402 <ulink url="&url.libxml2.download.win32;">here</ulink>.
405 Libxslt has other dependencies, but these can all be downloaded
406 from the same site. Get the following:
407 iconv, zlib, libxml2, libxslt.
411 <section id="installation.windows.yaz">
414 <ulink url="&url.yaz;">YAZ</ulink> can be downloaded
416 <ulink url="&url.yaz.download.win32;">here</ulink>.
420 <section id="installation.windows.yazplusplus">
423 Get <ulink url="&url.yazplusplus;">YAZ++</ulink> as well.
424 Version 1.0 or later is required. For now get it from
426 <ulink url="&url.snapshot.download;">Snapshot area</ulink>.
429 YAZ++ includes NMAKE makefiles, similar to those found in the
434 <section id="installation.windows.metaproxy">
435 <title>Metaproxy</title>
437 Metaproxy is shipped with NMAKE makefiles as well - similar
438 to those found in the YAZ++/YAZ packages. Adjust this Makefile
439 to point to the proper locations of Boost, Libxslt, Libxml2,
440 zlib, iconv, yaz and yazpp.
444 <varlistentry><term><literal>DEBUG</literal></term>
446 If set to 1, the software is
447 compiled with debugging libraries (code generation is
448 multi-threaded debug DLL).
449 If set to 0, the software is compiled with release libraries
450 (code generation is multi-threaded DLL).
455 <term><literal>BOOST</literal></term>
458 Boost install location
464 <term><literal>BOOST_VERSION</literal></term>
467 Boost version (replace . with _).
473 <term><literal>BOOST_TOOLSET</literal></term>
482 <term><literal>LIBXSLT_DIR</literal>,
483 <literal>LIBXML2_DIR</literal> ..</term>
486 Specify the locations of Libxslt, libiconv, libxml2 and
495 After successful compilation you'll find
496 <literal>metaproxy.exe</literal> in the
497 <literal>bin</literal> directory.
505 <chapter id="architecture">
506 <title>The Metaproxy Architecture</title>
508 The Metaproxy architecture is based on three concepts:
509 the <emphasis>package</emphasis>,
510 the <emphasis>route</emphasis>
511 and the <emphasis>filter</emphasis>.
515 <term>Packages</term>
518 A package is request or response, encoded in some protocol,
519 issued by a client, making its way through Metaproxy, send to or
520 received from a server, or sent back to the client.
523 The core of a package is the protocol unit - for example, a
524 Z39.50 Init Request or Search Response, or an SRU searchRetrieve
525 URL or Explain Response. In addition to this core, a package
526 also carries some extra information added and used by Metaproxy
530 In general, packages are doctored as they pass through
531 Metaproxy. For example, when the proxy performs authentication
532 and authorization on a Z39.50 Init request, it removes the
533 authentication credentials from the package so that they are not
534 passed onto the back-end server; and when search-response
535 packages are obtained from multiple servers, they are merged
536 into a single unified package that makes its way back to the
545 Packages make their way through routes, which can be thought of
546 as programs that operate on the package data-type. Each
547 incoming package initially makes its way through a default
548 route, but may be switched to a different route based on various
549 considerations. Routes are made up of sequences of filters (see
558 Filters provide the individual instructions within a route, and
559 effect the necessary transformations on packages. A particular
560 configuration of Metaproxy is essentially a set of filters,
561 described by configuration details and arranged in order in one
562 or more routes. There are many kinds of filter - about a dozen
563 at the time of writing with more appearing all the time - each
564 performing a specific function and configured by different
568 The word ``filter'' is sometimes used rather loosely, in two
569 different ways: it may be used to mean a particular
570 <emphasis>type</emphasis> of filter, as when we speak of ``the
571 auth_simple filter'' or ``the multi filter''; or it may be used
572 to be a specific <emphasis>instance</emphasis> of a filter
573 within a Metaproxy configuration. For example, a single
574 configuration will often contain multiple instances of the
575 <literal>z3950_client</literal> filter. In
576 operational terms, of these is a separate filter. In practice,
577 context always make it clear which sense of the word ``filter''
581 Extensibility of Metaproxy is primarily through the creation of
582 plugins that provide new filters. The filter API is small and
583 conceptually simple, but there are many details to master. See
585 <link linkend="extensions">extensions</link>.
591 Since packages are created and handled by the system itself, and
592 routes are conceptually simple, most of the remainder of this
593 document concentrates on filters. After a brief overview of the
594 filter types follows, along with some thoughts on possible future
601 <chapter id="filters">
602 <title>Filters</title>
606 <title>Introductory notes</title>
608 It's useful to think of Metaproxy as an interpreter providing a small
609 number of primitives and operations, but operating on a very
610 complex data type, namely the ``package''.
613 A package represents a Z39.50 or SRU/W request (whether for Init,
614 Search, Scan, etc.) together with information about where it came
615 from. Packages are created by front-end filters such as
616 <literal>frontend_net</literal> (see below), which reads them from
617 the network; other front-end filters are possible. They then pass
618 along a route consisting of a sequence of filters, each of which
619 transforms the package and may also have side-effects such as
620 generating logging. Eventually, the route will yield a response,
621 which is sent back to the origin.
624 There are many kinds of filter: some that are defined statically
625 as part of Metaproxy, and others may be provided by third parties
626 and dynamically loaded. They all conform to the same simple API
627 of essentially two methods: <function>configure()</function> is
628 called at startup time, and is passed an XML DOM tree representing that
629 part of the configuration file that pertains to this filter
630 instance: it is expected to walk that tree extracting relevant
631 information; and <function>process()</function> is called every
632 time the filter has to processes a package.
635 While all filters provide the same API, there are different modes
636 of functionality. Some filters are sources: they create
638 (<literal>frontend_net</literal>);
639 others are sinks: they consume packages and return a result
640 (<literal>backend_test</literal>,
641 <literal>bounce</literal>,
642 <literal>http_file</literal>,
643 <literal>z3950_client</literal>);
644 the others are true filters, that read, process and pass on the
645 packages they are fed
646 (<literal>auth_simple</literal>,
647 <literal>log</literal>,
648 <literal>multi</literal>,
649 <literal>query_rewrite</literal>,
650 <literal>record_transform</literal>,
651 <literal>session_shared</literal>,
652 <literal>sru_z3950</literal>,
653 <literal>template</literal>,
654 <literal>virt_db</literal>).
659 <section id="overview.filter.types">
660 <title>Overview of filter types</title>
662 We now briefly consider each of the types of filter supported by
663 the core Metaproxy binary. This overview is intended to give a
664 flavor of the available functionality; more detailed information
665 about each type of filter is included below in
666 <xref linkend="reference"/>.
669 The filters are here named by the string that is used as the
670 <literal>type</literal> attribute of a
671 <literal><filter></literal> element in the configuration
672 file to request them, with the name of the class that implements
673 them in parentheses. (The classname is not needed for normal
674 configuration and use of Metaproxy; it is useful only to
678 The filters are here listed in alphabetical order:
685 New virt_db-alike that does inteligent peer choice, explain merging,
686 adds FD&N to explain. Keeps init responses (like "virt_db Classic"),
687 makes routing choices based on local explain knowledge. Ref IDDI
690 Filter to convert Explain Classic to ZeeRex.
692 CQL2PQF (which needs augmented ZeeRex) - MARC for Talis.
696 Figure out what additional information we need in:
697 ZeeRex (check against D3.1)
698 Init request (e.g. loop detection)
699 Query package (e.g. number of hops)
700 Query response (e.g. record source)
705 <title><literal>auth_simple</literal>
706 (mp::filter::AuthSimple)</title>
708 Simple authentication and authorization. The configuration
709 specifies the name of a file that is the user register, which
710 lists <varname>username</varname>:<varname>password</varname>
711 pairs, one per line, colon separated. When a session begins, it
712 is rejected unless username and passsword are supplied, and match
713 a pair in the register. The configuration file may also specific
714 the name of another file that is the target register: this lists
715 lists <varname>username</varname>:<varname>dbname</varname>,<varname>dbname</varname>...
716 sets, one per line, with multiple database names separated by
717 commas. When a search is processed, it is rejected unless the
718 database to be searched is one of those listed as available to
724 <title><literal>backend_test</literal>
725 (mp::filter::Backend_test)</title>
727 A partial sink that provides dummy responses in the manner of the
728 <literal>yaz-ztest</literal> Z39.50 server. This is useful only
729 for testing. Seriously, you don't need this. Pretend you didn't
730 even read this section.
735 <title><literal>bounce</literal>
736 (mp::filter::Bounce)</title>
738 A sink that swallows <emphasis>all packages</emphasis>,
739 and returns them almost unprocessed.
740 It never sends any package of any type further down the row, but
741 sets Z39.50 packages to Z_Close, and HTTP_Request packages to
742 HTTP_Response err code 400 packages, and adds a suitable bounce
744 The bounce filter is usually added at end of each filter chain
745 config.xml to prevent infinite hanging of for example HTTP
746 requests packages when only the Z39.50 client partial sink
747 filter is found in the
753 <title><literal>frontend_net</literal>
754 (mp::filter::FrontendNet)</title>
756 A source that accepts Z39.50 connections from a port
757 specified in the configuration, reads protocol units, and
758 feeds them into the next filter in the route. When the result is
759 received, it is returned to the original origin.
764 <title><literal>http_file</literal>
765 (mp::filter::HttpFile)</title>
767 A partial sink which swallows only HTTP_Request packages, and
768 returns the contents of files from the local
769 filesystem in response to HTTP requests.
770 It lets Z39.50 packages and all other forthcoming package types
773 does mean that Metaproxy is also a Web-server in its spare time. So
774 far it does not contain either an email-reader or a Lisp
775 interpreter, but that day is surely coming.)
780 <title><literal>load_balance</literal>
781 (mp::filter::LoadBalance)</title>
783 Performs load balancing for incoming Z39.50 init requests.
784 It is used together with the <literal>virt_db</literal> filter,
785 but unlike the <literal>multi</literal> filter it does send an
786 entire session to only one of the virtual backends. The
787 <literal>load_balance</literal> filter is assuming that
788 all backend targets have equal content, and chooses the backend
789 with least load cost for a new session.
792 This filter is experimental and yet not mature for heavy load
800 <title><literal>log</literal>
801 (mp::filter::Log)</title>
803 Writes logging information to standard output, and passes on
804 the package unchanged. A log file name can be specified, as well
805 as multiple different logging formats.
810 <title><literal>multi</literal>
811 (mp::filter::Multi)</title>
813 Performs multi-database searching.
815 <link linkend="multidb">the extended discussion</link>
816 of virtual databases and multi-database searching below.
821 <title><literal>query_rewrite</literal>
822 (mp::filter::QueryRewrite)</title>
824 Rewrites Z39.50 Type-1 and Type-101 (``RPN'') queries by a
825 three-step process: the query is transliterated from Z39.50
826 packet structures into an XML representation; that XML
827 representation is transformed by an XSLT stylesheet; and the
828 resulting XML is transliterated back into the Z39.50 packet
835 <title><literal>record_transform</literal>
836 (mp::filter::RecordTransform)</title>
838 This filter acts only on Z3950 present requests, and let all
839 other types of packages and requests pass untouched. It's use is
840 twofold: blocking Z3950 present requests, which the backend
841 server does not understand and can not honor, and transforming
842 the present syntax and elementset name according to the rules
843 specified, to fetch only existing record formats, and transform
844 them on the fly to requested record syntaxes.
849 <title><literal>session_shared</literal>
850 (mp::filter::SessionShared)</title>
852 This filter implements global sharing of
853 result sets (i.e. between threads and therefore between
854 clients), yielding performance improvements especially when
855 incoming requests are from a stateless environment such as a
856 web-server, in which the client process representing a session
857 might be any one of many.
862 <title><literal>sru_z3950</literal>
863 (mp::filter::SRUtoZ3950)</title>
865 This filter transforms valid
866 SRU GET/POST/SOAP searchRetrieve requests to Z3950 init, search,
867 and present requests, and wraps the
868 received hit counts and XML records into suitable SRU response
870 The <literal>sru_z3950</literal> filter processes also SRU
871 GET/POST/SOAP explain requests, returning
872 either the absolute minimum required by the standard, or a full
873 pre-defined ZeeReX explain record.
875 <ulink url="&url.zeerex.explain;">ZeeReX Explain</ulink>
876 standard pages and the
877 <ulink url="&url.sru.explain;">SRU Explain</ulink> pages
878 for more information on the correct explain syntax.
879 SRU scan requests are not supported yet.
884 <title><literal>template</literal>
885 (mp::filter::Template)</title>
887 Does nothing at all, merely passing the packet on. (Maybe it
888 should be called <literal>nop</literal> or
889 <literal>passthrough</literal>?) This exists not to be used, but
890 to be copied - to become the skeleton of new filters as they are
891 written. As with <literal>backend_test</literal>, this is not
892 intended for civilians.
897 <title><literal>virt_db</literal>
898 (mp::filter::VirtualDB)</title>
900 Performs virtual database selection: based on the name of the
901 database in the search request, a server is selected, and its
902 address added to the request in a <literal>VAL_PROXY</literal>
903 otherInfo packet. It will subsequently be used by a
904 <literal>z3950_client</literal> filter.
906 <link linkend="multidb">the extended discussion</link>
907 of virtual databases and multi-database searching below.
912 <title><literal>z3950_client</literal>
913 (mp::filter::Z3950Client)</title>
915 A partial sink which swallows only Z39.50 packages.
916 It performs Z39.50 searching and retrieval by proxying the
917 packages that are passed to it. Init requests are sent to the
918 address specified in the <literal>VAL_PROXY</literal> otherInfo
919 attached to the request: this may have been specified by client,
920 or generated by a <literal>virt_db</literal> filter earlier in
921 the route. Subsequent requests are sent to the same address,
922 which is remembered at Init time in a Session object.
923 HTTP_Request packages and all other forthcoming package types
924 are passed untouched.
930 <title><literal>zeerex_explain</literal>
931 (mp::filter::ZeerexExplain)</title>
933 This filter acts as a sink for
934 Z39.50 explain requests, returning a static ZeeReX
935 Explain XML record from the config section. All other packages
938 <ulink url="&url.zeerex.explain;">ZeeReX Explain</ulink>
940 for more information on the correct explain syntax.
944 This filter is not yet completed.
953 <section id="future.directions">
954 <title>Future directions</title>
956 Some other filters that do not yet exist, but which would be
957 useful, are briefly described. These may be added in future
958 releases (or may be created by third parties, as loadable
964 <term><literal>frontend_cli</literal> (source)</term>
967 Command-line interface for generating requests.
972 <term><literal>sru_client</literal> (sink)</term>
975 SRU/GET and SRU/SOAP searching and retrieval.
980 <term><literal>opensearch_client</literal> (sink)</term>
983 A9 OpenSearch searching and retrieval.
993 <chapter id="configuration">
994 <title>Configuration: the Metaproxy configuration file format</title>
998 <title>Introductory notes</title>
1000 If Metaproxy is an interpreter providing operations on packages, then
1001 its configuration file can be thought of as a program for that
1002 interpreter. Configuration is by means of a single XML file, the name
1003 of which is supplied as the sole command-line argument to the
1004 <command>metaproxy</command> program. (See
1005 <xref linkend="reference"/> below for more information on invoking
1010 <section id="overview.xml.structure">
1011 <title>Overview of the config file XML structure</title>
1013 All elements and attributes are in the namespace
1014 <ulink url="http://indexdata.com/metaproxy"/>.
1015 This is most easily achieved by setting the default namespace on
1016 the top-level element, as here:
1019 <metaproxy xmlns="http://indexdata.com/metaproxy" version="1.0">
1022 The top-level element is <metaproxy>. This contains a
1023 <start> element, a <filters> element and a
1024 <routes> element, in that order. <filters> is
1025 optional; the other two are mandatory. All three are
1029 The <start> element is empty, but carries a
1030 <literal>route</literal> attribute, whose value is the name of
1031 route at which to start running - analogous to the name of the
1032 start production in a formal grammar.
1035 If present, <filters> contains zero or more <filter>
1036 elements. Each filter carries a <literal>type</literal> attribute
1037 which specifies what kind of filter is being defined
1038 (<literal>frontend_net</literal>, <literal>log</literal>, etc.)
1039 and contain various elements that provide suitable configuration
1040 for a filter of its type. The filter-specific elements are
1042 <xref linkend="reference"/>.
1043 Filters defined in this part of the file must carry an
1044 <literal>id</literal> attribute so that they can be referenced
1048 <routes> contains one or more <route> elements, each
1049 of which must carry an <literal>id</literal> element. One of the
1050 routes must have the ID value that was specified as the start
1051 route in the <start> element's <literal>route</literal>
1052 attribute. Each route contains zero or more <filter>
1053 elements. These are of two types. They may be empty, but carry a
1054 <literal>refid</literal> attribute whose value is the same as the
1055 <literal>id</literal> of a filter previously defined in the
1056 <filters> section. Alternatively, a route within a filter
1057 may omit the <literal>refid</literal> attribute, but contain
1058 configuration elements similar to those used for filters defined
1059 in the <filters> section. (In other words, each filter in a
1060 route may be included either by reference or by physical
1066 <section id="example.configuration">
1067 <title>An example configuration</title>
1069 The following is a small, but complete, Metaproxy configuration
1070 file (included in the distribution as
1071 <literal>metaproxy/etc/config1.xml</literal>).
1072 This file defines a very simple configuration that simply proxies
1073 to whatever back-end server the client requests, but logs each
1074 request and response. This can be useful for debugging complex
1075 client-server dialogues.
1077 <screen><![CDATA[<?xml version="1.0"?>
1078 <metaproxy xmlns="http://indexdata.com/metaproxy" version="1.0">
1079 <start route="start"/>
1081 <filter id="frontend" type="frontend_net">
1084 <filter id="backend" type="z3950_client">
1089 <filter refid="frontend"/>
1090 <filter type="log"/>
1091 <filter refid="backend"/>
1092 <filter type="bounce"/>
1098 It works by defining a single route, called
1099 <literal>start</literal>, which consists of a sequence of four
1100 filters. The first and last of these are included by reference:
1101 their <literal><filter></literal> elements have
1102 <literal>refid</literal> attributes that refer to filters defined
1103 within the prior <literal><filters></literal> section. The
1104 middle filter is included inline in the route.
1107 The four filters in the route are as follows: first, a
1108 <literal>frontend_net</literal> filter accepts Z39.50 requests
1109 from any host on port 9000; then these requests are passed through
1110 a <literal>log</literal> filter that emits a message for each
1111 request; they are then fed into a <literal>z3950_client</literal>
1112 filter, which forwards all Z39.50 requests to the client-specified
1113 back-end Z39.509 server. Those Z39.50 packages are returned by the
1114 <literal>z3950_client</literal> filter, with the response data
1115 filled by the external Z39.50 server targeted.
1116 All non-Z39.50 packages are passed through to the
1117 <literal>bounce</literal> filter, which definitely bounces
1118 everything, including fish, bananas, cold pyjamas,
1119 mutton, beef and trout packages.
1120 When the response arrives, it is handed
1121 back to the <literal>log</literal> filter, which emits another
1122 message; and then to the <literal>frontend_net</literal> filter,
1123 which returns the response to the client.
1126 <section id="checking.xml.syntax">
1127 <title>Config file syntax checking</title>
1129 The distribution contains RelaxNG Compact and XML syntax checking
1130 files, as well as XML Schema files. These are found in the
1133 xml/schema/metaproxy.rnc
1134 xml/schema/metaproxy.rng
1135 xml/schema/metaproxy.xsd
1137 and can be used to verify or debug the XML structure of
1138 configuration files. For example, using the utility
1139 <filename>xmllint</filename>, syntax checking is done like this:
1141 xmllint --noout --schema xml/schema/metaproxy.xsd etc/config-local.xml
1142 xmllint --noout --relaxng xml/schema/metaproxy.rng etc/config-local.xml
1144 (A recent version of <literal>libxml2</literal> is required, as
1145 support for XML Schemas is a relatively recent addition.)
1148 You can of course use any other RelaxNG or XML Schema compliant tool
1156 <chapter id="multidb">
1157 <title>Virtual databases and multi-database searching</title>
1161 <title>Introductory notes</title>
1163 Two of Metaproxy's filters are concerned with multiple-database
1164 operations. Of these, <literal>virt_db</literal> can work alone
1165 to control the routing of searches to one of a number of servers,
1166 while <literal>multi</literal> can work together with
1167 <literal>virt_db</literal> to perform multi-database searching, merging
1168 the results into a unified result-set - ``metasearch in a box''.
1171 The interaction between
1172 these two filters is necessarily complex: it reflects the real,
1173 irreducible complexity of multi-database searching in a protocol such
1174 as Z39.50 that separates initialization from searching, and in
1175 which the database to be searched is not known at initialization
1179 It's possible to use these filters without understanding the
1180 details of their functioning and the interaction between them; the
1181 next two sections of this chapter are ``HOW-TO'' guides for doing
1182 just that. However, debugging complex configurations will require
1183 a deeper understanding, which the last two sections of this
1184 chapters attempt to provide.
1189 <section id="multidb.virt_db">
1190 <title>Virtual databases with the <literal>virt_db</literal> filter</title>
1192 Working alone, the purpose of the
1193 <literal>virt_db</literal>
1194 filter is to route search requests to one of a selection of
1195 back-end databases. In this way, a single Z39.50 endpoint
1196 (running Metaproxy) can provide access to several different
1197 underlying services, including those that would otherwise be
1198 inaccessible due to firewalls. In many useful configurations, the
1199 back-end databases are local to the Metaproxy installation, but
1200 the software does not enforce this, and any valid Z39.50 servers
1201 may be used as back-ends.
1204 For example, a <literal>virt_db</literal>
1205 filter could be set up so that searches in the virtual database
1206 ``lc'' are forwarded to the Library of Congress bibliographic
1207 catalogue server, and searches in the virtual database ``marc''
1208 are forwarded to the toy database of MARC records that Index Data
1209 hosts for testing purposes. A <literal>virt_db</literal>
1210 configuration to make this switch would look like this:
1212 <screen><![CDATA[<filter type="virt_db">
1214 <database>lc</database>
1215 <target>z3950.loc.gov:7090/voyager</target>
1218 <database>marc</database>
1219 <target>indexdata.com/marc</target>
1221 </filter>]]></screen>
1223 As well as being useful in it own right, this filter also provides
1224 the foundation for multi-database searching.
1229 <section id="multidb.multi">
1230 <title>Multi-database search with the <literal>multi</literal> filter</title>
1232 To arrange for Metaproxy to broadcast searches to multiple back-end
1233 servers, the configuration needs to include two components: a
1234 <literal>virt_db</literal>
1235 filter that specifies multiple
1236 <literal><target></literal>
1237 elements, and a subsequent
1238 <literal>multi</literal>
1239 filter. Here, for example, is a complete configuration that
1240 broadcasts searches to both the Library of Congress catalogue and
1241 Index Data's tiny testing database of MARC records:
1243 <screen><![CDATA[<?xml version="1.0"?>
1244 <metaproxy xmlns="http://indexdata.com/metaproxy" version="1.0">
1245 <start route="start"/>
1248 <filter type="frontend_net">
1249 <threads>10</threads>
1252 <filter type="virt_db">
1254 <database>lc</database>
1255 <target>z3950.loc.gov:7090/voyager</target>
1258 <database>marc</database>
1259 <target>indexdata.com/marc</target>
1262 <database>all</database>
1263 <target>z3950.loc.gov:7090/voyager</target>
1264 <target>indexdata.com/marc</target>
1267 <filter type="multi"/>
1268 <filter type="z3950_client">
1269 <timeout>30</timeout>
1271 <filter type="bounce"/>
1274 </metaproxy>]]></screen>
1277 <literal>virt_db</literal>
1278 filter that specifies multiple
1279 <literal><target></literal>
1280 elements but without a subsequent
1281 <literal>multi</literal>
1282 filter yields surprising and undesirable results, as will be
1283 described below. Don't do that.)
1286 Metaproxy can be invoked with this configuration as follows:
1288 <screen>../src/metaproxy --config config-simple-multi.xml</screen>
1290 And thereafter, Z39.50 clients can connect to the running server
1291 (on port 9000, as specified in the configuration) and search in
1292 any of the databases
1293 <literal>lc</literal> (the Library of Congress catalogue),
1294 <literal>marc</literal> (Index Data's test database of MARC records)
1296 <literal>all</literal> (both of these). As an example, a session
1297 using the YAZ command-line client <literal>yaz-client</literal> is
1298 here included (edited for brevity and clarity):
1300 <screen><![CDATA[$ yaz-client @:9000
1304 Search was a success.
1305 Number of hits: 10000, setno 1
1309 Search was a success.
1310 Number of hits: 10, setno 3
1314 Search was a success.
1315 Number of hits: 10010, setno 4
1318 [marc]Record type: USmarc
1321 005 00000000000000.0
1322 008 910710c19910701nju 00010 eng
1326 100 10 $a Jack Collins
1327 245 10 $a How to program a computer
1333 [VOYAGER]Record type: USmarc
1335 005 20041229102447.0
1336 008 030910s2004 caua 000 0 eng
1337 035 $a (DLC) 2003112666
1338 906 $a 7 $b cbc $c orignew $d 4 $e epcn $f 20 $g y-gencatlg
1339 925 0 $a acquire $b 1 shelf copy $x policy default
1340 955 $a pc10 2003-09-10 $a pv12 2004-06-23 to SSCD; $h sj05 2004-11-30 $e sj05 2004-11-30 to Shelf.
1343 040 $a DLC $c DLC $d DLC
1344 050 00 $a MLCM 2004/03312 (G)
1345 245 10 $a 007, everything or nothing : $b Prima's official strategy guide / $c created by Kaizen Media Group.
1346 246 3 $a Double-O-seven, everything or nothing
1347 246 30 $a Prima's official strategy guide
1348 260 $a Roseville, CA : $b Prima Games, $c c2004.
1349 300 $a 161 p. : $b col. ill. ; $c 28 cm.
1350 500 $a "Platforms: Nintendo GameCube, Macintosh, PC, PlayStation 2 computer entertainment system, Xbox"--P. [4] of cover.
1351 650 0 $a Video games.
1352 710 2 $a Kaizen Media Group.
1353 856 42 $3 Publisher description $u http://www.loc.gov/catdir/description/random052/2003112666.html
1358 As can be seen, the first record in the result set is from the
1359 Index Data test database, and the second from the Library of
1360 Congress database. The result-set continues alternating records
1361 round-robin style until the point where one of the databases'
1362 records are exhausted.
1365 This example uses only two back-end databases; more may be used.
1366 There is no limitation imposed on the number of databases that may
1367 be metasearched in this way: issues of resource usage and
1368 administrative complexity dictate the practical limits.
1371 What happens when one of the databases doesn't respond? By default,
1372 the entire multi-database search fails, and the appropriate
1373 diagnostic is returned to the client. This is usually appropriate
1374 during development, when technicians need maximum information, but
1375 can be inconvenient in deployment, when users typically don't want
1376 to be bothered with problems of this kind and prefer just to get
1377 the records from the databases that are available. To obtain this
1378 latter behavior add an empty
1379 <literal><hideunavailable></literal>
1381 <literal>multi</literal> filter:
1383 <screen><![CDATA[ <filter type="multi">
1385 </filter>]]></screen>
1387 Under this regime, an error is reported to the client only if
1388 <emphasis>all</emphasis> the databases in a multi-database search
1394 <section id="multidb.what">
1395 <title>What's going on?</title>
1397 <title>Lark's vomit</title>
1399 This section goes into a level of technical detail that is
1400 probably not necessary in order to configure and use Metaproxy.
1401 It is provided only for those who like to know how things work.
1402 You should feel free to skip on to the next section if this one
1403 doesn't seem like fun.
1407 Hold on tight - this may get a little hairy.
1410 In the general course of things, a Z39.50 Init request may carry
1411 with it an otherInfo packet of type <literal>VAL_PROXY</literal>,
1412 whose value indicates the address of a Z39.50 server to which the
1413 ultimate connection is to be made. (This otherInfo packet is
1414 supported by YAZ-based Z39.50 clients and servers, but has not yet
1415 been ratified by the Maintenance Agency and so is not widely used
1416 in non-Index Data software. We're working on it.)
1417 The <literal>VAL_PROXY</literal> packet functions
1418 analogously to the absoluteURI-style Request-URI used with the GET
1419 method when a web browser asks a proxy to forward its request: see
1421 <ulink url="http://www.w3.org/Protocols/rfc2616/rfc2616-sec5.html#sec5.1.2"
1422 >Request-URI</ulink>
1424 <ulink url="http://www.w3.org/Protocols/rfc2616/rfc2616.html"
1425 >the HTTP 1.1 specification</ulink>.
1428 Within Metaproxy, Search requests that are part of the same
1429 session as an Init request that carries a
1430 <literal>VAL_PROXY</literal> otherInfo are also annotated with the
1431 same information. The role of the <literal>virt_db</literal>
1432 filter is to rewrite this otherInfo packet dependent on the
1433 virtual database that the client wants to search.
1436 When Metaproxy receives a Z39.50 Init request from a client, it
1437 doesn't immediately forward that request to the back-end server.
1438 Why not? Because it doesn't know <emphasis>which</emphasis>
1439 back-end server to forward it to until the client sends a Search
1440 request that specifies the database that it wants to search in.
1441 Instead, it just treasures the Init request up in its heart; and,
1442 later, the first time the client does a search on one of the
1443 specified virtual databases, a connection is forged to the
1444 appropriate server and the Init request is forwarded to it. If,
1445 later in the session, the same client searches in a different
1446 virtual database, then a connection is forged to the server that
1447 hosts it, and the same cached Init request is forwarded there,
1451 All of this clever Init-delaying is done by the
1452 <literal>frontend_net</literal> filter. The
1453 <literal>virt_db</literal> filter knows nothing about it; in
1454 fact, because the Init request that is received from the client
1455 doesn't get forwarded until a Search request is received, the
1456 <literal>virt_db</literal> filter (and the
1457 <literal>z3950_client</literal> filter behind it) doesn't even get
1458 invoked at Init time. The <emphasis>only</emphasis> thing that a
1459 <literal>virt_db</literal> filter ever does is rewrite the
1460 <literal>VAL_PROXY</literal> otherInfo in the requests that pass
1464 It is possible for a <literal>virt_db</literal> filter to contain
1466 <literal><target></literal>
1467 elements. What does this mean? Only that the filter will add
1468 multiple <literal>VAL_PROXY</literal> otherInfo packets to the
1469 Search requests that pass through it. That's because the virtual
1470 DB filter is dumb, and does exactly what it's told - no more, no
1472 If a Search request with multiple <literal>VAL_PROXY</literal>
1473 otherInfo packets reaches a <literal>z3950_client</literal>
1474 filter, this is an error. That filter doesn't know how to deal
1475 with multiple targets, so it will either just pick one and search
1476 in it, or (better) fail with an error message.
1479 The <literal>multi</literal> filter comes to the rescue! This is
1480 the only filter that knows how to deal with multiple
1481 <literal>VAL_PROXY</literal> otherInfo packets, and it does so by
1482 making multiple copies of the entire Search request: one for each
1483 <literal>VAL_PROXY</literal>. Each of these new copies is then
1484 passed down through the remaining filters in the route. (The
1485 copies are handled in parallel though the
1486 spawning of new threads.) Since the copies each have only one
1487 <literal>VAL_PROXY</literal> otherInfo, they can be handled by the
1488 <literal>z3950_client</literal> filter, which happily deals with
1489 each one individually. When the results of the individual
1490 searches come back up to the <literal>multi</literal> filter, it
1491 merges them into a single Search response, which is what
1492 eventually makes it back to the client.
1497 <section id="multidb.picture">
1498 <title>A picture is worth a thousand words (but only five hundred on 64-bit architectures)</title>
1502 <imagedata fileref="multi.pdf" format="PDF" scale="50"/>
1505 <imagedata fileref="multi.png" format="PNG"/>
1508 <!-- Fall back if none of the images can be used -->
1510 [Here there should be a diagram showing the progress of
1511 packages through the filters during a simple virtual-database
1512 search and a multi-database search, but is seems that your
1513 tool chain has not been able to include the diagram in this
1517 <!-- ### This used to work with an older version of DocBook
1519 <para>Caption: progress of packages through filters.</para>
1522 </inlinemediaobject>
1529 <chapter id="extensions">
1530 <title>Writing extensions for Metaproxy</title>
1531 <para>### To be written</para>
1537 <chapter id="classes">
1538 <title>Classes in the Metaproxy source code</title>
1542 <title>Introductory notes</title>
1544 <emphasis>Stop! Do not read this!</emphasis>
1545 You won't enjoy it at all. You should just skip ahead to
1546 <xref linkend="reference"/>,
1548 <!-- The remainder of this paragraph is lifted verbatim from
1549 Douglas Adams' _Hitch Hiker's Guide to the Galaxy_, chapter 8 -->
1550 you things you really need to know, like the fact that the
1551 fabulously beautiful planet Bethselamin is now so worried about
1552 the cumulative erosion by ten billion visiting tourists a year
1553 that any net imbalance between the amount you eat and the amount
1554 you excrete whilst on the planet is surgically removed from your
1555 bodyweight when you leave: so every time you go to the lavatory it
1556 is vitally important to get a receipt.
1559 This chapter contains documentation of the Metaproxy source code, and is
1560 of interest only to maintainers and developers. If you need to
1561 change Metaproxy's behavior or write a new filter, then you will most
1562 likely find this chapter helpful. Otherwise it's a waste of your
1563 good time. Seriously: go and watch a film or something.
1564 <citetitle>This is Spinal Tap</citetitle> is particularly good.
1567 Still here? OK, let's continue.
1570 In general, classes seem to be named big-endianly, so that
1571 <literal>FactoryFilter</literal> is not a filter that filters
1572 factories, but a factory that produces filters; and
1573 <literal>FactoryStatic</literal> is a factory for the statically
1574 registered filters (as opposed to those that are dynamically
1579 <section id="individual.classes">
1580 <title>Individual classes</title>
1582 The classes making up the Metaproxy application are here listed by
1583 class-name, with the names of the source files that define them in
1588 <title><literal>mp::FactoryFilter</literal>
1589 (<filename>factory_filter.cpp</filename>)</title>
1591 A factory class that exists primarily to provide the
1592 <literal>create()</literal> method, which takes the name of a
1593 filter class as its argument and returns a new filter of that
1594 type. To enable this, the factory must first be populated by
1595 calling <literal>add_creator()</literal> for static filters (this
1596 is done by the <literal>FactoryStatic</literal> class, see below)
1597 and <literal>add_creator_dyn()</literal> for filters loaded
1603 <title><literal>mp::FactoryStatic</literal>
1604 (<filename>factory_static.cpp</filename>)</title>
1606 A subclass of <literal>FactoryFilter</literal> which is
1607 responsible for registering all the statically defined filter
1608 types. It does this by knowing about all those filters'
1609 structures, which are listed in its constructor. Merely
1610 instantiating this class registers all the static classes. It is
1611 for the benefit of this class that <literal>struct
1612 metaproxy_1_filter_struct</literal> exists, and that all the filter
1613 classes provide a static object of that type.
1618 <title><literal>mp::filter::Base</literal>
1619 (<filename>filter.cpp</filename>)</title>
1621 The virtual base class of all filters. The filter API is, on the
1622 surface at least, extremely simple: two methods.
1623 <literal>configure()</literal> is passed an XML DOM tree representing
1624 that part of the configuration file that pertains to this filter
1625 instance, and is expected to walk that tree extracting relevant
1626 information. And <literal>process()</literal> processes a
1627 package (see below). That surface simplicity is a bit
1628 misleading, as <literal>process()</literal> needs to know a lot
1629 about the <literal>Package</literal> class in order to do
1635 <title><literal>mp::filter::AuthSimple</literal>,
1636 <literal>Backend_test</literal>, etc.
1637 (<filename>filter_auth_simple.cpp</filename>,
1638 <filename>filter_backend_test.cpp</filename>, etc.)</title>
1640 Individual filters. Each of these is implemented by a header and
1641 a source file, named <filename>filter_*.hpp</filename> and
1642 <filename>filter_*.cpp</filename> respectively. All the header
1643 files should be pretty much identical, in that they declare the
1644 class, including a private <literal>Rep</literal> class and a
1645 member pointer to it, and the two public methods.
1648 The source file for each filter needs to supply:
1653 A definition of the private <literal>Rep</literal> class.
1658 Some boilerplate constructors and destructors.
1663 A <literal>configure()</literal> method that uses the
1664 appropriate XML fragment.
1669 Most important, the <literal>process()</literal> method that
1670 does all the actual work.
1677 <title><literal>mp::Package</literal>
1678 (<filename>package.cpp</filename>)</title>
1680 Represents a package on its way through the series of filters
1681 that make up a route. This is essentially a Z39.50 or SRU APDU
1682 together with information about where it came from, which is
1683 modified as it passes through the various filters.
1688 <title><literal>mp::Pipe</literal>
1689 (<filename>pipe.cpp</filename>)</title>
1691 This class provides a compatibility layer so that we have an IPC
1692 mechanism that works the same under Unix and Windows. It's not
1693 particularly exciting.
1698 <title><literal>mp::RouterChain</literal>
1699 (<filename>router_chain.cpp</filename>)</title>
1706 <title><literal>mp::RouterFleXML</literal>
1707 (<filename>router_flexml.cpp</filename>)</title>
1714 <title><literal>mp::Session</literal>
1715 (<filename>session.cpp</filename>)</title>
1722 <title><literal>mp::ThreadPoolSocketObserver</literal>
1723 (<filename>thread_pool_observer.cpp</filename>)</title>
1730 <title><literal>mp::util</literal>
1731 (<filename>util.cpp</filename>)</title>
1733 A namespace of various small utility functions and classes,
1734 collected together for convenience. Most importantly, includes
1735 the <literal>mp::util::odr</literal> class, a wrapper for YAZ's
1741 <title><literal>mp::xml</literal>
1742 (<filename>xmlutil.cpp</filename>)</title>
1744 A namespace of various XML utility functions and classes,
1745 collected together for convenience.
1751 <section id="other.source.files">
1752 <title>Other Source Files</title>
1754 In addition to the Metaproxy source files that define the classes
1755 described above, there are a few additional files which are
1756 briefly described here:
1760 <term><literal>metaproxy_prog.cpp</literal></term>
1763 The main function of the <command>metaproxy</command> program.
1768 <term><literal>ex_router_flexml.cpp</literal></term>
1771 Identical to <literal>metaproxy_prog.cpp</literal>: it's not clear why.
1776 <term><literal>test_*.cpp</literal></term>
1779 Unit-tests for various modules.
1785 ### Still to be described:
1786 <literal>ex_filter_frontend_net.cpp</literal>,
1787 <literal>filter_dl.cpp</literal>,
1788 <literal>plainfile.cpp</literal>,
1789 <literal>tstdl.cpp</literal>.
1795 <reference id="reference">
1796 <title>Reference</title>
1799 The material in this chapter is drawn directly from the individual
1800 manual entries. In particular, the Metaproxy invocation section is
1801 available using <command>man metaproxy</command>, and the section
1802 on each individual filter is available using the name of the filter
1803 as the argument to the <command>man</command> command.
1810 <!-- Keep this comment at the end of the file
1815 sgml-minimize-attributes:nil
1816 sgml-always-quote-attributes:t
1819 sgml-parent-document: nil
1820 sgml-local-catalogs: nil
1821 sgml-namecase-general:t