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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 retrieval
52 web service <ulink url="&url.sru;">SRU</ulink>
53 as well as functioning as a limited
54 <ulink url="&url.http;">HTTP</ulink> server.
57 Metaproxy is configured by an XML file which
58 specifies how the software should function in terms of routes that
59 the request packets can take through the proxy, each step on a
60 route being an instantiation of a filter. Filters come in many
61 types, one for each operation: accepting Z39.50 packets, logging,
62 query transformation, multiplexing, etc. Further filter-types can
63 be added as loadable modules to extend Metaproxy functionality,
67 Metaproxy is covered by the GNU General Public License version 2.
72 <imagedata fileref="common/id.png" format="PNG"/>
75 <imagedata fileref="common/id.eps" format="EPS"/>
82 <chapter id="introduction">
83 <title>Introduction</title>
86 <ulink url="&url.metaproxy;">Metaproxy</ulink>
87 is a stand alone program that acts as a universal router, proxy and
88 encapsulated metasearcher for information retrieval protocols such
89 as <ulink url="&url.z39.50;">Z39.50</ulink> and
90 <ulink url="&url.sru;">SRU</ulink>.
91 To clients, it acts as a server of these protocols: it can be searched,
92 records can be retrieved from it, etc.
93 To servers, it acts as a client: it searches in them,
94 retrieves records from them, etc. it satisfies its clients'
95 requests by transforming them, multiplexing them, forwarding them
96 on to zero or more servers, merging the results, transforming
97 them, and delivering them back to the client. In addition, it
98 acts as a simple <ulink url="&url.http;">HTTP</ulink> server; support
99 for further protocols can be added in a modular fashion, through the
100 creation of new filters.
105 Fish, bananas, cold pyjamas,
106 Mutton, beef and trout!
107 - attributed to Cole Porter.
110 Metaproxy is a more capable alternative to
111 <ulink url="&url.yazproxy;">YAZ Proxy</ulink>,
112 being more powerful, flexible, configurable and extensible. Among
113 its many advantages over the older, more pedestrian work are
114 support for multiplexing (encapsulated metasearching), routing by
115 database name, authentication and authorization and serving local
116 files via HTTP. Equally significant, its modular architecture
117 facilitites the creation of pluggable modules implementing further
121 This manual will describe how to install Metaproxy
122 before giving an overview of its architecture, then discussing the
123 key concept of a filter in some depth and giving an overview of
124 the various filter types, then discussing the configuration file
125 format. After this come several optional chapters which may be
126 freely skipped: a detailed discussion of virtual databases and
127 multi-database searching, some notes on writing extensions
128 (additional filter types) and a high-level description of the
129 source code. Finally comes the reference guide, which contains
130 instructions for invoking the <command>metaproxy</command>
131 program, and detailed information on each type of filter,
136 <chapter id="installation">
137 <title>Installation</title>
139 Metaproxy depends on the following tools/libraries:
141 <varlistentry><term><ulink url="&url.yazplusplus;">YAZ++</ulink></term>
144 This is a C++ library based on <ulink url="&url.yaz;">YAZ</ulink>.
148 <varlistentry><term><ulink url="&url.libxslt;">Libxslt</ulink></term>
150 <para>This is an XSLT processor - based on
151 <ulink url="&url.libxml2;">Libxml2</ulink>. Both Libxml2 and
152 Libxslt must be installed with the development components
153 (header files, etc.) as well as the run-time libraries.
157 <varlistentry><term><ulink url="&url.boost;">Boost</ulink></term>
160 The popular C++ library. Initial versions of Metaproxy
161 was built with 1.33.0. Version 1.33.1 works too. Metaproxy
162 also compiles with Boost 1.34.1 but the tests do
163 <emphasis>not</emphasis> compile, because the Test API
164 of Boost has changed.
171 In order to compile Metaproxy a modern C++ compiler is
172 required. Boost, in particular, requires the C++ compiler
173 to facilitate the newest features. Refer to Boost
174 <ulink url="&url.boost.compilers.status;">Compiler Status</ulink>
175 for more information.
178 We have successfully built Metaproxy using the compilers
179 <ulink url="&url.gcc;">GCC</ulink> version 4.0 and
180 <ulink url="&url.vstudio;">Microsoft Visual Studio</ulink> 2003/2005.
183 <section id="installation.unix">
184 <title>Installation on Unix (from Source)</title>
186 Here is a quick step-by-step guide on how to compile all the
187 tools that Metaproxy uses. Only few systems have none of the required
188 tools binary packages. If, for example, Libxml2/libxslt are already
189 installed as development packages use those (and omit compilation).
196 gunzip -c libxml2-version.tar.gz|tar xf -
204 gunzip -c libxslt-version.tar.gz|tar xf -
215 gunzip -c yaz-version.tar.gz|tar xf -
223 gunzip -c yazpp-version.tar.gz|tar xf -
234 gunzip -c boost-version.tar.gz|tar xf -
245 gunzip -c metaproxy-version.tar.gz|tar xf -
254 <section id="installation.debian">
255 <title>Installation on Debian GNU/Linux</title>
257 All dependencies for Metaproxy are available as
258 <ulink url="&url.debian;">Debian</ulink>
259 packages for the sarge (stable in 2005) and etch (testing in 2005)
263 The procedures for Debian based systems, such as
264 <ulink url="&url.ubuntu;">Ubuntu</ulink> is probably similar
267 There is currently no official Debian package for YAZ++.
268 And the official Debian package for YAZ is probably too old.
269 But Index Data bulds "new" versions of those for Debian (i386 only).
272 Update the <filename>/etc/apt/sources.list</filename>
273 to include the Index Data repository.
274 See YAZ' <ulink url="&url.yaz.download.debian;">Download Debian</ulink>
275 for more information.
278 apt-get install libxslt1-dev
279 apt-get install libyazpp2-dev
280 apt-get install libboost-dev
281 apt-get install libboost-thread-dev
282 apt-get install libboost-date-time-dev
283 apt-get install libboost-program-options-dev
284 apt-get install libboost-test-dev
287 With these packages installed, the usual configure + make
288 procedure can be used for Metaproxy as outlined in
289 <xref linkend="installation.unix"/>.
293 <section id="installation.rpm">
294 <title>Installation on RPM based Linux Systems</title>
296 All external dependencies for Metaproxy are available as
297 RPM packages, either from your distribution site, or from the
298 <ulink url="http://fr.rpmfind.net/">RPMfind</ulink> site.
301 For example, an installation of the requires Boost C++ development
302 libraries on RedHat Fedora C4 and C5 can be done like this:
304 wget ftp://fr.rpmfind.net/wlinux/fedora/core/updates/testing/4/SRPMS/boost-1.33.0-3.fc4.src.rpm
305 sudo rpmbuild --buildroot src/ --rebuild -p fc4/boost-1.33.0-3.fc4.src.rpm
306 sudo rpm -U /usr/src/redhat/RPMS/i386/boost-*rpm
310 The <ulink url="&url.yaz;">YAZ</ulink> library is needed to
311 compile &metaproxy;, see there
312 for more information on available RPM packages.
315 There is currently no official RPM package for YAZ++.
316 See the <ulink url="&url.yazplusplus;">YAZ++</ulink> pages
317 for more information on a Unix tarball install.
320 With these packages installed, the usual configure + make
321 procedure can be used for Metaproxy as outlined in
322 <xref linkend="installation.unix"/>.
326 <section id="installation.windows">
327 <title>Installation on Windows</title>
329 Metaproxy can be compiled with Microsoft
330 <ulink url="&url.vstudio;">Visual Studio</ulink>.
331 Version 2003 (C 7.1) and 2005 (C 8.0) is known to work.
333 <section id="installation.windows.boost">
336 For Windows, it's easiest to get the precompiled Boost
337 package from <ulink url="&url.boost.windows.download;">here</ulink>.
338 Several versions of the Boost libraries may be selected when
339 installing Boost for windows. Please choose at least the
340 <emphasis>multithreaded</emphasis> (non-DLL) version because
341 the Metaproxy makefile uses that.
344 For more information about installing Boost refer to the
345 <ulink url="&url.boost.getting.started;">getting started</ulink>
350 <section id="installation.windows.libxslt">
351 <title>Libxslt</title>
353 <ulink url="&url.libxslt;">Libxslt</ulink> can be downloaded
355 <ulink url="&url.libxml2.download.win32;">here</ulink>.
358 Libxslt has other dependencies, but these can all be downloaded
359 from the same site. Get the following package:
360 iconv, zlib, libxml2, libxslt.
364 <section id="installation.windows.yaz">
367 <ulink url="&url.yaz;">YAZ</ulink> can be downloaded
369 <ulink url="&url.yaz.download.win32;">here</ulink>.
373 <section id="installation.windows.yazplusplus">
376 Get <ulink url="&url.yazplusplus;">YAZ++</ulink> as well.
377 Version 1.0.3 or later is required.
380 YAZ++ includes NMAKE makefiles, similar to those found in the
385 <section id="installation.windows.metaproxy">
386 <title>Metaproxy</title>
388 Metaproxy is shipped with NMAKE makefiles as well - similar
389 to those found in the YAZ++/YAZ packages. Adjust this Makefile
390 to point to the proper locations of Boost, Libxslt, Libxml2,
391 zlib, iconv, yaz and yazpp.
395 <varlistentry><term><literal>DEBUG</literal></term>
397 If set to 1, the software is
398 compiled with debugging libraries (code generation is
399 multi-threaded debug DLL).
400 If set to 0, the software is compiled with release libraries
401 (code generation is multi-threaded DLL).
406 <term><literal>BOOST</literal></term>
409 Boost install location
415 <term><literal>BOOST_VERSION</literal></term>
418 Boost version (replace . with _).
424 <term><literal>BOOST_TOOLSET</literal></term>
433 <term><literal>LIBXSLT_DIR</literal>,
434 <literal>LIBXML2_DIR</literal> ..</term>
437 Specify the locations of Libxslt, libiconv, libxml2 and
446 After successful compilation you'll find
447 <literal>metaproxy.exe</literal> in the
448 <literal>bin</literal> directory.
456 <chapter id="yazproxy-comparison">
457 <title>YAZ Proxy Comparison</title>
459 The table below lists facilities either supported by either
460 <ulink url="&url.yazproxy;">YAZ Proxy</ulink> or Metaproxy.
462 <table id="yazproxy-comparison-table">
463 <title>Metaproxy / YAZ Proxy comparison</title>
467 <entry>Facility</entry>
468 <entry>Metaproxy</entry>
469 <entry>YAZ Proxy</entry>
474 <entry>Z39.50 server</entry>
475 <entry>Using filter <literal>frontend_net</literal></entry>
476 <entry>Supported</entry>
479 <entry>SRU server</entry>
480 <entry>Supported with filter <literal>sru_z3950</literal></entry>
481 <entry>Supported</entry>
484 <entry>Z39.50 client</entry>
485 <entry>Supported with filter <literal>z3950_client</literal></entry>
486 <entry>Supported</entry>
489 <entry>SRU client</entry>
490 <entry>Unsupported</entry>
491 <entry>Unsupported</entry>
494 <entry>Connection reuse</entry>
495 <entry>Supported with filter <literal>session_shared</literal></entry>
496 <entry>Supported</entry>
499 <entry>Connection share</entry>
500 <entry>Supported with filter <literal>session_shared</literal></entry>
501 <entry>Unsupported</entry>
504 <entry>Result set reuse</entry>
505 <entry>Supported with filter <literal>session_shared</literal></entry>
506 <entry>Within one Z39.50 session / HTTP keep-alive</entry>
509 <entry>Record cache</entry>
510 <entry>Unsupported</entry>
511 <entry>Supported for last result set within one Z39.50/HTTP-keep alive session</entry>
514 <entry>Z39.50 Virtual database, i.e. select any Z39.50 target for database</entry>
515 <entry>Supported with filter <literal>virt_db</literal></entry>
516 <entry>Unsupported</entry>
519 <entry>SRU Virtual database, i.e. select any Z39.50 target for path</entry>
520 <entry>Supported with filter <literal>virt_db</literal>,
521 <literal>sru_z3950</literal></entry>
522 <entry>Supported</entry>
525 <entry>Multi target search</entry>
526 <entry>Supported with filter <literal>multi</literal> (round-robin)</entry>
527 <entry>Unsupported</entry>
530 <entry>Retrieval and search limits</entry>
531 <entry>Unsupported</entry>
532 <entry>Supported</entry>
535 <entry>Bandwidth limits</entry>
536 <entry>Unsupported</entry>
537 <entry>Supported</entry>
540 <entry>Connect limits</entry>
541 <entry>Unsupported</entry>
542 <entry>Supported</entry>
545 <entry>Retrieval sanity check and conversions</entry>
546 <entry>Supported using filter <literal>record_transform</literal></entry>
547 <entry>Supported</entry>
550 <entry>Query check</entry>
552 Supported in a limited way using <literal>query_rewrite</literal>
554 <entry>Supported</entry>
557 <entry>Query rewrite</entry>
558 <entry>Supported with <literal>query_rewrite</literal></entry>
559 <entry>Unsupported</entry>
562 <entry>Session invalidate for -1 hits</entry>
563 <entry>Unsupported</entry>
564 <entry>Supported</entry>
567 <entry>Architecture</entry>
568 <entry>Multi-threaded + select for networked modules such as
569 <literal>frontend_net</literal>)</entry>
570 <entry>Single-threaded using select</entry>
574 <entry>Extensability</entry>
575 <entry>Most functionality implemented as loadable modules</entry>
576 <entry>Unsupported and experimental</entry>
580 <entry><ulink url="&url.usemarcon;">USEMARCON</ulink></entry>
581 <entry>Unsupported</entry>
582 <entry>Supported</entry>
586 <entry>Portability</entry>
588 Requires YAZ, YAZ++ and modern C++ compiler supporting
589 <ulink url="&url.boost;">Boost</ulink>.
592 Requires YAZ and YAZ++.
593 STL is not required so pretty much any C++ compiler out there should work.
602 <chapter id="architecture">
603 <title>The Metaproxy Architecture</title>
605 The Metaproxy architecture is based on three concepts:
606 the <emphasis>package</emphasis>,
607 the <emphasis>route</emphasis>
608 and the <emphasis>filter</emphasis>.
612 <term>Packages</term>
615 A package is request or response, encoded in some protocol,
616 issued by a client, making its way through Metaproxy, send to or
617 received from a server, or sent back to the client.
620 The core of a package is the protocol unit - for example, a
621 Z39.50 Init Request or Search Response, or an SRU searchRetrieve
622 URL or Explain Response. In addition to this core, a package
623 also carries some extra information added and used by Metaproxy
627 In general, packages are doctored as they pass through
628 Metaproxy. For example, when the proxy performs authentication
629 and authorization on a Z39.50 Init request, it removes the
630 authentication credentials from the package so that they are not
631 passed onto the back-end server; and when search-response
632 packages are obtained from multiple servers, they are merged
633 into a single unified package that makes its way back to the
642 Packages make their way through routes, which can be thought of
643 as programs that operate on the package data-type. Each
644 incoming package initially makes its way through a default
645 route, but may be switched to a different route based on various
646 considerations. Routes are made up of sequences of filters (see
655 Filters provide the individual instructions within a route, and
656 effect the necessary transformations on packages. A particular
657 configuration of Metaproxy is essentially a set of filters,
658 described by configuration details and arranged in order in one
659 or more routes. There are many kinds of filter - about a dozen
660 at the time of writing with more appearing all the time - each
661 performing a specific function and configured by different
665 The word ``filter'' is sometimes used rather loosely, in two
666 different ways: it may be used to mean a particular
667 <emphasis>type</emphasis> of filter, as when we speak of ``the
668 auth_simple filter'' or ``the multi filter''; or it may be used
669 to be a specific <emphasis>instance</emphasis> of a filter
670 within a Metaproxy configuration. For example, a single
671 configuration will often contain multiple instances of the
672 <literal>z3950_client</literal> filter. In
673 operational terms, of these is a separate filter. In practice,
674 context always make it clear which sense of the word ``filter''
678 Extensibility of Metaproxy is primarily through the creation of
679 plugins that provide new filters. The filter API is small and
680 conceptually simple, but there are many details to master. See
682 <link linkend="filters">Filters</link>.
688 Since packages are created and handled by the system itself, and
689 routes are conceptually simple, most of the remainder of this
690 document concentrates on filters. After a brief overview of the
691 filter types follows, along with some thoughts on possible future
698 <chapter id="filters">
699 <title>Filters</title>
702 <section id="filters-introductory-notes">
703 <title>Introductory notes</title>
705 It's useful to think of Metaproxy as an interpreter providing a small
706 number of primitives and operations, but operating on a very
707 complex data type, namely the ``package''.
710 A package represents a Z39.50 or SRU/W request (whether for Init,
711 Search, Scan, etc.) together with information about where it came
712 from. Packages are created by front-end filters such as
713 <literal>frontend_net</literal> (see below), which reads them from
714 the network; other front-end filters are possible. They then pass
715 along a route consisting of a sequence of filters, each of which
716 transforms the package and may also have side-effects such as
717 generating logging. Eventually, the route will yield a response,
718 which is sent back to the origin.
721 There are many kinds of filter: some that are defined statically
722 as part of Metaproxy, and others may be provided by third parties
723 and dynamically loaded. They all conform to the same simple API
724 of essentially two methods: <function>configure()</function> is
725 called at startup time, and is passed an XML DOM tree representing that
726 part of the configuration file that pertains to this filter
727 instance: it is expected to walk that tree extracting relevant
728 information; and <function>process()</function> is called every
729 time the filter has to processes a package.
732 While all filters provide the same API, there are different modes
733 of functionality. Some filters are sources: they create
735 (<literal>frontend_net</literal>);
736 others are sinks: they consume packages and return a result
737 (<literal>backend_test</literal>,
738 <literal>bounce</literal>,
739 <literal>http_file</literal>,
740 <literal>z3950_client</literal>);
741 the others are true filters, that read, process and pass on the
742 packages they are fed
743 (<literal>auth_simple</literal>,
744 <literal>log</literal>,
745 <literal>multi</literal>,
746 <literal>query_rewrite</literal>,
747 <literal>record_transform</literal>,
748 <literal>session_shared</literal>,
749 <literal>sru_z3950</literal>,
750 <literal>template</literal>,
751 <literal>virt_db</literal>).
756 <section id="overview.filter.types">
757 <title>Overview of filter types</title>
759 We now briefly consider each of the types of filter supported by
760 the core Metaproxy binary. This overview is intended to give a
761 flavor of the available functionality; more detailed information
762 about each type of filter is included below in
763 <xref linkend="reference"/>.
766 The filters are here named by the string that is used as the
767 <literal>type</literal> attribute of a
768 <literal><filter></literal> element in the configuration
769 file to request them, with the name of the class that implements
770 them in parentheses. (The classname is not needed for normal
771 configuration and use of Metaproxy; it is useful only to
775 The filters are here listed in alphabetical order:
782 New virt_db-alike that does inteligent peer choice, explain merging,
783 adds FD&N to explain. Keeps init responses (like "virt_db Classic"),
784 makes routing choices based on local explain knowledge. Ref IDDI
787 Filter to convert Explain Classic to ZeeRex.
789 CQL2PQF (which needs augmented ZeeRex) - MARC for Talis.
793 Figure out what additional information we need in:
794 ZeeRex (check against D3.1)
795 Init request (e.g. loop detection)
796 Query package (e.g. number of hops)
797 Query response (e.g. record source)
801 <section id="auth_simple">
802 <title><literal>auth_simple</literal>
803 (mp::filter::AuthSimple)</title>
805 Simple authentication and authorization. The configuration
806 specifies the name of a file that is the user register, which
807 lists <varname>username</varname>:<varname>password</varname>
808 pairs, one per line, colon separated. When a session begins, it
809 is rejected unless username and passsword are supplied, and match
810 a pair in the register. The configuration file may also specific
811 the name of another file that is the target register: this lists
812 lists <varname>username</varname>:<varname>dbname</varname>,<varname>dbname</varname>...
813 sets, one per line, with multiple database names separated by
814 commas. When a search is processed, it is rejected unless the
815 database to be searched is one of those listed as available to
820 <section id="backend_test">
821 <title><literal>backend_test</literal>
822 (mp::filter::Backend_test)</title>
824 A partial sink that provides dummy responses in the manner of the
825 <literal>yaz-ztest</literal> Z39.50 server. This is useful only
826 for testing. Seriously, you don't need this. Pretend you didn't
827 even read this section.
831 <section id="bounce">
832 <title><literal>bounce</literal>
833 (mp::filter::Bounce)</title>
835 A sink that swallows <emphasis>all packages</emphasis>,
836 and returns them almost unprocessed.
837 It never sends any package of any type further down the row, but
838 sets Z39.50 packages to Z_Close, and HTTP_Request packages to
839 HTTP_Response err code 400 packages, and adds a suitable bounce
841 The bounce filter is usually added at end of each filter chain route
842 to prevent infinite hanging of for example HTTP
843 requests packages when only the Z39.50 client partial sink
844 filter is found in the
849 <section id="cql_rpn">
850 <title><literal>cql_rpn</literal>
851 (mp::filter::CQLtoRPN)</title>
853 A query language transforming filter which catches Z39.50
854 <literal>searchRequest</literal>
855 packages containing <literal>CQL</literal> queries, transforms
856 those to <literal>RPN</literal> queries,
857 and sends the <literal>searchRequests</literal> on to the next
858 filters. It is among other things useful in a SRU context.
862 <section id="frontend_net">
863 <title><literal>frontend_net</literal>
864 (mp::filter::FrontendNet)</title>
866 A source that accepts Z39.50 connections from a port
867 specified in the configuration, reads protocol units, and
868 feeds them into the next filter in the route. When the result is
869 received, it is returned to the original origin.
873 <section id="http_file">
874 <title><literal>http_file</literal>
875 (mp::filter::HttpFile)</title>
877 A partial sink which swallows only
878 <literal>HTTP_Request</literal> packages, and
879 returns the contents of files from the local
880 filesystem in response to HTTP requests.
881 It lets Z39.50 packages and all other forthcoming package types
884 does mean that Metaproxy is also a Web-server in its spare time. So
885 far it does not contain either an email-reader or a Lisp
886 interpreter, but that day is surely coming.)
890 <section id="load_balance">
891 <title><literal>load_balance</literal>
892 (mp::filter::LoadBalance)</title>
894 Performs load balancing for incoming Z39.50 init requests.
895 It is used together with the <literal>virt_db</literal> filter,
896 but unlike the <literal>multi</literal> filter it does send an
897 entire session to only one of the virtual backends. The
898 <literal>load_balance</literal> filter is assuming that
899 all backend targets have equal content, and chooses the backend
900 with least load cost for a new session.
903 This filter is experimental and yet not mature for heavy load
911 <title><literal>log</literal>
912 (mp::filter::Log)</title>
914 Writes logging information to standard output, and passes on
915 the package unchanged. A log file name can be specified, as well
916 as multiple different logging formats.
921 <title><literal>multi</literal>
922 (mp::filter::Multi)</title>
924 Performs multi-database searching.
926 <link linkend="multidb">the extended discussion</link>
927 of virtual databases and multi-database searching below.
931 <section id="query_rewrite">
932 <title><literal>query_rewrite</literal>
933 (mp::filter::QueryRewrite)</title>
935 Rewrites Z39.50 <literal>Type-1</literal>
936 and <literal>Type-101</literal> (``<literal>RPN</literal>'')
938 three-step process: the query is transliterated from Z39.50
939 packet structures into an XML representation; that XML
940 representation is transformed by an XSLT stylesheet; and the
941 resulting XML is transliterated back into the Z39.50 packet
947 <section id="record_transform">
948 <title><literal>record_transform</literal>
949 (mp::filter::RecordTransform)</title>
951 This filter acts only on Z3950 present requests, and let all
952 other types of packages and requests pass untouched. It's use is
953 twofold: blocking Z3950 present requests, which the backend
954 server does not understand and can not honor, and transforming
955 the present syntax and elementset name according to the rules
956 specified, to fetch only existing record formats, and transform
957 them on the fly to requested record syntaxes.
961 <section id="session_shared">
962 <title><literal>session_shared</literal>
963 (mp::filter::SessionShared)</title>
965 This filter implements global sharing of
966 result sets (i.e. between threads and therefore between
967 clients), yielding performance improvements by clever resource
972 <section id="sru_z3950">
973 <title><literal>sru_z3950</literal>
974 (mp::filter::SRUtoZ3950)</title>
976 This filter transforms valid
977 SRU GET/POST/SOAP searchRetrieve requests to Z3950 init, search,
978 and present requests, and wraps the
979 received hit counts and XML records into suitable SRU response
981 The <literal>sru_z3950</literal> filter processes also SRU
982 GET/POST/SOAP explain requests, returning
983 either the absolute minimum required by the standard, or a full
984 pre-defined ZeeReX explain record.
986 <ulink url="&url.zeerex.explain;">ZeeReX Explain</ulink>
987 standard pages and the
988 <ulink url="&url.sru.explain;">SRU Explain</ulink> pages
989 for more information on the correct explain syntax.
990 SRU scan requests are not supported yet.
994 <section id="template">
995 <title><literal>template</literal>
996 (mp::filter::Template)</title>
998 Does nothing at all, merely passing the packet on. (Maybe it
999 should be called <literal>nop</literal> or
1000 <literal>passthrough</literal>?) This exists not to be used, but
1001 to be copied - to become the skeleton of new filters as they are
1002 written. As with <literal>backend_test</literal>, this is not
1003 intended for civilians.
1007 <section id="virt_db">
1008 <title><literal>virt_db</literal>
1009 (mp::filter::VirtualDB)</title>
1011 Performs virtual database selection: based on the name of the
1012 database in the search request, a server is selected, and its
1013 address added to the request in a <literal>VAL_PROXY</literal>
1014 otherInfo packet. It will subsequently be used by a
1015 <literal>z3950_client</literal> filter.
1017 <link linkend="multidb">the extended discussion</link>
1018 of virtual databases and multi-database searching below.
1022 <section id="z3950_client">
1023 <title><literal>z3950_client</literal>
1024 (mp::filter::Z3950Client)</title>
1026 A partial sink which swallows only Z39.50 packages.
1027 It performs Z39.50 searching and retrieval by proxying the
1028 packages that are passed to it. Init requests are sent to the
1029 address specified in the <literal>VAL_PROXY</literal> otherInfo
1030 attached to the request: this may have been specified by client,
1031 or generated by a <literal>virt_db</literal> filter earlier in
1032 the route. Subsequent requests are sent to the same address,
1033 which is remembered at Init time in a Session object.
1034 HTTP_Request packages and all other forthcoming package types
1035 are passed untouched.
1040 <section id="zeerex_explain">
1041 <title><literal>zeerex_explain</literal>
1042 (mp::filter::ZeerexExplain)</title>
1044 This filter acts as a sink for
1045 Z39.50 explain requests, returning a static ZeeReX
1046 Explain XML record from the config section. All other packages
1049 <ulink url="&url.zeerex.explain;">ZeeReX Explain</ulink>
1051 for more information on the correct explain syntax.
1055 This filter is not yet completed.
1064 <section id="future.directions">
1065 <title>Future directions</title>
1067 Some other filters that do not yet exist, but which would be
1068 useful, are briefly described. These may be added in future
1069 releases (or may be created by third parties, as loadable
1075 <term><literal>frontend_cli</literal> (source)</term>
1078 Command-line interface for generating requests.
1083 <term><literal>sru_client</literal> (sink)</term>
1086 SRU/GET and SRU/SOAP searching and retrieval.
1091 <term><literal>opensearch_client</literal> (sink)</term>
1094 A9 OpenSearch searching and retrieval.
1104 <chapter id="configuration">
1105 <title>Configuration: the Metaproxy configuration file format</title>
1108 <section id="configuration-introductory-notes">
1109 <title>Introductory notes</title>
1111 If Metaproxy is an interpreter providing operations on packages, then
1112 its configuration file can be thought of as a program for that
1113 interpreter. Configuration is by means of a single XML file, the name
1114 of which is supplied as the sole command-line argument to the
1115 <command>metaproxy</command> program. (See
1116 <xref linkend="reference"/> below for more information on invoking
1121 <section id="overview.xml.structure">
1122 <title>Overview of the config file XML structure</title>
1124 All elements and attributes are in the namespace
1125 <ulink url="http://indexdata.com/metaproxy"/>.
1126 This is most easily achieved by setting the default namespace on
1127 the top-level element, as here:
1130 <metaproxy xmlns="http://indexdata.com/metaproxy" version="1.0">
1133 The top-level element is <metaproxy>. This contains a
1134 <start> element, a <filters> element and a
1135 <routes> element, in that order. <filters> is
1136 optional; the other two are mandatory. All three are
1140 The <start> element is empty, but carries a
1141 <literal>route</literal> attribute, whose value is the name of
1142 route at which to start running - analogous to the name of the
1143 start production in a formal grammar.
1146 If present, <filters> contains zero or more <filter>
1147 elements. Each filter carries a <literal>type</literal> attribute
1148 which specifies what kind of filter is being defined
1149 (<literal>frontend_net</literal>, <literal>log</literal>, etc.)
1150 and contain various elements that provide suitable configuration
1151 for a filter of its type. The filter-specific elements are
1153 <xref linkend="reference"/>.
1154 Filters defined in this part of the file must carry an
1155 <literal>id</literal> attribute so that they can be referenced
1159 <routes> contains one or more <route> elements, each
1160 of which must carry an <literal>id</literal> element. One of the
1161 routes must have the ID value that was specified as the start
1162 route in the <start> element's <literal>route</literal>
1163 attribute. Each route contains zero or more <filter>
1164 elements. These are of two types. They may be empty, but carry a
1165 <literal>refid</literal> attribute whose value is the same as the
1166 <literal>id</literal> of a filter previously defined in the
1167 <filters> section. Alternatively, a route within a filter
1168 may omit the <literal>refid</literal> attribute, but contain
1169 configuration elements similar to those used for filters defined
1170 in the <filters> section. (In other words, each filter in a
1171 route may be included either by reference or by physical
1177 <section id="example.configuration">
1178 <title>An example configuration</title>
1180 The following is a small, but complete, Metaproxy configuration
1181 file (included in the distribution as
1182 <literal>metaproxy/etc/config1.xml</literal>).
1183 This file defines a very simple configuration that simply proxies
1184 to whatever back-end server the client requests, but logs each
1185 request and response. This can be useful for debugging complex
1186 client-server dialogues.
1188 <screen><![CDATA[<?xml version="1.0"?>
1189 <metaproxy xmlns="http://indexdata.com/metaproxy" version="1.0">
1190 <start route="start"/>
1192 <filter id="frontend" type="frontend_net">
1195 <filter id="backend" type="z3950_client">
1200 <filter refid="frontend"/>
1201 <filter type="log"/>
1202 <filter refid="backend"/>
1203 <filter type="bounce"/>
1209 It works by defining a single route, called
1210 <literal>start</literal>, which consists of a sequence of four
1211 filters. The first and last of these are included by reference:
1212 their <literal><filter></literal> elements have
1213 <literal>refid</literal> attributes that refer to filters defined
1214 within the prior <literal><filters></literal> section. The
1215 middle filter is included inline in the route.
1218 The four filters in the route are as follows: first, a
1219 <literal>frontend_net</literal> filter accepts Z39.50 requests
1220 from any host on port 9000; then these requests are passed through
1221 a <literal>log</literal> filter that emits a message for each
1222 request; they are then fed into a <literal>z3950_client</literal>
1223 filter, which forwards all Z39.50 requests to the client-specified
1224 back-end Z39.509 server. Those Z39.50 packages are returned by the
1225 <literal>z3950_client</literal> filter, with the response data
1226 filled by the external Z39.50 server targeted.
1227 All non-Z39.50 packages are passed through to the
1228 <literal>bounce</literal> filter, which definitely bounces
1229 everything, including fish, bananas, cold pyjamas,
1230 mutton, beef and trout packages.
1231 When the response arrives, it is handed
1232 back to the <literal>log</literal> filter, which emits another
1233 message; and then to the <literal>frontend_net</literal> filter,
1234 which returns the response to the client.
1238 <section id="config-file-modularity">
1239 <title>Config file modularity</title>
1241 Metaproxy XML configuration snippets can be reused by other
1242 filters using the <literal>XInclude</literal> standard, as seen in
1243 the <literal>/etc/config-sru-to-z3950.xml</literal> example SRU
1246 <filter id="sru" type="sru_z3950">
1247 <database name="Default">
1248 <xi:include xmlns:xi="http://www.w3.org/2001/XInclude"
1249 href="explain.xml"/>
1256 <section id="config-file-syntax-check">
1257 <title>Config file syntax checking</title>
1259 The distribution contains RelaxNG Compact and XML syntax checking
1260 files, as well as XML Schema files. These are found in the
1263 xml/schema/metaproxy.rnc
1264 xml/schema/metaproxy.rng
1265 xml/schema/metaproxy.xsd
1267 and can be used to verify or debug the XML structure of
1268 configuration files. For example, using the utility
1269 <filename>xmllint</filename>, syntax checking is done like this:
1271 xmllint --noout --schema xml/schema/metaproxy.xsd etc/config-local.xml
1272 xmllint --noout --relaxng xml/schema/metaproxy.rng etc/config-local.xml
1274 (A recent version of <literal>libxml2</literal> is required, as
1275 support for XML Schemas is a relatively recent addition.)
1278 You can of course use any other RelaxNG or XML Schema compliant tool
1286 <chapter id="multidb">
1287 <title>Virtual databases and multi-database searching</title>
1290 <section id="multidb-introductory-notes">
1291 <title>Introductory notes</title>
1293 Two of Metaproxy's filters are concerned with multiple-database
1294 operations. Of these, <literal>virt_db</literal> can work alone
1295 to control the routing of searches to one of a number of servers,
1296 while <literal>multi</literal> can work together with
1297 <literal>virt_db</literal> to perform multi-database searching, merging
1298 the results into a unified result-set - ``metasearch in a box''.
1301 The interaction between
1302 these two filters is necessarily complex: it reflects the real,
1303 irreducible complexity of multi-database searching in a protocol such
1304 as Z39.50 that separates initialization from searching, and in
1305 which the database to be searched is not known at initialization
1309 It's possible to use these filters without understanding the
1310 details of their functioning and the interaction between them; the
1311 next two sections of this chapter are ``HOW-TO'' guides for doing
1312 just that. However, debugging complex configurations will require
1313 a deeper understanding, which the last two sections of this
1314 chapters attempt to provide.
1319 <section id="multidb.virt_db">
1320 <title>Virtual databases with the <literal>virt_db</literal> filter</title>
1322 Working alone, the purpose of the
1323 <literal>virt_db</literal>
1324 filter is to route search requests to one of a selection of
1325 back-end databases. In this way, a single Z39.50 endpoint
1326 (running Metaproxy) can provide access to several different
1327 underlying services, including those that would otherwise be
1328 inaccessible due to firewalls. In many useful configurations, the
1329 back-end databases are local to the Metaproxy installation, but
1330 the software does not enforce this, and any valid Z39.50 servers
1331 may be used as back-ends.
1334 For example, a <literal>virt_db</literal>
1335 filter could be set up so that searches in the virtual database
1336 ``lc'' are forwarded to the Library of Congress bibliographic
1337 catalogue server, and searches in the virtual database ``marc''
1338 are forwarded to the toy database of MARC records that Index Data
1339 hosts for testing purposes. A <literal>virt_db</literal>
1340 configuration to make this switch would look like this:
1342 <screen><![CDATA[<filter type="virt_db">
1344 <database>lc</database>
1345 <target>z3950.loc.gov:7090/voyager</target>
1348 <database>marc</database>
1349 <target>indexdata.com/marc</target>
1351 </filter>]]></screen>
1353 As well as being useful in it own right, this filter also provides
1354 the foundation for multi-database searching.
1359 <section id="multidb.multi">
1360 <title>Multi-database search with the <literal>multi</literal> filter</title>
1362 To arrange for Metaproxy to broadcast searches to multiple back-end
1363 servers, the configuration needs to include two components: a
1364 <literal>virt_db</literal>
1365 filter that specifies multiple
1366 <literal><target></literal>
1367 elements, and a subsequent
1368 <literal>multi</literal>
1369 filter. Here, for example, is a complete configuration that
1370 broadcasts searches to both the Library of Congress catalogue and
1371 Index Data's tiny testing database of MARC records:
1373 <screen><![CDATA[<?xml version="1.0"?>
1374 <metaproxy xmlns="http://indexdata.com/metaproxy" version="1.0">
1375 <start route="start"/>
1378 <filter type="frontend_net">
1379 <threads>10</threads>
1382 <filter type="virt_db">
1384 <database>lc</database>
1385 <target>z3950.loc.gov:7090/voyager</target>
1388 <database>marc</database>
1389 <target>indexdata.com/marc</target>
1392 <database>all</database>
1393 <target>z3950.loc.gov:7090/voyager</target>
1394 <target>indexdata.com/marc</target>
1397 <filter type="multi"/>
1398 <filter type="z3950_client">
1399 <timeout>30</timeout>
1401 <filter type="bounce"/>
1404 </metaproxy>]]></screen>
1407 <literal>virt_db</literal>
1408 filter that specifies multiple
1409 <literal><target></literal>
1410 elements but without a subsequent
1411 <literal>multi</literal>
1412 filter yields surprising and undesirable results, as will be
1413 described below. Don't do that.)
1416 Metaproxy can be invoked with this configuration as follows:
1418 <screen>../src/metaproxy --config config-simple-multi.xml</screen>
1420 And thereafter, Z39.50 clients can connect to the running server
1421 (on port 9000, as specified in the configuration) and search in
1422 any of the databases
1423 <literal>lc</literal> (the Library of Congress catalogue),
1424 <literal>marc</literal> (Index Data's test database of MARC records)
1426 <literal>all</literal> (both of these). As an example, a session
1427 using the YAZ command-line client <literal>yaz-client</literal> is
1428 here included (edited for brevity and clarity):
1430 <screen><![CDATA[$ yaz-client @:9000
1434 Search was a success.
1435 Number of hits: 10000, setno 1
1439 Search was a success.
1440 Number of hits: 10, setno 3
1444 Search was a success.
1445 Number of hits: 10010, setno 4
1448 [marc]Record type: USmarc
1451 005 00000000000000.0
1452 008 910710c19910701nju 00010 eng
1456 100 10 $a Jack Collins
1457 245 10 $a How to program a computer
1463 [VOYAGER]Record type: USmarc
1465 005 20041229102447.0
1466 008 030910s2004 caua 000 0 eng
1467 035 $a (DLC) 2003112666
1468 906 $a 7 $b cbc $c orignew $d 4 $e epcn $f 20 $g y-gencatlg
1469 925 0 $a acquire $b 1 shelf copy $x policy default
1470 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.
1473 040 $a DLC $c DLC $d DLC
1474 050 00 $a MLCM 2004/03312 (G)
1475 245 10 $a 007, everything or nothing : $b Prima's official strategy guide / $c created by Kaizen Media Group.
1476 246 3 $a Double-O-seven, everything or nothing
1477 246 30 $a Prima's official strategy guide
1478 260 $a Roseville, CA : $b Prima Games, $c c2004.
1479 300 $a 161 p. : $b col. ill. ; $c 28 cm.
1480 500 $a "Platforms: Nintendo GameCube, Macintosh, PC, PlayStation 2 computer entertainment system, Xbox"--P. [4] of cover.
1481 650 0 $a Video games.
1482 710 2 $a Kaizen Media Group.
1483 856 42 $3 Publisher description $u http://www.loc.gov/catdir/description/random052/2003112666.html
1488 As can be seen, the first record in the result set is from the
1489 Index Data test database, and the second from the Library of
1490 Congress database. The result-set continues alternating records
1491 round-robin style until the point where one of the databases'
1492 records are exhausted.
1495 This example uses only two back-end databases; more may be used.
1496 There is no limitation imposed on the number of databases that may
1497 be metasearched in this way: issues of resource usage and
1498 administrative complexity dictate the practical limits.
1501 What happens when one of the databases doesn't respond? By default,
1502 the entire multi-database search fails, and the appropriate
1503 diagnostic is returned to the client. This is usually appropriate
1504 during development, when technicians need maximum information, but
1505 can be inconvenient in deployment, when users typically don't want
1506 to be bothered with problems of this kind and prefer just to get
1507 the records from the databases that are available. To obtain this
1508 latter behavior add an empty
1509 <literal><hideunavailable></literal>
1511 <literal>multi</literal> filter:
1513 <screen><![CDATA[ <filter type="multi">
1515 </filter>]]></screen>
1517 Under this regime, an error is reported to the client only if
1518 <emphasis>all</emphasis> the databases in a multi-database search
1524 <section id="multidb.what">
1525 <title>What's going on?</title>
1527 <title>Lark's vomit</title>
1529 This section goes into a level of technical detail that is
1530 probably not necessary in order to configure and use Metaproxy.
1531 It is provided only for those who like to know how things work.
1532 You should feel free to skip on to the next section if this one
1533 doesn't seem like fun.
1537 Hold on tight - this may get a little hairy.
1540 In the general course of things, a Z39.50 Init request may carry
1541 with it an otherInfo packet of type <literal>VAL_PROXY</literal>,
1542 whose value indicates the address of a Z39.50 server to which the
1543 ultimate connection is to be made. (This otherInfo packet is
1544 supported by YAZ-based Z39.50 clients and servers, but has not yet
1545 been ratified by the Maintenance Agency and so is not widely used
1546 in non-Index Data software. We're working on it.)
1547 The <literal>VAL_PROXY</literal> packet functions
1548 analogously to the absoluteURI-style Request-URI used with the GET
1549 method when a web browser asks a proxy to forward its request: see
1551 <ulink url="http://www.w3.org/Protocols/rfc2616/rfc2616-sec5.html#sec5.1.2"
1552 >Request-URI</ulink>
1554 <ulink url="http://www.w3.org/Protocols/rfc2616/rfc2616.html"
1555 >the HTTP 1.1 specification</ulink>.
1558 Within Metaproxy, Search requests that are part of the same
1559 session as an Init request that carries a
1560 <literal>VAL_PROXY</literal> otherInfo are also annotated with the
1561 same information. The role of the <literal>virt_db</literal>
1562 filter is to rewrite this otherInfo packet dependent on the
1563 virtual database that the client wants to search.
1566 When Metaproxy receives a Z39.50 Init request from a client, it
1567 doesn't immediately forward that request to the back-end server.
1568 Why not? Because it doesn't know <emphasis>which</emphasis>
1569 back-end server to forward it to until the client sends a Search
1570 request that specifies the database that it wants to search in.
1571 Instead, it just treasures the Init request up in its heart; and,
1572 later, the first time the client does a search on one of the
1573 specified virtual databases, a connection is forged to the
1574 appropriate server and the Init request is forwarded to it. If,
1575 later in the session, the same client searches in a different
1576 virtual database, then a connection is forged to the server that
1577 hosts it, and the same cached Init request is forwarded there,
1581 All of this clever Init-delaying is done by the
1582 <literal>frontend_net</literal> filter. The
1583 <literal>virt_db</literal> filter knows nothing about it; in
1584 fact, because the Init request that is received from the client
1585 doesn't get forwarded until a Search request is received, the
1586 <literal>virt_db</literal> filter (and the
1587 <literal>z3950_client</literal> filter behind it) doesn't even get
1588 invoked at Init time. The <emphasis>only</emphasis> thing that a
1589 <literal>virt_db</literal> filter ever does is rewrite the
1590 <literal>VAL_PROXY</literal> otherInfo in the requests that pass
1594 It is possible for a <literal>virt_db</literal> filter to contain
1596 <literal><target></literal>
1597 elements. What does this mean? Only that the filter will add
1598 multiple <literal>VAL_PROXY</literal> otherInfo packets to the
1599 Search requests that pass through it. That's because the virtual
1600 DB filter is dumb, and does exactly what it's told - no more, no
1602 If a Search request with multiple <literal>VAL_PROXY</literal>
1603 otherInfo packets reaches a <literal>z3950_client</literal>
1604 filter, this is an error. That filter doesn't know how to deal
1605 with multiple targets, so it will either just pick one and search
1606 in it, or (better) fail with an error message.
1609 The <literal>multi</literal> filter comes to the rescue! This is
1610 the only filter that knows how to deal with multiple
1611 <literal>VAL_PROXY</literal> otherInfo packets, and it does so by
1612 making multiple copies of the entire Search request: one for each
1613 <literal>VAL_PROXY</literal>. Each of these new copies is then
1614 passed down through the remaining filters in the route. (The
1615 copies are handled in parallel though the
1616 spawning of new threads.) Since the copies each have only one
1617 <literal>VAL_PROXY</literal> otherInfo, they can be handled by the
1618 <literal>z3950_client</literal> filter, which happily deals with
1619 each one individually. When the results of the individual
1620 searches come back up to the <literal>multi</literal> filter, it
1621 merges them into a single Search response, which is what
1622 eventually makes it back to the client.
1627 <section id="multidb.picture">
1628 <title>A picture is worth a thousand words (but only five hundred on 64-bit architectures)</title>
1632 <imagedata fileref="multi.pdf" format="PDF" scale="50"/>
1635 <imagedata fileref="multi.png" format="PNG"/>
1638 <!-- Fall back if none of the images can be used -->
1640 [Here there should be a diagram showing the progress of
1641 packages through the filters during a simple virtual-database
1642 search and a multi-database search, but is seems that your
1643 tool chain has not been able to include the diagram in this
1647 <!-- ### This used to work with an older version of DocBook
1649 <para>Caption: progress of packages through filters.</para>
1652 </inlinemediaobject>
1658 <chapter id="sru-server">
1659 <title>Combined SRU webservice and Z39.50 server configuration</title>
1661 Metaproxy can act as
1662 <ulink url="&url.sru;">SRU</ulink> and
1663 web service server, which translates web service requests to
1664 <ulink url="&url.z39.50;">ANSI/NISO Z39.50</ulink> packages and
1665 sends them off to common available targets.
1668 A typical setup for this operation needs a filter route including the
1672 <table id="sru-server-table-config" frame="top">
1673 <title>SRU/Z39.50 Server Filter Route Configuration</title>
1677 <entry>Filter</entry>
1678 <entry>Importance</entry>
1679 <entry>Purpose</entry>
1685 <entry><literal>frontend_net</literal></entry>
1686 <entry>required</entry>
1687 <entry>Accepting HTTP connections and passing them to following
1688 filters. Since this filter also accepts Z39.50 connections, the
1689 server works as SRU and Z39.50 server on the same port.</entry>
1692 <entry><literal>sru_z3950</literal></entry>
1693 <entry>required</entry>
1694 <entry>Accepting SRU GET/POST/SOAP explain and
1695 searchRetrieve requests for the the configured databases.
1696 Explain requests are directly served from the static XML configuration.
1697 SearchRetrieve requests are
1698 transformed to Z39.50 search and present packages.
1699 All other HTTP and Z39.50 packages are passed unaltered.</entry>
1702 <entry><literal>http_file</literal></entry>
1703 <entry>optional</entry>
1704 <entry>Serving HTTP requests from the filesystem. This is only
1705 needed if the server should serve XSLT stylesheets, static HTML
1706 files or Java Script for thin browser based clients.
1707 Z39.50 packages are passed unaltered.</entry>
1710 <entry><literal>cql_rpn</literal></entry>
1711 <entry>required</entry>
1712 <entry>Usually, Z39.50 servers do not talk CQL, hence the
1713 translation of the CQL query language to RPN is mandatory in
1714 most cases. Affects only Z39.50 search packages.</entry>
1717 <entry><literal>record_transform</literal></entry>
1718 <entry>optional</entry>
1719 <entry>Some Z39.50 backend targets can not present XML record
1720 syntaxes in common wanted element sets. using this filter, one
1721 can transform binary MARC records to MARCXML records, and
1722 further transform those to any needed XML schema/format by XSLT
1723 transformations. Changes only Z39.50 present packages.</entry>
1726 <entry><literal>session_shared</literal></entry>
1727 <entry>optional</entry>
1728 <entry>The stateless nature of web services requires frequent
1729 re-searching of the same targets for display of paged result set
1730 records. This might be an unacceptable burden for the accessed
1731 backend Z39.50 targets, and this mosule can be added for
1732 efficient backend target resource pooling.</entry>
1735 <entry><literal>z3950_client</literal></entry>
1736 <entry>required</entry>
1737 <entry>Finally, a Z39.50 package sink is needed in the filter
1738 chain to provide the response packages. The Z39.50 client module
1739 is used to access external targets over the network, but any
1740 coming local Z39.50 package sink could be used instead of.</entry>
1743 <entry><literal>bounce</literal></entry>
1744 <entry>required</entry>
1745 <entry>Any Metaproxy package arriving here did not do so by
1746 purpose, and is bounced back with connection closure. this
1747 prevents inifinite package hanging inside the SRU server.</entry>
1753 A typical minimal example <ulink url="&url.sru;">SRU</ulink>
1754 server configuration file is found in the tarball distribution at
1755 <literal>etc/config-sru-to-z3950.xml</literal>.
1758 Off course, any other metaproxy modules can be integrated into a
1759 SRU server solution, including, but not limited to, load balancing,
1760 multiple target querying
1761 (see <xref linkend="multidb"/>), and complex RPN query rewrites.
1768 <chapter id="extensions">
1769 <title>Writing extensions for Metaproxy</title>
1770 <para>### To be written</para>
1776 <chapter id="classes">
1777 <title>Classes in the Metaproxy source code</title>
1780 <section id="classes-introductory-notes">
1781 <title>Introductory notes</title>
1783 <emphasis>Stop! Do not read this!</emphasis>
1784 You won't enjoy it at all. You should just skip ahead to
1785 <xref linkend="reference"/>,
1787 <!-- The remainder of this paragraph is lifted verbatim from
1788 Douglas Adams' _Hitch Hiker's Guide to the Galaxy_, chapter 8 -->
1789 you things you really need to know, like the fact that the
1790 fabulously beautiful planet Bethselamin is now so worried about
1791 the cumulative erosion by ten billion visiting tourists a year
1792 that any net imbalance between the amount you eat and the amount
1793 you excrete whilst on the planet is surgically removed from your
1794 bodyweight when you leave: so every time you go to the lavatory it
1795 is vitally important to get a receipt.
1798 This chapter contains documentation of the Metaproxy source code, and is
1799 of interest only to maintainers and developers. If you need to
1800 change Metaproxy's behavior or write a new filter, then you will most
1801 likely find this chapter helpful. Otherwise it's a waste of your
1802 good time. Seriously: go and watch a film or something.
1803 <citetitle>This is Spinal Tap</citetitle> is particularly good.
1806 Still here? OK, let's continue.
1809 In general, classes seem to be named big-endianly, so that
1810 <literal>FactoryFilter</literal> is not a filter that filters
1811 factories, but a factory that produces filters; and
1812 <literal>FactoryStatic</literal> is a factory for the statically
1813 registered filters (as opposed to those that are dynamically
1818 <section id="individual.classes">
1819 <title>Individual classes</title>
1821 The classes making up the Metaproxy application are here listed by
1822 class-name, with the names of the source files that define them in
1826 <section id="class-FactoryFilter">
1827 <title><literal>mp::FactoryFilter</literal>
1828 (<filename>factory_filter.cpp</filename>)</title>
1830 A factory class that exists primarily to provide the
1831 <literal>create()</literal> method, which takes the name of a
1832 filter class as its argument and returns a new filter of that
1833 type. To enable this, the factory must first be populated by
1834 calling <literal>add_creator()</literal> for static filters (this
1835 is done by the <literal>FactoryStatic</literal> class, see below)
1836 and <literal>add_creator_dyn()</literal> for filters loaded
1841 <section id="class-FactoryStatic">
1842 <title><literal>mp::FactoryStatic</literal>
1843 (<filename>factory_static.cpp</filename>)</title>
1845 A subclass of <literal>FactoryFilter</literal> which is
1846 responsible for registering all the statically defined filter
1847 types. It does this by knowing about all those filters'
1848 structures, which are listed in its constructor. Merely
1849 instantiating this class registers all the static classes. It is
1850 for the benefit of this class that <literal>struct
1851 metaproxy_1_filter_struct</literal> exists, and that all the filter
1852 classes provide a static object of that type.
1856 <section id="class-filter-Base">
1857 <title><literal>mp::filter::Base</literal>
1858 (<filename>filter.cpp</filename>)</title>
1860 The virtual base class of all filters. The filter API is, on the
1861 surface at least, extremely simple: two methods.
1862 <literal>configure()</literal> is passed an XML DOM tree representing
1863 that part of the configuration file that pertains to this filter
1864 instance, and is expected to walk that tree extracting relevant
1865 information. And <literal>process()</literal> processes a
1866 package (see below). That surface simplicity is a bit
1867 misleading, as <literal>process()</literal> needs to know a lot
1868 about the <literal>Package</literal> class in order to do
1873 <section id="class-AuthSimple">
1874 <title><literal>mp::filter::AuthSimple</literal>,
1875 <literal>Backend_test</literal>, etc.
1876 (<filename>filter_auth_simple.cpp</filename>,
1877 <filename>filter_backend_test.cpp</filename>, etc.)</title>
1879 Individual filters. Each of these is implemented by a header and
1880 a source file, named <filename>filter_*.hpp</filename> and
1881 <filename>filter_*.cpp</filename> respectively. All the header
1882 files should be pretty much identical, in that they declare the
1883 class, including a private <literal>Rep</literal> class and a
1884 member pointer to it, and the two public methods.
1887 The source file for each filter needs to supply:
1892 A definition of the private <literal>Rep</literal> class.
1897 Some boilerplate constructors and destructors.
1902 A <literal>configure()</literal> method that uses the
1903 appropriate XML fragment.
1908 Most important, the <literal>process()</literal> method that
1909 does all the actual work.
1915 <section id="class-Package">
1916 <title><literal>mp::Package</literal>
1917 (<filename>package.cpp</filename>)</title>
1919 Represents a package on its way through the series of filters
1920 that make up a route. This is essentially a Z39.50 or SRU APDU
1921 together with information about where it came from, which is
1922 modified as it passes through the various filters.
1926 <section id="class-Pipe">
1927 <title><literal>mp::Pipe</literal>
1928 (<filename>pipe.cpp</filename>)</title>
1930 This class provides a compatibility layer so that we have an IPC
1931 mechanism that works the same under Unix and Windows. It's not
1932 particularly exciting.
1936 <section id="class-RouterChain">
1937 <title><literal>mp::RouterChain</literal>
1938 (<filename>router_chain.cpp</filename>)</title>
1944 <section id="class-RouterFleXML">
1945 <title><literal>mp::RouterFleXML</literal>
1946 (<filename>router_flexml.cpp</filename>)</title>
1952 <section id="class-Session">
1953 <title><literal>mp::Session</literal>
1954 (<filename>session.cpp</filename>)</title>
1960 <section id="class-ThreadPoolSocketObserver">
1961 <title><literal>mp::ThreadPoolSocketObserver</literal>
1962 (<filename>thread_pool_observer.cpp</filename>)</title>
1968 <section id="class-util">
1969 <title><literal>mp::util</literal>
1970 (<filename>util.cpp</filename>)</title>
1972 A namespace of various small utility functions and classes,
1973 collected together for convenience. Most importantly, includes
1974 the <literal>mp::util::odr</literal> class, a wrapper for YAZ's
1979 <section id="class-xml">
1980 <title><literal>mp::xml</literal>
1981 (<filename>xmlutil.cpp</filename>)</title>
1983 A namespace of various XML utility functions and classes,
1984 collected together for convenience.
1990 <section id="other.source.files">
1991 <title>Other Source Files</title>
1993 In addition to the Metaproxy source files that define the classes
1994 described above, there are a few additional files which are
1995 briefly described here:
1999 <term><literal>metaproxy_prog.cpp</literal></term>
2002 The main function of the <command>metaproxy</command> program.
2007 <term><literal>ex_router_flexml.cpp</literal></term>
2010 Identical to <literal>metaproxy_prog.cpp</literal>: it's not clear why.
2015 <term><literal>test_*.cpp</literal></term>
2018 Unit-tests for various modules.
2024 ### Still to be described:
2025 <literal>ex_filter_frontend_net.cpp</literal>,
2026 <literal>filter_dl.cpp</literal>,
2027 <literal>plainfile.cpp</literal>,
2028 <literal>tstdl.cpp</literal>.
2034 <reference id="reference">
2035 <title>Reference</title>
2036 <partintro id="reference-introduction">
2038 The material in this chapter is drawn directly from the individual
2039 manual entries. In particular, the Metaproxy invocation section is
2040 available using <command>man metaproxy</command>, and the section
2041 on each individual filter is available using the name of the filter
2042 as the argument to the <command>man</command> command.
2048 <appendix id="license">
2049 <title>License</title>
2052 Metaproxy, Copyright © 1995-2007 Index Data ApS.
2056 Metaproxy is free software; you can redistribute it and/or modify it under
2057 the terms of the GNU General Public License as published by the Free
2058 Software Foundation; either version 2, or (at your option) any later
2063 Metaproxy is distributed in the hope that it will be useful, but WITHOUT ANY
2064 WARRANTY; without even the implied warranty of MERCHANTABILITY or
2065 FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License
2070 You should have received a copy of the GNU General Public License
2071 along with Metaproxy; see the file LICENSE. If not, write to the
2072 Free Software Foundation,
2073 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA
2077 <title>GNU General Public License</title>
2079 GNU GENERAL PUBLIC LICENSE
2080 Version 2, June 1991
2082 Copyright (C) 1989, 1991 Free Software Foundation, Inc.
2083 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA
2084 w Everyone is permitted to copy and distribute verbatim copies
2085 of this license document, but changing it is not allowed.
2089 The licenses for most software are designed to take away your
2090 freedom to share and change it. By contrast, the GNU General Public
2091 License is intended to guarantee your freedom to share and change free
2092 software--to make sure the software is free for all its users. This
2093 General Public License applies to most of the Free Software
2094 Foundation's software and to any other program whose authors commit to
2095 using it. (Some other Free Software Foundation software is covered by
2096 the GNU Library General Public License instead.) You can apply it to
2099 When we speak of free software, we are referring to freedom, not
2100 price. Our General Public Licenses are designed to make sure that you
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2103 if you want it, that you can change the software or use pieces of it
2104 in new free programs; and that you know you can do these things.
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2108 These restrictions translate to certain responsibilities for you if you
2109 distribute copies of the software, or if you modify it.
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2138 TERMS AND CONDITIONS FOR COPYING, DISTRIBUTION AND MODIFICATION
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