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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-test-dev
285 With these packages installed, the usual configure + make
286 procedure can be used for Metaproxy as outlined in
287 <xref linkend="installation.unix"/>.
291 <section id="installation.rpm">
292 <title>Installation on RPM based Linux Systems</title>
294 All external dependencies for Metaproxy are available as
295 RPM packages, either from your distribution site, or from the
296 <ulink url="http://fr.rpmfind.net/">RPMfind</ulink> site.
299 For example, an installation of the requires Boost C++ development
300 libraries on RedHat Fedora C4 and C5 can be done like this:
302 wget ftp://fr.rpmfind.net/wlinux/fedora/core/updates/testing/4/SRPMS/boost-1.33.0-3.fc4.src.rpm
303 sudo rpmbuild --buildroot src/ --rebuild -p fc4/boost-1.33.0-3.fc4.src.rpm
304 sudo rpm -U /usr/src/redhat/RPMS/i386/boost-*rpm
308 The <ulink url="&url.yaz;">YAZ</ulink> library is needed to
309 compile &metaproxy;, see there
310 for more information on available RPM packages.
313 There is currently no official RPM package for YAZ++.
314 See the <ulink url="&url.yazplusplus;">YAZ++</ulink> pages
315 for more information on a Unix tarball install.
318 With these packages installed, the usual configure + make
319 procedure can be used for Metaproxy as outlined in
320 <xref linkend="installation.unix"/>.
324 <section id="installation.windows">
325 <title>Installation on Windows</title>
327 Metaproxy can be compiled with Microsoft
328 <ulink url="&url.vstudio;">Visual Studio</ulink>.
329 Version 2003 (C 7.1) and 2005 (C 8.0) is known to work.
331 <section id="installation.windows.boost">
334 For Windows, it's easiest to get the precompiled Boost
335 package from <ulink url="&url.boost.windows.download;">here</ulink>.
336 Several versions of the Boost libraries may be selected when
337 installing Boost for windows. Please choose at least the
338 <emphasis>multithreaded</emphasis> (non-DLL) version because
339 the Metaproxy makefile uses that.
342 For more information about installing Boost refer to the
343 <ulink url="&url.boost.getting.started;">getting started</ulink>
348 <section id="installation.windows.libxslt">
349 <title>Libxslt</title>
351 <ulink url="&url.libxslt;">Libxslt</ulink> can be downloaded
353 <ulink url="&url.libxml2.download.win32;">here</ulink>.
356 Libxslt has other dependencies, but these can all be downloaded
357 from the same site. Get the following package:
358 iconv, zlib, libxml2, libxslt.
362 <section id="installation.windows.yaz">
365 <ulink url="&url.yaz;">YAZ</ulink> can be downloaded
367 <ulink url="&url.yaz.download.win32;">here</ulink>.
371 <section id="installation.windows.yazplusplus">
374 Get <ulink url="&url.yazplusplus;">YAZ++</ulink> as well.
375 Version 1.0.3 or later is required.
378 YAZ++ includes NMAKE makefiles, similar to those found in the
383 <section id="installation.windows.metaproxy">
384 <title>Metaproxy</title>
386 Metaproxy is shipped with NMAKE makefiles as well - similar
387 to those found in the YAZ++/YAZ packages. Adjust this Makefile
388 to point to the proper locations of Boost, Libxslt, Libxml2,
389 zlib, iconv, yaz and yazpp.
393 <varlistentry><term><literal>DEBUG</literal></term>
395 If set to 1, the software is
396 compiled with debugging libraries (code generation is
397 multi-threaded debug DLL).
398 If set to 0, the software is compiled with release libraries
399 (code generation is multi-threaded DLL).
404 <term><literal>BOOST</literal></term>
407 Boost install location
413 <term><literal>BOOST_VERSION</literal></term>
416 Boost version (replace . with _).
422 <term><literal>BOOST_TOOLSET</literal></term>
431 <term><literal>LIBXSLT_DIR</literal>,
432 <literal>LIBXML2_DIR</literal> ..</term>
435 Specify the locations of Libxslt, libiconv, libxml2 and
444 After successful compilation you'll find
445 <literal>metaproxy.exe</literal> in the
446 <literal>bin</literal> directory.
454 <chapter id="yazproxy-comparison">
455 <title>YAZ Proxy Comparison</title>
457 The table below lists facilities either supported by either
458 <ulink url="&url.yazproxy;">YAZ Proxy</ulink> or Metaproxy.
460 <table id="yazproxy-comparison-table">
461 <title>Metaproxy / YAZ Proxy comparison</title>
465 <entry>Facility</entry>
466 <entry>Metaproxy</entry>
467 <entry>YAZ Proxy</entry>
472 <entry>Z39.50 server</entry>
473 <entry>Using filter <literal>frontend_net</literal></entry>
474 <entry>Supported</entry>
477 <entry>SRU server</entry>
478 <entry>Supported with filter <literal>sru_z3950</literal></entry>
479 <entry>Supported</entry>
482 <entry>Z39.50 client</entry>
483 <entry>Supported with filter <literal>z3950_client</literal></entry>
484 <entry>Supported</entry>
487 <entry>SRU client</entry>
488 <entry>Unsupported</entry>
489 <entry>Unsupported</entry>
492 <entry>Connection reuse</entry>
493 <entry>Supported with filter <literal>session_shared</literal></entry>
494 <entry>Supported</entry>
497 <entry>Connection share</entry>
498 <entry>Supported with filter <literal>session_shared</literal></entry>
499 <entry>Unsupported</entry>
502 <entry>Result set reuse</entry>
503 <entry>Supported with filter <literal>session_shared</literal></entry>
504 <entry>Within one Z39.50 session / HTTP keep-alive</entry>
507 <entry>Record cache</entry>
508 <entry>Unsupported</entry>
509 <entry>Supported for last result set within one Z39.50/HTTP-keep alive session</entry>
512 <entry>Z39.50 Virtual database, i.e. select any Z39.50 target for database</entry>
513 <entry>Supported with filter <literal>virt_db</literal></entry>
514 <entry>Unsupported</entry>
517 <entry>SRU Virtual database, i.e. select any Z39.50 target for path</entry>
518 <entry>Supported with filter <literal>virt_db</literal>,
519 <literal>sru_z3950</literal></entry>
520 <entry>Supported</entry>
523 <entry>Multi target search</entry>
524 <entry>Supported with filter <literal>multi</literal> (round-robin)</entry>
525 <entry>Unsupported</entry>
528 <entry>Retrieval and search limits</entry>
529 <entry>Unsupported</entry>
530 <entry>Supported</entry>
533 <entry>Bandwidth limits</entry>
534 <entry>Unsupported</entry>
535 <entry>Supported</entry>
538 <entry>Connect limits</entry>
539 <entry>Unsupported</entry>
540 <entry>Supported</entry>
543 <entry>Retrieval sanity check and conversions</entry>
544 <entry>Supported using filter <literal>record_transform</literal></entry>
545 <entry>Supported</entry>
548 <entry>Query check</entry>
550 Supported in a limited way using <literal>query_rewrite</literal>
552 <entry>Supported</entry>
555 <entry>Query rewrite</entry>
556 <entry>Supported with <literal>query_rewrite</literal></entry>
557 <entry>Unsupported</entry>
560 <entry>Session invalidate for -1 hits</entry>
561 <entry>Unsupported</entry>
562 <entry>Supported</entry>
565 <entry>Architecture</entry>
566 <entry>Multi-threaded + select for networked modules such as
567 <literal>frontend_net</literal>)</entry>
568 <entry>Single-threaded using select</entry>
572 <entry>Extensability</entry>
573 <entry>Most functionality implemented as loadable modules</entry>
574 <entry>Unsupported and experimental</entry>
578 <entry><ulink url="&url.usemarcon;">USEMARCON</ulink></entry>
579 <entry>Unsupported</entry>
580 <entry>Supported</entry>
584 <entry>Portability</entry>
586 Requires YAZ, YAZ++ and modern C++ compiler supporting
587 <ulink url="&url.boost;">Boost</ulink>.
590 Requires YAZ and YAZ++.
591 STL is not required so pretty much any C++ compiler out there should work.
600 <chapter id="architecture">
601 <title>The Metaproxy Architecture</title>
603 The Metaproxy architecture is based on three concepts:
604 the <emphasis>package</emphasis>,
605 the <emphasis>route</emphasis>
606 and the <emphasis>filter</emphasis>.
610 <term>Packages</term>
613 A package is request or response, encoded in some protocol,
614 issued by a client, making its way through Metaproxy, send to or
615 received from a server, or sent back to the client.
618 The core of a package is the protocol unit - for example, a
619 Z39.50 Init Request or Search Response, or an SRU searchRetrieve
620 URL or Explain Response. In addition to this core, a package
621 also carries some extra information added and used by Metaproxy
625 In general, packages are doctored as they pass through
626 Metaproxy. For example, when the proxy performs authentication
627 and authorization on a Z39.50 Init request, it removes the
628 authentication credentials from the package so that they are not
629 passed onto the back-end server; and when search-response
630 packages are obtained from multiple servers, they are merged
631 into a single unified package that makes its way back to the
640 Packages make their way through routes, which can be thought of
641 as programs that operate on the package data-type. Each
642 incoming package initially makes its way through a default
643 route, but may be switched to a different route based on various
644 considerations. Routes are made up of sequences of filters (see
653 Filters provide the individual instructions within a route, and
654 effect the necessary transformations on packages. A particular
655 configuration of Metaproxy is essentially a set of filters,
656 described by configuration details and arranged in order in one
657 or more routes. There are many kinds of filter - about a dozen
658 at the time of writing with more appearing all the time - each
659 performing a specific function and configured by different
663 The word ``filter'' is sometimes used rather loosely, in two
664 different ways: it may be used to mean a particular
665 <emphasis>type</emphasis> of filter, as when we speak of ``the
666 auth_simple filter'' or ``the multi filter''; or it may be used
667 to be a specific <emphasis>instance</emphasis> of a filter
668 within a Metaproxy configuration. For example, a single
669 configuration will often contain multiple instances of the
670 <literal>z3950_client</literal> filter. In
671 operational terms, of these is a separate filter. In practice,
672 context always make it clear which sense of the word ``filter''
676 Extensibility of Metaproxy is primarily through the creation of
677 plugins that provide new filters. The filter API is small and
678 conceptually simple, but there are many details to master. See
680 <link linkend="filters">Filters</link>.
686 Since packages are created and handled by the system itself, and
687 routes are conceptually simple, most of the remainder of this
688 document concentrates on filters. After a brief overview of the
689 filter types follows, along with some thoughts on possible future
696 <chapter id="filters">
697 <title>Filters</title>
700 <section id="filters-introductory-notes">
701 <title>Introductory notes</title>
703 It's useful to think of Metaproxy as an interpreter providing a small
704 number of primitives and operations, but operating on a very
705 complex data type, namely the ``package''.
708 A package represents a Z39.50 or SRU/W request (whether for Init,
709 Search, Scan, etc.) together with information about where it came
710 from. Packages are created by front-end filters such as
711 <literal>frontend_net</literal> (see below), which reads them from
712 the network; other front-end filters are possible. They then pass
713 along a route consisting of a sequence of filters, each of which
714 transforms the package and may also have side-effects such as
715 generating logging. Eventually, the route will yield a response,
716 which is sent back to the origin.
719 There are many kinds of filter: some that are defined statically
720 as part of Metaproxy, and others may be provided by third parties
721 and dynamically loaded. They all conform to the same simple API
722 of essentially two methods: <function>configure()</function> is
723 called at startup time, and is passed an XML DOM tree representing that
724 part of the configuration file that pertains to this filter
725 instance: it is expected to walk that tree extracting relevant
726 information; and <function>process()</function> is called every
727 time the filter has to processes a package.
730 While all filters provide the same API, there are different modes
731 of functionality. Some filters are sources: they create
733 (<literal>frontend_net</literal>);
734 others are sinks: they consume packages and return a result
735 (<literal>backend_test</literal>,
736 <literal>bounce</literal>,
737 <literal>http_file</literal>,
738 <literal>z3950_client</literal>);
739 the others are true filters, that read, process and pass on the
740 packages they are fed
741 (<literal>auth_simple</literal>,
742 <literal>log</literal>,
743 <literal>multi</literal>,
744 <literal>query_rewrite</literal>,
745 <literal>record_transform</literal>,
746 <literal>session_shared</literal>,
747 <literal>sru_z3950</literal>,
748 <literal>template</literal>,
749 <literal>virt_db</literal>).
754 <section id="overview.filter.types">
755 <title>Overview of filter types</title>
757 We now briefly consider each of the types of filter supported by
758 the core Metaproxy binary. This overview is intended to give a
759 flavor of the available functionality; more detailed information
760 about each type of filter is included below in
761 <xref linkend="reference"/>.
764 The filters are here named by the string that is used as the
765 <literal>type</literal> attribute of a
766 <literal><filter></literal> element in the configuration
767 file to request them, with the name of the class that implements
768 them in parentheses. (The classname is not needed for normal
769 configuration and use of Metaproxy; it is useful only to
773 The filters are here listed in alphabetical order:
780 New virt_db-alike that does inteligent peer choice, explain merging,
781 adds FD&N to explain. Keeps init responses (like "virt_db Classic"),
782 makes routing choices based on local explain knowledge. Ref IDDI
785 Filter to convert Explain Classic to ZeeRex.
787 CQL2PQF (which needs augmented ZeeRex) - MARC for Talis.
791 Figure out what additional information we need in:
792 ZeeRex (check against D3.1)
793 Init request (e.g. loop detection)
794 Query package (e.g. number of hops)
795 Query response (e.g. record source)
799 <section id="auth_simple">
800 <title><literal>auth_simple</literal>
801 (mp::filter::AuthSimple)</title>
803 Simple authentication and authorization. The configuration
804 specifies the name of a file that is the user register, which
805 lists <varname>username</varname>:<varname>password</varname>
806 pairs, one per line, colon separated. When a session begins, it
807 is rejected unless username and passsword are supplied, and match
808 a pair in the register. The configuration file may also specific
809 the name of another file that is the target register: this lists
810 lists <varname>username</varname>:<varname>dbname</varname>,<varname>dbname</varname>...
811 sets, one per line, with multiple database names separated by
812 commas. When a search is processed, it is rejected unless the
813 database to be searched is one of those listed as available to
818 <section id="backend_test">
819 <title><literal>backend_test</literal>
820 (mp::filter::Backend_test)</title>
822 A partial sink that provides dummy responses in the manner of the
823 <literal>yaz-ztest</literal> Z39.50 server. This is useful only
824 for testing. Seriously, you don't need this. Pretend you didn't
825 even read this section.
829 <section id="bounce">
830 <title><literal>bounce</literal>
831 (mp::filter::Bounce)</title>
833 A sink that swallows <emphasis>all packages</emphasis>,
834 and returns them almost unprocessed.
835 It never sends any package of any type further down the row, but
836 sets Z39.50 packages to Z_Close, and HTTP_Request packages to
837 HTTP_Response err code 400 packages, and adds a suitable bounce
839 The bounce filter is usually added at end of each filter chain route
840 to prevent infinite hanging of for example HTTP
841 requests packages when only the Z39.50 client partial sink
842 filter is found in the
847 <section id="cql_rpn">
848 <title><literal>cql_rpn</literal>
849 (mp::filter::CQLtoRPN)</title>
851 A query language transforming filter which catches Z39.50
852 <literal>searchRequest</literal>
853 packages containing <literal>CQL</literal> queries, transforms
854 those to <literal>RPN</literal> queries,
855 and sends the <literal>searchRequests</literal> on to the next
856 filters. It is among other things useful in a SRU context.
860 <section id="frontend_net">
861 <title><literal>frontend_net</literal>
862 (mp::filter::FrontendNet)</title>
864 A source that accepts Z39.50 connections from a port
865 specified in the configuration, reads protocol units, and
866 feeds them into the next filter in the route. When the result is
867 received, it is returned to the original origin.
871 <section id="http_file">
872 <title><literal>http_file</literal>
873 (mp::filter::HttpFile)</title>
875 A partial sink which swallows only
876 <literal>HTTP_Request</literal> packages, and
877 returns the contents of files from the local
878 filesystem in response to HTTP requests.
879 It lets Z39.50 packages and all other forthcoming package types
882 does mean that Metaproxy is also a Web-server in its spare time. So
883 far it does not contain either an email-reader or a Lisp
884 interpreter, but that day is surely coming.)
888 <section id="load_balance">
889 <title><literal>load_balance</literal>
890 (mp::filter::LoadBalance)</title>
892 Performs load balancing for incoming Z39.50 init requests.
893 It is used together with the <literal>virt_db</literal> filter,
894 but unlike the <literal>multi</literal> filter it does send an
895 entire session to only one of the virtual backends. The
896 <literal>load_balance</literal> filter is assuming that
897 all backend targets have equal content, and chooses the backend
898 with least load cost for a new session.
901 This filter is experimental and yet not mature for heavy load
909 <title><literal>log</literal>
910 (mp::filter::Log)</title>
912 Writes logging information to standard output, and passes on
913 the package unchanged. A log file name can be specified, as well
914 as multiple different logging formats.
919 <title><literal>multi</literal>
920 (mp::filter::Multi)</title>
922 Performs multi-database searching.
924 <link linkend="multidb">the extended discussion</link>
925 of virtual databases and multi-database searching below.
929 <section id="query_rewrite">
930 <title><literal>query_rewrite</literal>
931 (mp::filter::QueryRewrite)</title>
933 Rewrites Z39.50 <literal>Type-1</literal>
934 and <literal>Type-101</literal> (``<literal>RPN</literal>'')
936 three-step process: the query is transliterated from Z39.50
937 packet structures into an XML representation; that XML
938 representation is transformed by an XSLT stylesheet; and the
939 resulting XML is transliterated back into the Z39.50 packet
945 <section id="record_transform">
946 <title><literal>record_transform</literal>
947 (mp::filter::RecordTransform)</title>
949 This filter acts only on Z3950 present requests, and let all
950 other types of packages and requests pass untouched. It's use is
951 twofold: blocking Z3950 present requests, which the backend
952 server does not understand and can not honor, and transforming
953 the present syntax and elementset name according to the rules
954 specified, to fetch only existing record formats, and transform
955 them on the fly to requested record syntaxes.
959 <section id="session_shared">
960 <title><literal>session_shared</literal>
961 (mp::filter::SessionShared)</title>
963 This filter implements global sharing of
964 result sets (i.e. between threads and therefore between
965 clients), yielding performance improvements by clever resource
970 <section id="sru_z3950">
971 <title><literal>sru_z3950</literal>
972 (mp::filter::SRUtoZ3950)</title>
974 This filter transforms valid
975 SRU GET/POST/SOAP searchRetrieve requests to Z3950 init, search,
976 and present requests, and wraps the
977 received hit counts and XML records into suitable SRU response
979 The <literal>sru_z3950</literal> filter processes also SRU
980 GET/POST/SOAP explain requests, returning
981 either the absolute minimum required by the standard, or a full
982 pre-defined ZeeReX explain record.
984 <ulink url="&url.zeerex.explain;">ZeeReX Explain</ulink>
985 standard pages and the
986 <ulink url="&url.sru.explain;">SRU Explain</ulink> pages
987 for more information on the correct explain syntax.
988 SRU scan requests are not supported yet.
992 <section id="template">
993 <title><literal>template</literal>
994 (mp::filter::Template)</title>
996 Does nothing at all, merely passing the packet on. (Maybe it
997 should be called <literal>nop</literal> or
998 <literal>passthrough</literal>?) This exists not to be used, but
999 to be copied - to become the skeleton of new filters as they are
1000 written. As with <literal>backend_test</literal>, this is not
1001 intended for civilians.
1005 <section id="virt_db">
1006 <title><literal>virt_db</literal>
1007 (mp::filter::VirtualDB)</title>
1009 Performs virtual database selection: based on the name of the
1010 database in the search request, a server is selected, and its
1011 address added to the request in a <literal>VAL_PROXY</literal>
1012 otherInfo packet. It will subsequently be used by a
1013 <literal>z3950_client</literal> filter.
1015 <link linkend="multidb">the extended discussion</link>
1016 of virtual databases and multi-database searching below.
1020 <section id="z3950_client">
1021 <title><literal>z3950_client</literal>
1022 (mp::filter::Z3950Client)</title>
1024 A partial sink which swallows only Z39.50 packages.
1025 It performs Z39.50 searching and retrieval by proxying the
1026 packages that are passed to it. Init requests are sent to the
1027 address specified in the <literal>VAL_PROXY</literal> otherInfo
1028 attached to the request: this may have been specified by client,
1029 or generated by a <literal>virt_db</literal> filter earlier in
1030 the route. Subsequent requests are sent to the same address,
1031 which is remembered at Init time in a Session object.
1032 HTTP_Request packages and all other forthcoming package types
1033 are passed untouched.
1038 <section id="zeerex_explain">
1039 <title><literal>zeerex_explain</literal>
1040 (mp::filter::ZeerexExplain)</title>
1042 This filter acts as a sink for
1043 Z39.50 explain requests, returning a static ZeeReX
1044 Explain XML record from the config section. All other packages
1047 <ulink url="&url.zeerex.explain;">ZeeReX Explain</ulink>
1049 for more information on the correct explain syntax.
1053 This filter is not yet completed.
1062 <section id="future.directions">
1063 <title>Future directions</title>
1065 Some other filters that do not yet exist, but which would be
1066 useful, are briefly described. These may be added in future
1067 releases (or may be created by third parties, as loadable
1073 <term><literal>frontend_cli</literal> (source)</term>
1076 Command-line interface for generating requests.
1081 <term><literal>sru_client</literal> (sink)</term>
1084 SRU/GET and SRU/SOAP searching and retrieval.
1089 <term><literal>opensearch_client</literal> (sink)</term>
1092 A9 OpenSearch searching and retrieval.
1102 <chapter id="configuration">
1103 <title>Configuration: the Metaproxy configuration file format</title>
1106 <section id="configuration-introductory-notes">
1107 <title>Introductory notes</title>
1109 If Metaproxy is an interpreter providing operations on packages, then
1110 its configuration file can be thought of as a program for that
1111 interpreter. Configuration is by means of a single XML file, the name
1112 of which is supplied as the sole command-line argument to the
1113 <command>metaproxy</command> program. (See
1114 <xref linkend="reference"/> below for more information on invoking
1119 <section id="overview.xml.structure">
1120 <title>Overview of the config file XML structure</title>
1122 All elements and attributes are in the namespace
1123 <ulink url="http://indexdata.com/metaproxy"/>.
1124 This is most easily achieved by setting the default namespace on
1125 the top-level element, as here:
1128 <metaproxy xmlns="http://indexdata.com/metaproxy" version="1.0">
1131 The top-level element is <metaproxy>. This contains a
1132 <start> element, a <filters> element and a
1133 <routes> element, in that order. <filters> is
1134 optional; the other two are mandatory. All three are
1138 The <start> element is empty, but carries a
1139 <literal>route</literal> attribute, whose value is the name of
1140 route at which to start running - analogous to the name of the
1141 start production in a formal grammar.
1144 If present, <filters> contains zero or more <filter>
1145 elements. Each filter carries a <literal>type</literal> attribute
1146 which specifies what kind of filter is being defined
1147 (<literal>frontend_net</literal>, <literal>log</literal>, etc.)
1148 and contain various elements that provide suitable configuration
1149 for a filter of its type. The filter-specific elements are
1151 <xref linkend="reference"/>.
1152 Filters defined in this part of the file must carry an
1153 <literal>id</literal> attribute so that they can be referenced
1157 <routes> contains one or more <route> elements, each
1158 of which must carry an <literal>id</literal> element. One of the
1159 routes must have the ID value that was specified as the start
1160 route in the <start> element's <literal>route</literal>
1161 attribute. Each route contains zero or more <filter>
1162 elements. These are of two types. They may be empty, but carry a
1163 <literal>refid</literal> attribute whose value is the same as the
1164 <literal>id</literal> of a filter previously defined in the
1165 <filters> section. Alternatively, a route within a filter
1166 may omit the <literal>refid</literal> attribute, but contain
1167 configuration elements similar to those used for filters defined
1168 in the <filters> section. (In other words, each filter in a
1169 route may be included either by reference or by physical
1175 <section id="example.configuration">
1176 <title>An example configuration</title>
1178 The following is a small, but complete, Metaproxy configuration
1179 file (included in the distribution as
1180 <literal>metaproxy/etc/config1.xml</literal>).
1181 This file defines a very simple configuration that simply proxies
1182 to whatever back-end server the client requests, but logs each
1183 request and response. This can be useful for debugging complex
1184 client-server dialogues.
1186 <screen><![CDATA[<?xml version="1.0"?>
1187 <metaproxy xmlns="http://indexdata.com/metaproxy" version="1.0">
1188 <start route="start"/>
1190 <filter id="frontend" type="frontend_net">
1193 <filter id="backend" type="z3950_client">
1198 <filter refid="frontend"/>
1199 <filter type="log"/>
1200 <filter refid="backend"/>
1201 <filter type="bounce"/>
1207 It works by defining a single route, called
1208 <literal>start</literal>, which consists of a sequence of four
1209 filters. The first and last of these are included by reference:
1210 their <literal><filter></literal> elements have
1211 <literal>refid</literal> attributes that refer to filters defined
1212 within the prior <literal><filters></literal> section. The
1213 middle filter is included inline in the route.
1216 The four filters in the route are as follows: first, a
1217 <literal>frontend_net</literal> filter accepts Z39.50 requests
1218 from any host on port 9000; then these requests are passed through
1219 a <literal>log</literal> filter that emits a message for each
1220 request; they are then fed into a <literal>z3950_client</literal>
1221 filter, which forwards all Z39.50 requests to the client-specified
1222 back-end Z39.509 server. Those Z39.50 packages are returned by the
1223 <literal>z3950_client</literal> filter, with the response data
1224 filled by the external Z39.50 server targeted.
1225 All non-Z39.50 packages are passed through to the
1226 <literal>bounce</literal> filter, which definitely bounces
1227 everything, including fish, bananas, cold pyjamas,
1228 mutton, beef and trout packages.
1229 When the response arrives, it is handed
1230 back to the <literal>log</literal> filter, which emits another
1231 message; and then to the <literal>frontend_net</literal> filter,
1232 which returns the response to the client.
1236 <section id="config-file-modularity">
1237 <title>Config file modularity</title>
1239 Metaproxy XML configuration snippets can be reused by other
1240 filters using the <literal>XInclude</literal> standard, as seen in
1241 the <literal>/etc/config-sru-to-z3950.xml</literal> example SRU
1244 <filter id="sru" type="sru_z3950">
1245 <database name="Default">
1246 <xi:include xmlns:xi="http://www.w3.org/2001/XInclude"
1247 href="explain.xml"/>
1254 <section id="config-file-syntax-check">
1255 <title>Config file syntax checking</title>
1257 The distribution contains RelaxNG Compact and XML syntax checking
1258 files, as well as XML Schema files. These are found in the
1261 xml/schema/metaproxy.rnc
1262 xml/schema/metaproxy.rng
1263 xml/schema/metaproxy.xsd
1265 and can be used to verify or debug the XML structure of
1266 configuration files. For example, using the utility
1267 <filename>xmllint</filename>, syntax checking is done like this:
1269 xmllint --noout --schema xml/schema/metaproxy.xsd etc/config-local.xml
1270 xmllint --noout --relaxng xml/schema/metaproxy.rng etc/config-local.xml
1272 (A recent version of <literal>libxml2</literal> is required, as
1273 support for XML Schemas is a relatively recent addition.)
1276 You can of course use any other RelaxNG or XML Schema compliant tool
1284 <chapter id="multidb">
1285 <title>Virtual databases and multi-database searching</title>
1288 <section id="multidb-introductory-notes">
1289 <title>Introductory notes</title>
1291 Two of Metaproxy's filters are concerned with multiple-database
1292 operations. Of these, <literal>virt_db</literal> can work alone
1293 to control the routing of searches to one of a number of servers,
1294 while <literal>multi</literal> can work together with
1295 <literal>virt_db</literal> to perform multi-database searching, merging
1296 the results into a unified result-set - ``metasearch in a box''.
1299 The interaction between
1300 these two filters is necessarily complex: it reflects the real,
1301 irreducible complexity of multi-database searching in a protocol such
1302 as Z39.50 that separates initialization from searching, and in
1303 which the database to be searched is not known at initialization
1307 It's possible to use these filters without understanding the
1308 details of their functioning and the interaction between them; the
1309 next two sections of this chapter are ``HOW-TO'' guides for doing
1310 just that. However, debugging complex configurations will require
1311 a deeper understanding, which the last two sections of this
1312 chapters attempt to provide.
1317 <section id="multidb.virt_db">
1318 <title>Virtual databases with the <literal>virt_db</literal> filter</title>
1320 Working alone, the purpose of the
1321 <literal>virt_db</literal>
1322 filter is to route search requests to one of a selection of
1323 back-end databases. In this way, a single Z39.50 endpoint
1324 (running Metaproxy) can provide access to several different
1325 underlying services, including those that would otherwise be
1326 inaccessible due to firewalls. In many useful configurations, the
1327 back-end databases are local to the Metaproxy installation, but
1328 the software does not enforce this, and any valid Z39.50 servers
1329 may be used as back-ends.
1332 For example, a <literal>virt_db</literal>
1333 filter could be set up so that searches in the virtual database
1334 ``lc'' are forwarded to the Library of Congress bibliographic
1335 catalogue server, and searches in the virtual database ``marc''
1336 are forwarded to the toy database of MARC records that Index Data
1337 hosts for testing purposes. A <literal>virt_db</literal>
1338 configuration to make this switch would look like this:
1340 <screen><![CDATA[<filter type="virt_db">
1342 <database>lc</database>
1343 <target>z3950.loc.gov:7090/voyager</target>
1346 <database>marc</database>
1347 <target>indexdata.com/marc</target>
1349 </filter>]]></screen>
1351 As well as being useful in it own right, this filter also provides
1352 the foundation for multi-database searching.
1357 <section id="multidb.multi">
1358 <title>Multi-database search with the <literal>multi</literal> filter</title>
1360 To arrange for Metaproxy to broadcast searches to multiple back-end
1361 servers, the configuration needs to include two components: a
1362 <literal>virt_db</literal>
1363 filter that specifies multiple
1364 <literal><target></literal>
1365 elements, and a subsequent
1366 <literal>multi</literal>
1367 filter. Here, for example, is a complete configuration that
1368 broadcasts searches to both the Library of Congress catalogue and
1369 Index Data's tiny testing database of MARC records:
1371 <screen><![CDATA[<?xml version="1.0"?>
1372 <metaproxy xmlns="http://indexdata.com/metaproxy" version="1.0">
1373 <start route="start"/>
1376 <filter type="frontend_net">
1377 <threads>10</threads>
1380 <filter type="virt_db">
1382 <database>lc</database>
1383 <target>z3950.loc.gov:7090/voyager</target>
1386 <database>marc</database>
1387 <target>indexdata.com/marc</target>
1390 <database>all</database>
1391 <target>z3950.loc.gov:7090/voyager</target>
1392 <target>indexdata.com/marc</target>
1395 <filter type="multi"/>
1396 <filter type="z3950_client">
1397 <timeout>30</timeout>
1399 <filter type="bounce"/>
1402 </metaproxy>]]></screen>
1405 <literal>virt_db</literal>
1406 filter that specifies multiple
1407 <literal><target></literal>
1408 elements but without a subsequent
1409 <literal>multi</literal>
1410 filter yields surprising and undesirable results, as will be
1411 described below. Don't do that.)
1414 Metaproxy can be invoked with this configuration as follows:
1416 <screen>../src/metaproxy --config config-simple-multi.xml</screen>
1418 And thereafter, Z39.50 clients can connect to the running server
1419 (on port 9000, as specified in the configuration) and search in
1420 any of the databases
1421 <literal>lc</literal> (the Library of Congress catalogue),
1422 <literal>marc</literal> (Index Data's test database of MARC records)
1424 <literal>all</literal> (both of these). As an example, a session
1425 using the YAZ command-line client <literal>yaz-client</literal> is
1426 here included (edited for brevity and clarity):
1428 <screen><![CDATA[$ yaz-client @:9000
1432 Search was a success.
1433 Number of hits: 10000, setno 1
1437 Search was a success.
1438 Number of hits: 10, setno 3
1442 Search was a success.
1443 Number of hits: 10010, setno 4
1446 [marc]Record type: USmarc
1449 005 00000000000000.0
1450 008 910710c19910701nju 00010 eng
1454 100 10 $a Jack Collins
1455 245 10 $a How to program a computer
1461 [VOYAGER]Record type: USmarc
1463 005 20041229102447.0
1464 008 030910s2004 caua 000 0 eng
1465 035 $a (DLC) 2003112666
1466 906 $a 7 $b cbc $c orignew $d 4 $e epcn $f 20 $g y-gencatlg
1467 925 0 $a acquire $b 1 shelf copy $x policy default
1468 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.
1471 040 $a DLC $c DLC $d DLC
1472 050 00 $a MLCM 2004/03312 (G)
1473 245 10 $a 007, everything or nothing : $b Prima's official strategy guide / $c created by Kaizen Media Group.
1474 246 3 $a Double-O-seven, everything or nothing
1475 246 30 $a Prima's official strategy guide
1476 260 $a Roseville, CA : $b Prima Games, $c c2004.
1477 300 $a 161 p. : $b col. ill. ; $c 28 cm.
1478 500 $a "Platforms: Nintendo GameCube, Macintosh, PC, PlayStation 2 computer entertainment system, Xbox"--P. [4] of cover.
1479 650 0 $a Video games.
1480 710 2 $a Kaizen Media Group.
1481 856 42 $3 Publisher description $u http://www.loc.gov/catdir/description/random052/2003112666.html
1486 As can be seen, the first record in the result set is from the
1487 Index Data test database, and the second from the Library of
1488 Congress database. The result-set continues alternating records
1489 round-robin style until the point where one of the databases'
1490 records are exhausted.
1493 This example uses only two back-end databases; more may be used.
1494 There is no limitation imposed on the number of databases that may
1495 be metasearched in this way: issues of resource usage and
1496 administrative complexity dictate the practical limits.
1499 What happens when one of the databases doesn't respond? By default,
1500 the entire multi-database search fails, and the appropriate
1501 diagnostic is returned to the client. This is usually appropriate
1502 during development, when technicians need maximum information, but
1503 can be inconvenient in deployment, when users typically don't want
1504 to be bothered with problems of this kind and prefer just to get
1505 the records from the databases that are available. To obtain this
1506 latter behavior add an empty
1507 <literal><hideunavailable></literal>
1509 <literal>multi</literal> filter:
1511 <screen><![CDATA[ <filter type="multi">
1513 </filter>]]></screen>
1515 Under this regime, an error is reported to the client only if
1516 <emphasis>all</emphasis> the databases in a multi-database search
1522 <section id="multidb.what">
1523 <title>What's going on?</title>
1525 <title>Lark's vomit</title>
1527 This section goes into a level of technical detail that is
1528 probably not necessary in order to configure and use Metaproxy.
1529 It is provided only for those who like to know how things work.
1530 You should feel free to skip on to the next section if this one
1531 doesn't seem like fun.
1535 Hold on tight - this may get a little hairy.
1538 In the general course of things, a Z39.50 Init request may carry
1539 with it an otherInfo packet of type <literal>VAL_PROXY</literal>,
1540 whose value indicates the address of a Z39.50 server to which the
1541 ultimate connection is to be made. (This otherInfo packet is
1542 supported by YAZ-based Z39.50 clients and servers, but has not yet
1543 been ratified by the Maintenance Agency and so is not widely used
1544 in non-Index Data software. We're working on it.)
1545 The <literal>VAL_PROXY</literal> packet functions
1546 analogously to the absoluteURI-style Request-URI used with the GET
1547 method when a web browser asks a proxy to forward its request: see
1549 <ulink url="http://www.w3.org/Protocols/rfc2616/rfc2616-sec5.html#sec5.1.2"
1550 >Request-URI</ulink>
1552 <ulink url="http://www.w3.org/Protocols/rfc2616/rfc2616.html"
1553 >the HTTP 1.1 specification</ulink>.
1556 Within Metaproxy, Search requests that are part of the same
1557 session as an Init request that carries a
1558 <literal>VAL_PROXY</literal> otherInfo are also annotated with the
1559 same information. The role of the <literal>virt_db</literal>
1560 filter is to rewrite this otherInfo packet dependent on the
1561 virtual database that the client wants to search.
1564 When Metaproxy receives a Z39.50 Init request from a client, it
1565 doesn't immediately forward that request to the back-end server.
1566 Why not? Because it doesn't know <emphasis>which</emphasis>
1567 back-end server to forward it to until the client sends a Search
1568 request that specifies the database that it wants to search in.
1569 Instead, it just treasures the Init request up in its heart; and,
1570 later, the first time the client does a search on one of the
1571 specified virtual databases, a connection is forged to the
1572 appropriate server and the Init request is forwarded to it. If,
1573 later in the session, the same client searches in a different
1574 virtual database, then a connection is forged to the server that
1575 hosts it, and the same cached Init request is forwarded there,
1579 All of this clever Init-delaying is done by the
1580 <literal>frontend_net</literal> filter. The
1581 <literal>virt_db</literal> filter knows nothing about it; in
1582 fact, because the Init request that is received from the client
1583 doesn't get forwarded until a Search request is received, the
1584 <literal>virt_db</literal> filter (and the
1585 <literal>z3950_client</literal> filter behind it) doesn't even get
1586 invoked at Init time. The <emphasis>only</emphasis> thing that a
1587 <literal>virt_db</literal> filter ever does is rewrite the
1588 <literal>VAL_PROXY</literal> otherInfo in the requests that pass
1592 It is possible for a <literal>virt_db</literal> filter to contain
1594 <literal><target></literal>
1595 elements. What does this mean? Only that the filter will add
1596 multiple <literal>VAL_PROXY</literal> otherInfo packets to the
1597 Search requests that pass through it. That's because the virtual
1598 DB filter is dumb, and does exactly what it's told - no more, no
1600 If a Search request with multiple <literal>VAL_PROXY</literal>
1601 otherInfo packets reaches a <literal>z3950_client</literal>
1602 filter, this is an error. That filter doesn't know how to deal
1603 with multiple targets, so it will either just pick one and search
1604 in it, or (better) fail with an error message.
1607 The <literal>multi</literal> filter comes to the rescue! This is
1608 the only filter that knows how to deal with multiple
1609 <literal>VAL_PROXY</literal> otherInfo packets, and it does so by
1610 making multiple copies of the entire Search request: one for each
1611 <literal>VAL_PROXY</literal>. Each of these new copies is then
1612 passed down through the remaining filters in the route. (The
1613 copies are handled in parallel though the
1614 spawning of new threads.) Since the copies each have only one
1615 <literal>VAL_PROXY</literal> otherInfo, they can be handled by the
1616 <literal>z3950_client</literal> filter, which happily deals with
1617 each one individually. When the results of the individual
1618 searches come back up to the <literal>multi</literal> filter, it
1619 merges them into a single Search response, which is what
1620 eventually makes it back to the client.
1625 <section id="multidb.picture">
1626 <title>A picture is worth a thousand words (but only five hundred on 64-bit architectures)</title>
1630 <imagedata fileref="multi.pdf" format="PDF" scale="50"/>
1633 <imagedata fileref="multi.png" format="PNG"/>
1636 <!-- Fall back if none of the images can be used -->
1638 [Here there should be a diagram showing the progress of
1639 packages through the filters during a simple virtual-database
1640 search and a multi-database search, but is seems that your
1641 tool chain has not been able to include the diagram in this
1645 <!-- ### This used to work with an older version of DocBook
1647 <para>Caption: progress of packages through filters.</para>
1650 </inlinemediaobject>
1656 <chapter id="sru-server">
1657 <title>Combined SRU webservice and Z39.50 server configuration</title>
1659 Metaproxy can act as
1660 <ulink url="&url.sru;">SRU</ulink> and
1661 web service server, which translates web service requests to
1662 <ulink url="&url.z39.50;">ANSI/NISO Z39.50</ulink> packages and
1663 sends them off to common available targets.
1666 A typical setup for this operation needs a filter route including the
1670 <table id="sru-server-table-config" frame="top">
1671 <title>SRU/Z39.50 Server Filter Route Configuration</title>
1675 <entry>Filter</entry>
1676 <entry>Importance</entry>
1677 <entry>Purpose</entry>
1683 <entry><literal>frontend_net</literal></entry>
1684 <entry>required</entry>
1685 <entry>Accepting HTTP connections and passing them to following
1686 filters. Since this filter also accepts Z39.50 connections, the
1687 server works as SRU and Z39.50 server on the same port.</entry>
1690 <entry><literal>sru_z3950</literal></entry>
1691 <entry>required</entry>
1692 <entry>Accepting SRU GET/POST/SOAP explain and
1693 searchRetrieve requests for the the configured databases.
1694 Explain requests are directly served from the static XML configuration.
1695 SearchRetrieve requests are
1696 transformed to Z39.50 search and present packages.
1697 All other HTTP and Z39.50 packages are passed unaltered.</entry>
1700 <entry><literal>http_file</literal></entry>
1701 <entry>optional</entry>
1702 <entry>Serving HTTP requests from the filesystem. This is only
1703 needed if the server should serve XSLT stylesheets, static HTML
1704 files or Java Script for thin browser based clients.
1705 Z39.50 packages are passed unaltered.</entry>
1708 <entry><literal>cql_rpn</literal></entry>
1709 <entry>required</entry>
1710 <entry>Usually, Z39.50 servers do not talk CQL, hence the
1711 translation of the CQL query language to RPN is mandatory in
1712 most cases. Affects only Z39.50 search packages.</entry>
1715 <entry><literal>record_transform</literal></entry>
1716 <entry>optional</entry>
1717 <entry>Some Z39.50 backend targets can not present XML record
1718 syntaxes in common wanted element sets. using this filter, one
1719 can transform binary MARC records to MARCXML records, and
1720 further transform those to any needed XML schema/format by XSLT
1721 transformations. Changes only Z39.50 present packages.</entry>
1724 <entry><literal>session_shared</literal></entry>
1725 <entry>optional</entry>
1726 <entry>The stateless nature of web services requires frequent
1727 re-searching of the same targets for display of paged result set
1728 records. This might be an unacceptable burden for the accessed
1729 backend Z39.50 targets, and this mosule can be added for
1730 efficient backend target resource pooling.</entry>
1733 <entry><literal>z3950_client</literal></entry>
1734 <entry>required</entry>
1735 <entry>Finally, a Z39.50 package sink is needed in the filter
1736 chain to provide the response packages. The Z39.50 client module
1737 is used to access external targets over the network, but any
1738 coming local Z39.50 package sink could be used instead of.</entry>
1741 <entry><literal>bounce</literal></entry>
1742 <entry>required</entry>
1743 <entry>Any Metaproxy package arriving here did not do so by
1744 purpose, and is bounced back with connection closure. this
1745 prevents inifinite package hanging inside the SRU server.</entry>
1751 A typical minimal example <ulink url="&url.sru;">SRU</ulink>
1752 server configuration file is found in the tarball distribution at
1753 <literal>etc/config-sru-to-z3950.xml</literal>.
1756 Off course, any other metaproxy modules can be integrated into a
1757 SRU server solution, including, but not limited to, load balancing,
1758 multiple target querying
1759 (see <xref linkend="multidb"/>), and complex RPN query rewrites.
1766 <chapter id="extensions">
1767 <title>Writing extensions for Metaproxy</title>
1768 <para>### To be written</para>
1774 <chapter id="classes">
1775 <title>Classes in the Metaproxy source code</title>
1778 <section id="classes-introductory-notes">
1779 <title>Introductory notes</title>
1781 <emphasis>Stop! Do not read this!</emphasis>
1782 You won't enjoy it at all. You should just skip ahead to
1783 <xref linkend="reference"/>,
1785 <!-- The remainder of this paragraph is lifted verbatim from
1786 Douglas Adams' _Hitch Hiker's Guide to the Galaxy_, chapter 8 -->
1787 you things you really need to know, like the fact that the
1788 fabulously beautiful planet Bethselamin is now so worried about
1789 the cumulative erosion by ten billion visiting tourists a year
1790 that any net imbalance between the amount you eat and the amount
1791 you excrete whilst on the planet is surgically removed from your
1792 bodyweight when you leave: so every time you go to the lavatory it
1793 is vitally important to get a receipt.
1796 This chapter contains documentation of the Metaproxy source code, and is
1797 of interest only to maintainers and developers. If you need to
1798 change Metaproxy's behavior or write a new filter, then you will most
1799 likely find this chapter helpful. Otherwise it's a waste of your
1800 good time. Seriously: go and watch a film or something.
1801 <citetitle>This is Spinal Tap</citetitle> is particularly good.
1804 Still here? OK, let's continue.
1807 In general, classes seem to be named big-endianly, so that
1808 <literal>FactoryFilter</literal> is not a filter that filters
1809 factories, but a factory that produces filters; and
1810 <literal>FactoryStatic</literal> is a factory for the statically
1811 registered filters (as opposed to those that are dynamically
1816 <section id="individual.classes">
1817 <title>Individual classes</title>
1819 The classes making up the Metaproxy application are here listed by
1820 class-name, with the names of the source files that define them in
1824 <section id="class-FactoryFilter">
1825 <title><literal>mp::FactoryFilter</literal>
1826 (<filename>factory_filter.cpp</filename>)</title>
1828 A factory class that exists primarily to provide the
1829 <literal>create()</literal> method, which takes the name of a
1830 filter class as its argument and returns a new filter of that
1831 type. To enable this, the factory must first be populated by
1832 calling <literal>add_creator()</literal> for static filters (this
1833 is done by the <literal>FactoryStatic</literal> class, see below)
1834 and <literal>add_creator_dyn()</literal> for filters loaded
1839 <section id="class-FactoryStatic">
1840 <title><literal>mp::FactoryStatic</literal>
1841 (<filename>factory_static.cpp</filename>)</title>
1843 A subclass of <literal>FactoryFilter</literal> which is
1844 responsible for registering all the statically defined filter
1845 types. It does this by knowing about all those filters'
1846 structures, which are listed in its constructor. Merely
1847 instantiating this class registers all the static classes. It is
1848 for the benefit of this class that <literal>struct
1849 metaproxy_1_filter_struct</literal> exists, and that all the filter
1850 classes provide a static object of that type.
1854 <section id="class-filter-Base">
1855 <title><literal>mp::filter::Base</literal>
1856 (<filename>filter.cpp</filename>)</title>
1858 The virtual base class of all filters. The filter API is, on the
1859 surface at least, extremely simple: two methods.
1860 <literal>configure()</literal> is passed an XML DOM tree representing
1861 that part of the configuration file that pertains to this filter
1862 instance, and is expected to walk that tree extracting relevant
1863 information. And <literal>process()</literal> processes a
1864 package (see below). That surface simplicity is a bit
1865 misleading, as <literal>process()</literal> needs to know a lot
1866 about the <literal>Package</literal> class in order to do
1871 <section id="class-AuthSimple">
1872 <title><literal>mp::filter::AuthSimple</literal>,
1873 <literal>Backend_test</literal>, etc.
1874 (<filename>filter_auth_simple.cpp</filename>,
1875 <filename>filter_backend_test.cpp</filename>, etc.)</title>
1877 Individual filters. Each of these is implemented by a header and
1878 a source file, named <filename>filter_*.hpp</filename> and
1879 <filename>filter_*.cpp</filename> respectively. All the header
1880 files should be pretty much identical, in that they declare the
1881 class, including a private <literal>Rep</literal> class and a
1882 member pointer to it, and the two public methods.
1885 The source file for each filter needs to supply:
1890 A definition of the private <literal>Rep</literal> class.
1895 Some boilerplate constructors and destructors.
1900 A <literal>configure()</literal> method that uses the
1901 appropriate XML fragment.
1906 Most important, the <literal>process()</literal> method that
1907 does all the actual work.
1913 <section id="class-Package">
1914 <title><literal>mp::Package</literal>
1915 (<filename>package.cpp</filename>)</title>
1917 Represents a package on its way through the series of filters
1918 that make up a route. This is essentially a Z39.50 or SRU APDU
1919 together with information about where it came from, which is
1920 modified as it passes through the various filters.
1924 <section id="class-Pipe">
1925 <title><literal>mp::Pipe</literal>
1926 (<filename>pipe.cpp</filename>)</title>
1928 This class provides a compatibility layer so that we have an IPC
1929 mechanism that works the same under Unix and Windows. It's not
1930 particularly exciting.
1934 <section id="class-RouterChain">
1935 <title><literal>mp::RouterChain</literal>
1936 (<filename>router_chain.cpp</filename>)</title>
1942 <section id="class-RouterFleXML">
1943 <title><literal>mp::RouterFleXML</literal>
1944 (<filename>router_flexml.cpp</filename>)</title>
1950 <section id="class-Session">
1951 <title><literal>mp::Session</literal>
1952 (<filename>session.cpp</filename>)</title>
1958 <section id="class-ThreadPoolSocketObserver">
1959 <title><literal>mp::ThreadPoolSocketObserver</literal>
1960 (<filename>thread_pool_observer.cpp</filename>)</title>
1966 <section id="class-util">
1967 <title><literal>mp::util</literal>
1968 (<filename>util.cpp</filename>)</title>
1970 A namespace of various small utility functions and classes,
1971 collected together for convenience. Most importantly, includes
1972 the <literal>mp::util::odr</literal> class, a wrapper for YAZ's
1977 <section id="class-xml">
1978 <title><literal>mp::xml</literal>
1979 (<filename>xmlutil.cpp</filename>)</title>
1981 A namespace of various XML utility functions and classes,
1982 collected together for convenience.
1988 <section id="other.source.files">
1989 <title>Other Source Files</title>
1991 In addition to the Metaproxy source files that define the classes
1992 described above, there are a few additional files which are
1993 briefly described here:
1997 <term><literal>metaproxy_prog.cpp</literal></term>
2000 The main function of the <command>metaproxy</command> program.
2005 <term><literal>ex_router_flexml.cpp</literal></term>
2008 Identical to <literal>metaproxy_prog.cpp</literal>: it's not clear why.
2013 <term><literal>test_*.cpp</literal></term>
2016 Unit-tests for various modules.
2022 ### Still to be described:
2023 <literal>ex_filter_frontend_net.cpp</literal>,
2024 <literal>filter_dl.cpp</literal>,
2025 <literal>plainfile.cpp</literal>,
2026 <literal>tstdl.cpp</literal>.
2032 <reference id="reference">
2033 <title>Reference</title>
2034 <partintro id="reference-introduction">
2036 The material in this chapter is drawn directly from the individual
2037 manual entries. In particular, the Metaproxy invocation section is
2038 available using <command>man metaproxy</command>, and the section
2039 on each individual filter is available using the name of the filter
2040 as the argument to the <command>man</command> command.
2046 <appendix id="license">
2047 <title>License</title>
2050 Metaproxy, Copyright © 1995-2007 Index Data ApS.
2054 Metaproxy is free software; you can redistribute it and/or modify it under
2055 the terms of the GNU General Public License as published by the Free
2056 Software Foundation; either version 2, or (at your option) any later
2061 Metaproxy is distributed in the hope that it will be useful, but WITHOUT ANY
2062 WARRANTY; without even the implied warranty of MERCHANTABILITY or
2063 FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License
2068 You should have received a copy of the GNU General Public License
2069 along with Metaproxy; see the file LICENSE. If not, write to the
2070 Free Software Foundation,
2071 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA
2075 <title>GNU General Public License</title>
2077 GNU GENERAL PUBLIC LICENSE
2078 Version 2, June 1991
2080 Copyright (C) 1989, 1991 Free Software Foundation, Inc.
2081 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA
2082 w Everyone is permitted to copy and distribute verbatim copies
2083 of this license document, but changing it is not allowed.
2087 The licenses for most software are designed to take away your
2088 freedom to share and change it. By contrast, the GNU General Public
2089 License is intended to guarantee your freedom to share and change free
2090 software--to make sure the software is free for all its users. This
2091 General Public License applies to most of the Free Software
2092 Foundation's software and to any other program whose authors commit to
2093 using it. (Some other Free Software Foundation software is covered by
2094 the GNU Library General Public License instead.) You can apply it to
2097 When we speak of free software, we are referring to freedom, not
2098 price. Our General Public Licenses are designed to make sure that you
2099 have the freedom to distribute copies of free software (and charge for
2100 this service if you wish), that you receive source code or can get it
2101 if you want it, that you can change the software or use pieces of it
2102 in new free programs; and that you know you can do these things.
2104 To protect your rights, we need to make restrictions that forbid
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2106 These restrictions translate to certain responsibilities for you if you
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2109 For example, if you distribute copies of such a program, whether
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2112 source code. And you must show them these terms so they know their
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2126 Finally, any free program is threatened constantly by software
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2135 GNU GENERAL PUBLIC LICENSE
2136 TERMS AND CONDITIONS FOR COPYING, DISTRIBUTION AND MODIFICATION
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