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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 1.32 but this is no longer supported.
162 Metaproxy is known to work with Boost version 1.33 / 1.34.
169 In order to compile Metaproxy a modern C++ compiler is
170 required. Boost, in particular, requires the C++ compiler
171 to facilitate the newest features. Refer to Boost
172 <ulink url="&url.boost.compilers.status;">Compiler Status</ulink>
173 for more information.
176 We have successfully built Metaproxy using the compilers
177 <ulink url="&url.gcc;">GCC</ulink> version 4.0 and
178 <ulink url="&url.vstudio;">Microsoft Visual Studio</ulink> 2003/2005.
181 <section id="installation.unix">
182 <title>Installation on Unix (from Source)</title>
184 Here is a quick step-by-step guide on how to compile all the
185 tools that Metaproxy uses. Only few systems have none of the required
186 tools binary packages. If, for example, Libxml2/libxslt are already
187 installed as development packages use those (and omit compilation).
194 gunzip -c libxml2-version.tar.gz|tar xf -
202 gunzip -c libxslt-version.tar.gz|tar xf -
213 gunzip -c yaz-version.tar.gz|tar xf -
221 gunzip -c yazpp-version.tar.gz|tar xf -
232 gunzip -c boost-version.tar.gz|tar xf -
243 gunzip -c metaproxy-version.tar.gz|tar xf -
252 <section id="installation.debian">
253 <title>Installation on Debian GNU/Linux</title>
255 All dependencies for Metaproxy are available as
256 <ulink url="&url.debian;">Debian</ulink>
257 packages for the sarge (stable in 2005) and etch (testing in 2005)
261 The procedures for Debian based systems, such as
262 <ulink url="&url.ubuntu;">Ubuntu</ulink> is probably similar
265 There is currently no official Debian package for YAZ++.
266 And the official Debian package for YAZ is probably too old.
267 But Index Data bulds "new" versions of those for Debian (i386 only).
270 Update the <filename>/etc/apt/sources.list</filename>
271 to include the Index Data repository.
272 See YAZ' <ulink url="&url.yaz.download.debian;">Download Debian</ulink>
273 for more information.
276 apt-get install libxslt1-dev
277 apt-get install libyazpp2-dev
278 apt-get install libboost-dev
279 apt-get install libboost-thread-dev
280 apt-get install libboost-test-dev
283 With these packages installed, the usual configure + make
284 procedure can be used for Metaproxy as outlined in
285 <xref linkend="installation.unix"/>.
289 <section id="installation.rpm">
290 <title>Installation on RPM based Linux Systems</title>
292 All external dependencies for Metaproxy are available as
293 RPM packages, either from your distribution site, or from the
294 <ulink url="http://fr.rpmfind.net/">RPMfind</ulink> site.
297 For example, an installation of the requires Boost C++ development
298 libraries on RedHat Fedora C4 and C5 can be done like this:
300 wget ftp://fr.rpmfind.net/wlinux/fedora/core/updates/testing/4/SRPMS/boost-1.33.0-3.fc4.src.rpm
301 sudo rpmbuild --buildroot src/ --rebuild -p fc4/boost-1.33.0-3.fc4.src.rpm
302 sudo rpm -U /usr/src/redhat/RPMS/i386/boost-*rpm
306 The <ulink url="&url.yaz;">YAZ</ulink> library is needed to
307 compile &metaproxy;, see there
308 for more information on available RPM packages.
311 There is currently no official RPM package for YAZ++.
312 See the <ulink url="&url.yazplusplus;">YAZ++</ulink> pages
313 for more information on a Unix tarball install.
316 With these packages installed, the usual configure + make
317 procedure can be used for Metaproxy as outlined in
318 <xref linkend="installation.unix"/>.
322 <section id="installation.windows">
323 <title>Installation on Windows</title>
325 Metaproxy can be compiled with Microsoft
326 <ulink url="&url.vstudio;">Visual Studio</ulink>.
327 Version 2003 (C 7.1) and 2005 (C 8.0) is known to work.
329 <section id="installation.windows.boost">
332 For Windows, it's easiest to get the precompiled Boost
333 package from <ulink url="&url.boost.windows.download;">here</ulink>.
334 Several versions of the Boost libraries may be selected when
335 installing Boost for windows. Please choose at least the
336 <emphasis>multithreaded</emphasis> (non-DLL) version because
337 the Metaproxy makefile uses that.
340 For more information about installing Boost refer to the
341 <ulink url="&url.boost.getting.started;">getting started</ulink>
346 <section id="installation.windows.libxslt">
347 <title>Libxslt</title>
349 <ulink url="&url.libxslt;">Libxslt</ulink> can be downloaded
351 <ulink url="&url.libxml2.download.win32;">here</ulink>.
354 Libxslt has other dependencies, but these can all be downloaded
355 from the same site. Get the following package:
356 iconv, zlib, libxml2, libxslt.
360 <section id="installation.windows.yaz">
363 <ulink url="&url.yaz;">YAZ</ulink> can be downloaded
365 <ulink url="&url.yaz.download.win32;">here</ulink>.
369 <section id="installation.windows.yazplusplus">
372 Get <ulink url="&url.yazplusplus;">YAZ++</ulink> as well.
373 Version 1.0.3 or later is required.
376 YAZ++ includes NMAKE makefiles, similar to those found in the
381 <section id="installation.windows.metaproxy">
382 <title>Metaproxy</title>
384 Metaproxy is shipped with NMAKE makefiles as well - similar
385 to those found in the YAZ++/YAZ packages. Adjust this Makefile
386 to point to the proper locations of Boost, Libxslt, Libxml2,
387 zlib, iconv, yaz and yazpp.
391 <varlistentry><term><literal>DEBUG</literal></term>
393 If set to 1, the software is
394 compiled with debugging libraries (code generation is
395 multi-threaded debug DLL).
396 If set to 0, the software is compiled with release libraries
397 (code generation is multi-threaded DLL).
402 <term><literal>BOOST</literal></term>
405 Boost install location
411 <term><literal>BOOST_VERSION</literal></term>
414 Boost version (replace . with _).
420 <term><literal>BOOST_TOOLSET</literal></term>
429 <term><literal>LIBXSLT_DIR</literal>,
430 <literal>LIBXML2_DIR</literal> ..</term>
433 Specify the locations of Libxslt, libiconv, libxml2 and
442 After successful compilation you'll find
443 <literal>metaproxy.exe</literal> in the
444 <literal>bin</literal> directory.
452 <chapter id="yazproxy-comparison">
453 <title>YAZ Proxy Comparison</title>
455 The table below lists facilities either supported by either
456 <ulink url="&url.yazproxy;">YAZ Proxy</ulink> or Metaproxy.
458 <table id="yazproxy-comparison-table">
459 <title>Metaproxy / YAZ Proxy comparison</title>
463 <entry>Facility</entry>
464 <entry>Metaproxy</entry>
465 <entry>YAZ Proxy</entry>
470 <entry>Z39.50 server</entry>
471 <entry>Using filter <literal>frontend_net</literal></entry>
472 <entry>Supported</entry>
475 <entry>SRU server</entry>
476 <entry>Supported with filter <literal>sru_z3950</literal></entry>
477 <entry>Supported</entry>
480 <entry>Z39.50 client</entry>
481 <entry>Supported with filter <literal>z3950_client</literal></entry>
482 <entry>Supported</entry>
485 <entry>SRU client</entry>
486 <entry>Unsupported</entry>
487 <entry>Unsupported</entry>
490 <entry>Connection reuse</entry>
491 <entry>Supported with filter <literal>session_shared</literal></entry>
492 <entry>Supported</entry>
495 <entry>Connection share</entry>
496 <entry>Supported with filter <literal>session_shared</literal></entry>
497 <entry>Unsupported</entry>
500 <entry>Result set reuse</entry>
501 <entry>Supported with filter <literal>session_shared</literal></entry>
502 <entry>Within one Z39.50 session / HTTP keep-alive</entry>
505 <entry>Record cache</entry>
506 <entry>Unsupported</entry>
507 <entry>Supported for last result set within one Z39.50/HTTP-keep alive session</entry>
510 <entry>Z39.50 Virtual database, i.e. select any Z39.50 target for database</entry>
511 <entry>Supported with filter <literal>virt_db</literal></entry>
512 <entry>Unsupported</entry>
515 <entry>SRU Virtual database, i.e. select any Z39.50 target for path</entry>
516 <entry>Supported with filter <literal>virt_db</literal>,
517 <literal>sru_z3950</literal></entry>
518 <entry>Supported</entry>
521 <entry>Multi target search</entry>
522 <entry>Supported with filter <literal>multi</literal> (round-robin)</entry>
523 <entry>Unsupported</entry>
526 <entry>Retrieval and search limits</entry>
527 <entry>Unsupported</entry>
528 <entry>Supported</entry>
531 <entry>Bandwidth limits</entry>
532 <entry>Unsupported</entry>
533 <entry>Supported</entry>
536 <entry>Connect limits</entry>
537 <entry>Unsupported</entry>
538 <entry>Supported</entry>
541 <entry>Retrieval sanity check and conversions</entry>
542 <entry>Supported using filter <literal>record_transform</literal></entry>
543 <entry>Supported</entry>
546 <entry>Query check</entry>
548 Supported in a limited way using <literal>query_rewrite</literal>
550 <entry>Supported</entry>
553 <entry>Query rewrite</entry>
554 <entry>Supported with <literal>query_rewrite</literal></entry>
555 <entry>Unsupported</entry>
558 <entry>Session invalidate for -1 hits</entry>
559 <entry>Unsupported</entry>
560 <entry>Supported</entry>
563 <entry>Architecture</entry>
564 <entry>Multi-threaded + select for networked modules such as
565 <literal>frontend_net</literal>)</entry>
566 <entry>Single-threaded using select</entry>
570 <entry>Extensability</entry>
571 <entry>Most functionality implemented as loadable modules</entry>
572 <entry>Unsupported and experimental</entry>
576 <entry><ulink url="&url.usemarcon;">USEMARCON</ulink></entry>
577 <entry>Unsupported</entry>
578 <entry>Supported</entry>
582 <entry>Portability</entry>
584 Requires YAZ, YAZ++ and modern C++ compiler supporting
585 <ulink url="&url.boost;">Boost</ulink>.
588 Requires YAZ and YAZ++.
589 STL is not required so pretty much any C++ compiler out there should work.
598 <chapter id="architecture">
599 <title>The Metaproxy Architecture</title>
601 The Metaproxy architecture is based on three concepts:
602 the <emphasis>package</emphasis>,
603 the <emphasis>route</emphasis>
604 and the <emphasis>filter</emphasis>.
608 <term>Packages</term>
611 A package is request or response, encoded in some protocol,
612 issued by a client, making its way through Metaproxy, send to or
613 received from a server, or sent back to the client.
616 The core of a package is the protocol unit - for example, a
617 Z39.50 Init Request or Search Response, or an SRU searchRetrieve
618 URL or Explain Response. In addition to this core, a package
619 also carries some extra information added and used by Metaproxy
623 In general, packages are doctored as they pass through
624 Metaproxy. For example, when the proxy performs authentication
625 and authorization on a Z39.50 Init request, it removes the
626 authentication credentials from the package so that they are not
627 passed onto the back-end server; and when search-response
628 packages are obtained from multiple servers, they are merged
629 into a single unified package that makes its way back to the
638 Packages make their way through routes, which can be thought of
639 as programs that operate on the package data-type. Each
640 incoming package initially makes its way through a default
641 route, but may be switched to a different route based on various
642 considerations. Routes are made up of sequences of filters (see
651 Filters provide the individual instructions within a route, and
652 effect the necessary transformations on packages. A particular
653 configuration of Metaproxy is essentially a set of filters,
654 described by configuration details and arranged in order in one
655 or more routes. There are many kinds of filter - about a dozen
656 at the time of writing with more appearing all the time - each
657 performing a specific function and configured by different
661 The word ``filter'' is sometimes used rather loosely, in two
662 different ways: it may be used to mean a particular
663 <emphasis>type</emphasis> of filter, as when we speak of ``the
664 auth_simple filter'' or ``the multi filter''; or it may be used
665 to be a specific <emphasis>instance</emphasis> of a filter
666 within a Metaproxy configuration. For example, a single
667 configuration will often contain multiple instances of the
668 <literal>z3950_client</literal> filter. In
669 operational terms, of these is a separate filter. In practice,
670 context always make it clear which sense of the word ``filter''
674 Extensibility of Metaproxy is primarily through the creation of
675 plugins that provide new filters. The filter API is small and
676 conceptually simple, but there are many details to master. See
678 <link linkend="filters">Filters</link>.
684 Since packages are created and handled by the system itself, and
685 routes are conceptually simple, most of the remainder of this
686 document concentrates on filters. After a brief overview of the
687 filter types follows, along with some thoughts on possible future
694 <chapter id="filters">
695 <title>Filters</title>
698 <section id="filters-introductory-notes">
699 <title>Introductory notes</title>
701 It's useful to think of Metaproxy as an interpreter providing a small
702 number of primitives and operations, but operating on a very
703 complex data type, namely the ``package''.
706 A package represents a Z39.50 or SRU/W request (whether for Init,
707 Search, Scan, etc.) together with information about where it came
708 from. Packages are created by front-end filters such as
709 <literal>frontend_net</literal> (see below), which reads them from
710 the network; other front-end filters are possible. They then pass
711 along a route consisting of a sequence of filters, each of which
712 transforms the package and may also have side-effects such as
713 generating logging. Eventually, the route will yield a response,
714 which is sent back to the origin.
717 There are many kinds of filter: some that are defined statically
718 as part of Metaproxy, and others may be provided by third parties
719 and dynamically loaded. They all conform to the same simple API
720 of essentially two methods: <function>configure()</function> is
721 called at startup time, and is passed an XML DOM tree representing that
722 part of the configuration file that pertains to this filter
723 instance: it is expected to walk that tree extracting relevant
724 information; and <function>process()</function> is called every
725 time the filter has to processes a package.
728 While all filters provide the same API, there are different modes
729 of functionality. Some filters are sources: they create
731 (<literal>frontend_net</literal>);
732 others are sinks: they consume packages and return a result
733 (<literal>backend_test</literal>,
734 <literal>bounce</literal>,
735 <literal>http_file</literal>,
736 <literal>z3950_client</literal>);
737 the others are true filters, that read, process and pass on the
738 packages they are fed
739 (<literal>auth_simple</literal>,
740 <literal>log</literal>,
741 <literal>multi</literal>,
742 <literal>query_rewrite</literal>,
743 <literal>record_transform</literal>,
744 <literal>session_shared</literal>,
745 <literal>sru_z3950</literal>,
746 <literal>template</literal>,
747 <literal>virt_db</literal>).
752 <section id="overview.filter.types">
753 <title>Overview of filter types</title>
755 We now briefly consider each of the types of filter supported by
756 the core Metaproxy binary. This overview is intended to give a
757 flavor of the available functionality; more detailed information
758 about each type of filter is included below in
759 <xref linkend="reference"/>.
762 The filters are here named by the string that is used as the
763 <literal>type</literal> attribute of a
764 <literal><filter></literal> element in the configuration
765 file to request them, with the name of the class that implements
766 them in parentheses. (The classname is not needed for normal
767 configuration and use of Metaproxy; it is useful only to
771 The filters are here listed in alphabetical order:
778 New virt_db-alike that does inteligent peer choice, explain merging,
779 adds FD&N to explain. Keeps init responses (like "virt_db Classic"),
780 makes routing choices based on local explain knowledge. Ref IDDI
783 Filter to convert Explain Classic to ZeeRex.
785 CQL2PQF (which needs augmented ZeeRex) - MARC for Talis.
789 Figure out what additional information we need in:
790 ZeeRex (check against D3.1)
791 Init request (e.g. loop detection)
792 Query package (e.g. number of hops)
793 Query response (e.g. record source)
797 <section id="auth_simple">
798 <title><literal>auth_simple</literal>
799 (mp::filter::AuthSimple)</title>
801 Simple authentication and authorization. The configuration
802 specifies the name of a file that is the user register, which
803 lists <varname>username</varname>:<varname>password</varname>
804 pairs, one per line, colon separated. When a session begins, it
805 is rejected unless username and passsword are supplied, and match
806 a pair in the register. The configuration file may also specific
807 the name of another file that is the target register: this lists
808 lists <varname>username</varname>:<varname>dbname</varname>,<varname>dbname</varname>...
809 sets, one per line, with multiple database names separated by
810 commas. When a search is processed, it is rejected unless the
811 database to be searched is one of those listed as available to
816 <section id="backend_test">
817 <title><literal>backend_test</literal>
818 (mp::filter::Backend_test)</title>
820 A partial sink that provides dummy responses in the manner of the
821 <literal>yaz-ztest</literal> Z39.50 server. This is useful only
822 for testing. Seriously, you don't need this. Pretend you didn't
823 even read this section.
827 <section id="bounce">
828 <title><literal>bounce</literal>
829 (mp::filter::Bounce)</title>
831 A sink that swallows <emphasis>all packages</emphasis>,
832 and returns them almost unprocessed.
833 It never sends any package of any type further down the row, but
834 sets Z39.50 packages to Z_Close, and HTTP_Request packages to
835 HTTP_Response err code 400 packages, and adds a suitable bounce
837 The bounce filter is usually added at end of each filter chain route
838 to prevent infinite hanging of for example HTTP
839 requests packages when only the Z39.50 client partial sink
840 filter is found in the
845 <section id="cql_rpn">
846 <title><literal>cql_rpn</literal>
847 (mp::filter::CQLtoRPN)</title>
849 A query language transforming filter which catches Z39.50
850 <literal>searchRequest</literal>
851 packages containing <literal>CQL</literal> queries, transforms
852 those to <literal>RPN</literal> queries,
853 and sends the <literal>searchRequests</literal> on to the next
854 filters. It is among other things useful in a SRU context.
858 <section id="frontend_net">
859 <title><literal>frontend_net</literal>
860 (mp::filter::FrontendNet)</title>
862 A source that accepts Z39.50 connections from a port
863 specified in the configuration, reads protocol units, and
864 feeds them into the next filter in the route. When the result is
865 received, it is returned to the original origin.
869 <section id="http_file">
870 <title><literal>http_file</literal>
871 (mp::filter::HttpFile)</title>
873 A partial sink which swallows only
874 <literal>HTTP_Request</literal> packages, and
875 returns the contents of files from the local
876 filesystem in response to HTTP requests.
877 It lets Z39.50 packages and all other forthcoming package types
880 does mean that Metaproxy is also a Web-server in its spare time. So
881 far it does not contain either an email-reader or a Lisp
882 interpreter, but that day is surely coming.)
886 <section id="load_balance">
887 <title><literal>load_balance</literal>
888 (mp::filter::LoadBalance)</title>
890 Performs load balancing for incoming Z39.50 init requests.
891 It is used together with the <literal>virt_db</literal> filter,
892 but unlike the <literal>multi</literal> filter it does send an
893 entire session to only one of the virtual backends. The
894 <literal>load_balance</literal> filter is assuming that
895 all backend targets have equal content, and chooses the backend
896 with least load cost for a new session.
899 This filter is experimental and yet not mature for heavy load
907 <title><literal>log</literal>
908 (mp::filter::Log)</title>
910 Writes logging information to standard output, and passes on
911 the package unchanged. A log file name can be specified, as well
912 as multiple different logging formats.
917 <title><literal>multi</literal>
918 (mp::filter::Multi)</title>
920 Performs multi-database searching.
922 <link linkend="multidb">the extended discussion</link>
923 of virtual databases and multi-database searching below.
927 <section id="query_rewrite">
928 <title><literal>query_rewrite</literal>
929 (mp::filter::QueryRewrite)</title>
931 Rewrites Z39.50 <literal>Type-1</literal>
932 and <literal>Type-101</literal> (``<literal>RPN</literal>'')
934 three-step process: the query is transliterated from Z39.50
935 packet structures into an XML representation; that XML
936 representation is transformed by an XSLT stylesheet; and the
937 resulting XML is transliterated back into the Z39.50 packet
943 <section id="record_transform">
944 <title><literal>record_transform</literal>
945 (mp::filter::RecordTransform)</title>
947 This filter acts only on Z3950 present requests, and let all
948 other types of packages and requests pass untouched. It's use is
949 twofold: blocking Z3950 present requests, which the backend
950 server does not understand and can not honor, and transforming
951 the present syntax and elementset name according to the rules
952 specified, to fetch only existing record formats, and transform
953 them on the fly to requested record syntaxes.
957 <section id="session_shared">
958 <title><literal>session_shared</literal>
959 (mp::filter::SessionShared)</title>
961 This filter implements global sharing of
962 result sets (i.e. between threads and therefore between
963 clients), yielding performance improvements by clever resource
968 <section id="sru_z3950">
969 <title><literal>sru_z3950</literal>
970 (mp::filter::SRUtoZ3950)</title>
972 This filter transforms valid
973 SRU GET/POST/SOAP searchRetrieve requests to Z3950 init, search,
974 and present requests, and wraps the
975 received hit counts and XML records into suitable SRU response
977 The <literal>sru_z3950</literal> filter processes also SRU
978 GET/POST/SOAP explain requests, returning
979 either the absolute minimum required by the standard, or a full
980 pre-defined ZeeReX explain record.
982 <ulink url="&url.zeerex.explain;">ZeeReX Explain</ulink>
983 standard pages and the
984 <ulink url="&url.sru.explain;">SRU Explain</ulink> pages
985 for more information on the correct explain syntax.
986 SRU scan requests are not supported yet.
990 <section id="template">
991 <title><literal>template</literal>
992 (mp::filter::Template)</title>
994 Does nothing at all, merely passing the packet on. (Maybe it
995 should be called <literal>nop</literal> or
996 <literal>passthrough</literal>?) This exists not to be used, but
997 to be copied - to become the skeleton of new filters as they are
998 written. As with <literal>backend_test</literal>, this is not
999 intended for civilians.
1003 <section id="virt_db">
1004 <title><literal>virt_db</literal>
1005 (mp::filter::VirtualDB)</title>
1007 Performs virtual database selection: based on the name of the
1008 database in the search request, a server is selected, and its
1009 address added to the request in a <literal>VAL_PROXY</literal>
1010 otherInfo packet. It will subsequently be used by a
1011 <literal>z3950_client</literal> filter.
1013 <link linkend="multidb">the extended discussion</link>
1014 of virtual databases and multi-database searching below.
1018 <section id="z3950_client">
1019 <title><literal>z3950_client</literal>
1020 (mp::filter::Z3950Client)</title>
1022 A partial sink which swallows only Z39.50 packages.
1023 It performs Z39.50 searching and retrieval by proxying the
1024 packages that are passed to it. Init requests are sent to the
1025 address specified in the <literal>VAL_PROXY</literal> otherInfo
1026 attached to the request: this may have been specified by client,
1027 or generated by a <literal>virt_db</literal> filter earlier in
1028 the route. Subsequent requests are sent to the same address,
1029 which is remembered at Init time in a Session object.
1030 HTTP_Request packages and all other forthcoming package types
1031 are passed untouched.
1036 <section id="zeerex_explain">
1037 <title><literal>zeerex_explain</literal>
1038 (mp::filter::ZeerexExplain)</title>
1040 This filter acts as a sink for
1041 Z39.50 explain requests, returning a static ZeeReX
1042 Explain XML record from the config section. All other packages
1045 <ulink url="&url.zeerex.explain;">ZeeReX Explain</ulink>
1047 for more information on the correct explain syntax.
1051 This filter is not yet completed.
1060 <section id="future.directions">
1061 <title>Future directions</title>
1063 Some other filters that do not yet exist, but which would be
1064 useful, are briefly described. These may be added in future
1065 releases (or may be created by third parties, as loadable
1071 <term><literal>frontend_cli</literal> (source)</term>
1074 Command-line interface for generating requests.
1079 <term><literal>sru_client</literal> (sink)</term>
1082 SRU/GET and SRU/SOAP searching and retrieval.
1087 <term><literal>opensearch_client</literal> (sink)</term>
1090 A9 OpenSearch searching and retrieval.
1100 <chapter id="configuration">
1101 <title>Configuration: the Metaproxy configuration file format</title>
1104 <section id="configuration-introductory-notes">
1105 <title>Introductory notes</title>
1107 If Metaproxy is an interpreter providing operations on packages, then
1108 its configuration file can be thought of as a program for that
1109 interpreter. Configuration is by means of a single XML file, the name
1110 of which is supplied as the sole command-line argument to the
1111 <command>metaproxy</command> program. (See
1112 <xref linkend="reference"/> below for more information on invoking
1117 <section id="overview.xml.structure">
1118 <title>Overview of the config file XML structure</title>
1120 All elements and attributes are in the namespace
1121 <ulink url="http://indexdata.com/metaproxy"/>.
1122 This is most easily achieved by setting the default namespace on
1123 the top-level element, as here:
1126 <metaproxy xmlns="http://indexdata.com/metaproxy" version="1.0">
1129 The top-level element is <metaproxy>. This contains a
1130 <start> element, a <filters> element and a
1131 <routes> element, in that order. <filters> is
1132 optional; the other two are mandatory. All three are
1136 The <start> element is empty, but carries a
1137 <literal>route</literal> attribute, whose value is the name of
1138 route at which to start running - analogous to the name of the
1139 start production in a formal grammar.
1142 If present, <filters> contains zero or more <filter>
1143 elements. Each filter carries a <literal>type</literal> attribute
1144 which specifies what kind of filter is being defined
1145 (<literal>frontend_net</literal>, <literal>log</literal>, etc.)
1146 and contain various elements that provide suitable configuration
1147 for a filter of its type. The filter-specific elements are
1149 <xref linkend="reference"/>.
1150 Filters defined in this part of the file must carry an
1151 <literal>id</literal> attribute so that they can be referenced
1155 <routes> contains one or more <route> elements, each
1156 of which must carry an <literal>id</literal> element. One of the
1157 routes must have the ID value that was specified as the start
1158 route in the <start> element's <literal>route</literal>
1159 attribute. Each route contains zero or more <filter>
1160 elements. These are of two types. They may be empty, but carry a
1161 <literal>refid</literal> attribute whose value is the same as the
1162 <literal>id</literal> of a filter previously defined in the
1163 <filters> section. Alternatively, a route within a filter
1164 may omit the <literal>refid</literal> attribute, but contain
1165 configuration elements similar to those used for filters defined
1166 in the <filters> section. (In other words, each filter in a
1167 route may be included either by reference or by physical
1173 <section id="example.configuration">
1174 <title>An example configuration</title>
1176 The following is a small, but complete, Metaproxy configuration
1177 file (included in the distribution as
1178 <literal>metaproxy/etc/config1.xml</literal>).
1179 This file defines a very simple configuration that simply proxies
1180 to whatever back-end server the client requests, but logs each
1181 request and response. This can be useful for debugging complex
1182 client-server dialogues.
1184 <screen><![CDATA[<?xml version="1.0"?>
1185 <metaproxy xmlns="http://indexdata.com/metaproxy" version="1.0">
1186 <start route="start"/>
1188 <filter id="frontend" type="frontend_net">
1191 <filter id="backend" type="z3950_client">
1196 <filter refid="frontend"/>
1197 <filter type="log"/>
1198 <filter refid="backend"/>
1199 <filter type="bounce"/>
1205 It works by defining a single route, called
1206 <literal>start</literal>, which consists of a sequence of four
1207 filters. The first and last of these are included by reference:
1208 their <literal><filter></literal> elements have
1209 <literal>refid</literal> attributes that refer to filters defined
1210 within the prior <literal><filters></literal> section. The
1211 middle filter is included inline in the route.
1214 The four filters in the route are as follows: first, a
1215 <literal>frontend_net</literal> filter accepts Z39.50 requests
1216 from any host on port 9000; then these requests are passed through
1217 a <literal>log</literal> filter that emits a message for each
1218 request; they are then fed into a <literal>z3950_client</literal>
1219 filter, which forwards all Z39.50 requests to the client-specified
1220 back-end Z39.509 server. Those Z39.50 packages are returned by the
1221 <literal>z3950_client</literal> filter, with the response data
1222 filled by the external Z39.50 server targeted.
1223 All non-Z39.50 packages are passed through to the
1224 <literal>bounce</literal> filter, which definitely bounces
1225 everything, including fish, bananas, cold pyjamas,
1226 mutton, beef and trout packages.
1227 When the response arrives, it is handed
1228 back to the <literal>log</literal> filter, which emits another
1229 message; and then to the <literal>frontend_net</literal> filter,
1230 which returns the response to the client.
1234 <section id="config-file-modularity">
1235 <title>Config file modularity</title>
1237 Metaproxy XML configuration snippets can be reused by other
1238 filters using the <literal>XInclude</literal> standard, as seen in
1239 the <literal>/etc/config-sru-to-z3950.xml</literal> example SRU
1242 <filter id="sru" type="sru_z3950">
1243 <database name="Default">
1244 <xi:include xmlns:xi="http://www.w3.org/2001/XInclude"
1245 href="explain.xml"/>
1252 <section id="config-file-syntax-check">
1253 <title>Config file syntax checking</title>
1255 The distribution contains RelaxNG Compact and XML syntax checking
1256 files, as well as XML Schema files. These are found in the
1259 xml/schema/metaproxy.rnc
1260 xml/schema/metaproxy.rng
1261 xml/schema/metaproxy.xsd
1263 and can be used to verify or debug the XML structure of
1264 configuration files. For example, using the utility
1265 <filename>xmllint</filename>, syntax checking is done like this:
1267 xmllint --noout --schema xml/schema/metaproxy.xsd etc/config-local.xml
1268 xmllint --noout --relaxng xml/schema/metaproxy.rng etc/config-local.xml
1270 (A recent version of <literal>libxml2</literal> is required, as
1271 support for XML Schemas is a relatively recent addition.)
1274 You can of course use any other RelaxNG or XML Schema compliant tool
1282 <chapter id="multidb">
1283 <title>Virtual databases and multi-database searching</title>
1286 <section id="multidb-introductory-notes">
1287 <title>Introductory notes</title>
1289 Two of Metaproxy's filters are concerned with multiple-database
1290 operations. Of these, <literal>virt_db</literal> can work alone
1291 to control the routing of searches to one of a number of servers,
1292 while <literal>multi</literal> can work together with
1293 <literal>virt_db</literal> to perform multi-database searching, merging
1294 the results into a unified result-set - ``metasearch in a box''.
1297 The interaction between
1298 these two filters is necessarily complex: it reflects the real,
1299 irreducible complexity of multi-database searching in a protocol such
1300 as Z39.50 that separates initialization from searching, and in
1301 which the database to be searched is not known at initialization
1305 It's possible to use these filters without understanding the
1306 details of their functioning and the interaction between them; the
1307 next two sections of this chapter are ``HOW-TO'' guides for doing
1308 just that. However, debugging complex configurations will require
1309 a deeper understanding, which the last two sections of this
1310 chapters attempt to provide.
1315 <section id="multidb.virt_db">
1316 <title>Virtual databases with the <literal>virt_db</literal> filter</title>
1318 Working alone, the purpose of the
1319 <literal>virt_db</literal>
1320 filter is to route search requests to one of a selection of
1321 back-end databases. In this way, a single Z39.50 endpoint
1322 (running Metaproxy) can provide access to several different
1323 underlying services, including those that would otherwise be
1324 inaccessible due to firewalls. In many useful configurations, the
1325 back-end databases are local to the Metaproxy installation, but
1326 the software does not enforce this, and any valid Z39.50 servers
1327 may be used as back-ends.
1330 For example, a <literal>virt_db</literal>
1331 filter could be set up so that searches in the virtual database
1332 ``lc'' are forwarded to the Library of Congress bibliographic
1333 catalogue server, and searches in the virtual database ``marc''
1334 are forwarded to the toy database of MARC records that Index Data
1335 hosts for testing purposes. A <literal>virt_db</literal>
1336 configuration to make this switch would look like this:
1338 <screen><![CDATA[<filter type="virt_db">
1340 <database>lc</database>
1341 <target>z3950.loc.gov:7090/voyager</target>
1344 <database>marc</database>
1345 <target>indexdata.com/marc</target>
1347 </filter>]]></screen>
1349 As well as being useful in it own right, this filter also provides
1350 the foundation for multi-database searching.
1355 <section id="multidb.multi">
1356 <title>Multi-database search with the <literal>multi</literal> filter</title>
1358 To arrange for Metaproxy to broadcast searches to multiple back-end
1359 servers, the configuration needs to include two components: a
1360 <literal>virt_db</literal>
1361 filter that specifies multiple
1362 <literal><target></literal>
1363 elements, and a subsequent
1364 <literal>multi</literal>
1365 filter. Here, for example, is a complete configuration that
1366 broadcasts searches to both the Library of Congress catalogue and
1367 Index Data's tiny testing database of MARC records:
1369 <screen><![CDATA[<?xml version="1.0"?>
1370 <metaproxy xmlns="http://indexdata.com/metaproxy" version="1.0">
1371 <start route="start"/>
1374 <filter type="frontend_net">
1375 <threads>10</threads>
1378 <filter type="virt_db">
1380 <database>lc</database>
1381 <target>z3950.loc.gov:7090/voyager</target>
1384 <database>marc</database>
1385 <target>indexdata.com/marc</target>
1388 <database>all</database>
1389 <target>z3950.loc.gov:7090/voyager</target>
1390 <target>indexdata.com/marc</target>
1393 <filter type="multi"/>
1394 <filter type="z3950_client">
1395 <timeout>30</timeout>
1397 <filter type="bounce"/>
1400 </metaproxy>]]></screen>
1403 <literal>virt_db</literal>
1404 filter that specifies multiple
1405 <literal><target></literal>
1406 elements but without a subsequent
1407 <literal>multi</literal>
1408 filter yields surprising and undesirable results, as will be
1409 described below. Don't do that.)
1412 Metaproxy can be invoked with this configuration as follows:
1414 <screen>../src/metaproxy --config config-simple-multi.xml</screen>
1416 And thereafter, Z39.50 clients can connect to the running server
1417 (on port 9000, as specified in the configuration) and search in
1418 any of the databases
1419 <literal>lc</literal> (the Library of Congress catalogue),
1420 <literal>marc</literal> (Index Data's test database of MARC records)
1422 <literal>all</literal> (both of these). As an example, a session
1423 using the YAZ command-line client <literal>yaz-client</literal> is
1424 here included (edited for brevity and clarity):
1426 <screen><![CDATA[$ yaz-client @:9000
1430 Search was a success.
1431 Number of hits: 10000, setno 1
1435 Search was a success.
1436 Number of hits: 10, setno 3
1440 Search was a success.
1441 Number of hits: 10010, setno 4
1444 [marc]Record type: USmarc
1447 005 00000000000000.0
1448 008 910710c19910701nju 00010 eng
1452 100 10 $a Jack Collins
1453 245 10 $a How to program a computer
1459 [VOYAGER]Record type: USmarc
1461 005 20041229102447.0
1462 008 030910s2004 caua 000 0 eng
1463 035 $a (DLC) 2003112666
1464 906 $a 7 $b cbc $c orignew $d 4 $e epcn $f 20 $g y-gencatlg
1465 925 0 $a acquire $b 1 shelf copy $x policy default
1466 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.
1469 040 $a DLC $c DLC $d DLC
1470 050 00 $a MLCM 2004/03312 (G)
1471 245 10 $a 007, everything or nothing : $b Prima's official strategy guide / $c created by Kaizen Media Group.
1472 246 3 $a Double-O-seven, everything or nothing
1473 246 30 $a Prima's official strategy guide
1474 260 $a Roseville, CA : $b Prima Games, $c c2004.
1475 300 $a 161 p. : $b col. ill. ; $c 28 cm.
1476 500 $a "Platforms: Nintendo GameCube, Macintosh, PC, PlayStation 2 computer entertainment system, Xbox"--P. [4] of cover.
1477 650 0 $a Video games.
1478 710 2 $a Kaizen Media Group.
1479 856 42 $3 Publisher description $u http://www.loc.gov/catdir/description/random052/2003112666.html
1484 As can be seen, the first record in the result set is from the
1485 Index Data test database, and the second from the Library of
1486 Congress database. The result-set continues alternating records
1487 round-robin style until the point where one of the databases'
1488 records are exhausted.
1491 This example uses only two back-end databases; more may be used.
1492 There is no limitation imposed on the number of databases that may
1493 be metasearched in this way: issues of resource usage and
1494 administrative complexity dictate the practical limits.
1497 What happens when one of the databases doesn't respond? By default,
1498 the entire multi-database search fails, and the appropriate
1499 diagnostic is returned to the client. This is usually appropriate
1500 during development, when technicians need maximum information, but
1501 can be inconvenient in deployment, when users typically don't want
1502 to be bothered with problems of this kind and prefer just to get
1503 the records from the databases that are available. To obtain this
1504 latter behavior add an empty
1505 <literal><hideunavailable></literal>
1507 <literal>multi</literal> filter:
1509 <screen><![CDATA[ <filter type="multi">
1511 </filter>]]></screen>
1513 Under this regime, an error is reported to the client only if
1514 <emphasis>all</emphasis> the databases in a multi-database search
1520 <section id="multidb.what">
1521 <title>What's going on?</title>
1523 <title>Lark's vomit</title>
1525 This section goes into a level of technical detail that is
1526 probably not necessary in order to configure and use Metaproxy.
1527 It is provided only for those who like to know how things work.
1528 You should feel free to skip on to the next section if this one
1529 doesn't seem like fun.
1533 Hold on tight - this may get a little hairy.
1536 In the general course of things, a Z39.50 Init request may carry
1537 with it an otherInfo packet of type <literal>VAL_PROXY</literal>,
1538 whose value indicates the address of a Z39.50 server to which the
1539 ultimate connection is to be made. (This otherInfo packet is
1540 supported by YAZ-based Z39.50 clients and servers, but has not yet
1541 been ratified by the Maintenance Agency and so is not widely used
1542 in non-Index Data software. We're working on it.)
1543 The <literal>VAL_PROXY</literal> packet functions
1544 analogously to the absoluteURI-style Request-URI used with the GET
1545 method when a web browser asks a proxy to forward its request: see
1547 <ulink url="http://www.w3.org/Protocols/rfc2616/rfc2616-sec5.html#sec5.1.2"
1548 >Request-URI</ulink>
1550 <ulink url="http://www.w3.org/Protocols/rfc2616/rfc2616.html"
1551 >the HTTP 1.1 specification</ulink>.
1554 Within Metaproxy, Search requests that are part of the same
1555 session as an Init request that carries a
1556 <literal>VAL_PROXY</literal> otherInfo are also annotated with the
1557 same information. The role of the <literal>virt_db</literal>
1558 filter is to rewrite this otherInfo packet dependent on the
1559 virtual database that the client wants to search.
1562 When Metaproxy receives a Z39.50 Init request from a client, it
1563 doesn't immediately forward that request to the back-end server.
1564 Why not? Because it doesn't know <emphasis>which</emphasis>
1565 back-end server to forward it to until the client sends a Search
1566 request that specifies the database that it wants to search in.
1567 Instead, it just treasures the Init request up in its heart; and,
1568 later, the first time the client does a search on one of the
1569 specified virtual databases, a connection is forged to the
1570 appropriate server and the Init request is forwarded to it. If,
1571 later in the session, the same client searches in a different
1572 virtual database, then a connection is forged to the server that
1573 hosts it, and the same cached Init request is forwarded there,
1577 All of this clever Init-delaying is done by the
1578 <literal>frontend_net</literal> filter. The
1579 <literal>virt_db</literal> filter knows nothing about it; in
1580 fact, because the Init request that is received from the client
1581 doesn't get forwarded until a Search request is received, the
1582 <literal>virt_db</literal> filter (and the
1583 <literal>z3950_client</literal> filter behind it) doesn't even get
1584 invoked at Init time. The <emphasis>only</emphasis> thing that a
1585 <literal>virt_db</literal> filter ever does is rewrite the
1586 <literal>VAL_PROXY</literal> otherInfo in the requests that pass
1590 It is possible for a <literal>virt_db</literal> filter to contain
1592 <literal><target></literal>
1593 elements. What does this mean? Only that the filter will add
1594 multiple <literal>VAL_PROXY</literal> otherInfo packets to the
1595 Search requests that pass through it. That's because the virtual
1596 DB filter is dumb, and does exactly what it's told - no more, no
1598 If a Search request with multiple <literal>VAL_PROXY</literal>
1599 otherInfo packets reaches a <literal>z3950_client</literal>
1600 filter, this is an error. That filter doesn't know how to deal
1601 with multiple targets, so it will either just pick one and search
1602 in it, or (better) fail with an error message.
1605 The <literal>multi</literal> filter comes to the rescue! This is
1606 the only filter that knows how to deal with multiple
1607 <literal>VAL_PROXY</literal> otherInfo packets, and it does so by
1608 making multiple copies of the entire Search request: one for each
1609 <literal>VAL_PROXY</literal>. Each of these new copies is then
1610 passed down through the remaining filters in the route. (The
1611 copies are handled in parallel though the
1612 spawning of new threads.) Since the copies each have only one
1613 <literal>VAL_PROXY</literal> otherInfo, they can be handled by the
1614 <literal>z3950_client</literal> filter, which happily deals with
1615 each one individually. When the results of the individual
1616 searches come back up to the <literal>multi</literal> filter, it
1617 merges them into a single Search response, which is what
1618 eventually makes it back to the client.
1623 <section id="multidb.picture">
1624 <title>A picture is worth a thousand words (but only five hundred on 64-bit architectures)</title>
1628 <imagedata fileref="multi.pdf" format="PDF" scale="50"/>
1631 <imagedata fileref="multi.png" format="PNG"/>
1634 <!-- Fall back if none of the images can be used -->
1636 [Here there should be a diagram showing the progress of
1637 packages through the filters during a simple virtual-database
1638 search and a multi-database search, but is seems that your
1639 tool chain has not been able to include the diagram in this
1643 <!-- ### This used to work with an older version of DocBook
1645 <para>Caption: progress of packages through filters.</para>
1648 </inlinemediaobject>
1654 <chapter id="sru-server">
1655 <title>Combined SRU webservice and Z39.50 server configuration</title>
1657 Metaproxy can act as
1658 <ulink url="&url.sru;">SRU</ulink> and
1659 web service server, which translates web service requests to
1660 <ulink url="&url.z39.50;">ANSI/NISO Z39.50</ulink> packages and
1661 sends them off to common available targets.
1664 A typical setup for this operation needs a filter route including the
1668 <table id="sru-server-table-config" frame="top">
1669 <title>SRU/Z39.50 Server Filter Route Configuration</title>
1673 <entry>Filter</entry>
1674 <entry>Importance</entry>
1675 <entry>Purpose</entry>
1681 <entry><literal>frontend_net</literal></entry>
1682 <entry>required</entry>
1683 <entry>Accepting HTTP connections and passing them to following
1684 filters. Since this filter also accepts Z39.50 connections, the
1685 server works as SRU and Z39.50 server on the same port.</entry>
1688 <entry><literal>sru_z3950</literal></entry>
1689 <entry>required</entry>
1690 <entry>Accepting SRU GET/POST/SOAP explain and
1691 searchRetrieve requests for the the configured databases.
1692 Explain requests are directly served from the static XML configuration.
1693 SearchRetrieve requests are
1694 transformed to Z39.50 search and present packages.
1695 All other HTTP and Z39.50 packages are passed unaltered.</entry>
1698 <entry><literal>http_file</literal></entry>
1699 <entry>optional</entry>
1700 <entry>Serving HTTP requests from the filesystem. This is only
1701 needed if the server should serve XSLT stylesheets, static HTML
1702 files or Java Script for thin browser based clients.
1703 Z39.50 packages are passed unaltered.</entry>
1706 <entry><literal>cql_rpn</literal></entry>
1707 <entry>required</entry>
1708 <entry>Usually, Z39.50 servers do not talk CQL, hence the
1709 translation of the CQL query language to RPN is mandatory in
1710 most cases. Affects only Z39.50 search packages.</entry>
1713 <entry><literal>record_transform</literal></entry>
1714 <entry>optional</entry>
1715 <entry>Some Z39.50 backend targets can not present XML record
1716 syntaxes in common wanted element sets. using this filter, one
1717 can transform binary MARC records to MARCXML records, and
1718 further transform those to any needed XML schema/format by XSLT
1719 transformations. Changes only Z39.50 present packages.</entry>
1722 <entry><literal>session_shared</literal></entry>
1723 <entry>optional</entry>
1724 <entry>The stateless nature of web services requires frequent
1725 re-searching of the same targets for display of paged result set
1726 records. This might be an unacceptable burden for the accessed
1727 backend Z39.50 targets, and this mosule can be added for
1728 efficient backend target resource pooling.</entry>
1731 <entry><literal>z3950_client</literal></entry>
1732 <entry>required</entry>
1733 <entry>Finally, a Z39.50 package sink is needed in the filter
1734 chain to provide the response packages. The Z39.50 client module
1735 is used to access external targets over the network, but any
1736 coming local Z39.50 package sink could be used instead of.</entry>
1739 <entry><literal>bounce</literal></entry>
1740 <entry>required</entry>
1741 <entry>Any Metaproxy package arriving here did not do so by
1742 purpose, and is bounced back with connection closure. this
1743 prevents inifinite package hanging inside the SRU server.</entry>
1749 A typical minimal example <ulink url="&url.sru;">SRU</ulink>
1750 server configuration file is found in the tarball distribution at
1751 <literal>etc/config-sru-to-z3950.xml</literal>.
1754 Off course, any other metaproxy modules can be integrated into a
1755 SRU server solution, including, but not limited to, load balancing,
1756 multiple target querying
1757 (see <xref linkend="multidb"/>), and complex RPN query rewrites.
1764 <chapter id="extensions">
1765 <title>Writing extensions for Metaproxy</title>
1766 <para>### To be written</para>
1772 <chapter id="classes">
1773 <title>Classes in the Metaproxy source code</title>
1776 <section id="classes-introductory-notes">
1777 <title>Introductory notes</title>
1779 <emphasis>Stop! Do not read this!</emphasis>
1780 You won't enjoy it at all. You should just skip ahead to
1781 <xref linkend="reference"/>,
1783 <!-- The remainder of this paragraph is lifted verbatim from
1784 Douglas Adams' _Hitch Hiker's Guide to the Galaxy_, chapter 8 -->
1785 you things you really need to know, like the fact that the
1786 fabulously beautiful planet Bethselamin is now so worried about
1787 the cumulative erosion by ten billion visiting tourists a year
1788 that any net imbalance between the amount you eat and the amount
1789 you excrete whilst on the planet is surgically removed from your
1790 bodyweight when you leave: so every time you go to the lavatory it
1791 is vitally important to get a receipt.
1794 This chapter contains documentation of the Metaproxy source code, and is
1795 of interest only to maintainers and developers. If you need to
1796 change Metaproxy's behavior or write a new filter, then you will most
1797 likely find this chapter helpful. Otherwise it's a waste of your
1798 good time. Seriously: go and watch a film or something.
1799 <citetitle>This is Spinal Tap</citetitle> is particularly good.
1802 Still here? OK, let's continue.
1805 In general, classes seem to be named big-endianly, so that
1806 <literal>FactoryFilter</literal> is not a filter that filters
1807 factories, but a factory that produces filters; and
1808 <literal>FactoryStatic</literal> is a factory for the statically
1809 registered filters (as opposed to those that are dynamically
1814 <section id="individual.classes">
1815 <title>Individual classes</title>
1817 The classes making up the Metaproxy application are here listed by
1818 class-name, with the names of the source files that define them in
1822 <section id="class-FactoryFilter">
1823 <title><literal>mp::FactoryFilter</literal>
1824 (<filename>factory_filter.cpp</filename>)</title>
1826 A factory class that exists primarily to provide the
1827 <literal>create()</literal> method, which takes the name of a
1828 filter class as its argument and returns a new filter of that
1829 type. To enable this, the factory must first be populated by
1830 calling <literal>add_creator()</literal> for static filters (this
1831 is done by the <literal>FactoryStatic</literal> class, see below)
1832 and <literal>add_creator_dyn()</literal> for filters loaded
1837 <section id="class-FactoryStatic">
1838 <title><literal>mp::FactoryStatic</literal>
1839 (<filename>factory_static.cpp</filename>)</title>
1841 A subclass of <literal>FactoryFilter</literal> which is
1842 responsible for registering all the statically defined filter
1843 types. It does this by knowing about all those filters'
1844 structures, which are listed in its constructor. Merely
1845 instantiating this class registers all the static classes. It is
1846 for the benefit of this class that <literal>struct
1847 metaproxy_1_filter_struct</literal> exists, and that all the filter
1848 classes provide a static object of that type.
1852 <section id="class-filter-Base">
1853 <title><literal>mp::filter::Base</literal>
1854 (<filename>filter.cpp</filename>)</title>
1856 The virtual base class of all filters. The filter API is, on the
1857 surface at least, extremely simple: two methods.
1858 <literal>configure()</literal> is passed an XML DOM tree representing
1859 that part of the configuration file that pertains to this filter
1860 instance, and is expected to walk that tree extracting relevant
1861 information. And <literal>process()</literal> processes a
1862 package (see below). That surface simplicity is a bit
1863 misleading, as <literal>process()</literal> needs to know a lot
1864 about the <literal>Package</literal> class in order to do
1869 <section id="class-AuthSimple">
1870 <title><literal>mp::filter::AuthSimple</literal>,
1871 <literal>Backend_test</literal>, etc.
1872 (<filename>filter_auth_simple.cpp</filename>,
1873 <filename>filter_backend_test.cpp</filename>, etc.)</title>
1875 Individual filters. Each of these is implemented by a header and
1876 a source file, named <filename>filter_*.hpp</filename> and
1877 <filename>filter_*.cpp</filename> respectively. All the header
1878 files should be pretty much identical, in that they declare the
1879 class, including a private <literal>Rep</literal> class and a
1880 member pointer to it, and the two public methods.
1883 The source file for each filter needs to supply:
1888 A definition of the private <literal>Rep</literal> class.
1893 Some boilerplate constructors and destructors.
1898 A <literal>configure()</literal> method that uses the
1899 appropriate XML fragment.
1904 Most important, the <literal>process()</literal> method that
1905 does all the actual work.
1911 <section id="class-Package">
1912 <title><literal>mp::Package</literal>
1913 (<filename>package.cpp</filename>)</title>
1915 Represents a package on its way through the series of filters
1916 that make up a route. This is essentially a Z39.50 or SRU APDU
1917 together with information about where it came from, which is
1918 modified as it passes through the various filters.
1922 <section id="class-Pipe">
1923 <title><literal>mp::Pipe</literal>
1924 (<filename>pipe.cpp</filename>)</title>
1926 This class provides a compatibility layer so that we have an IPC
1927 mechanism that works the same under Unix and Windows. It's not
1928 particularly exciting.
1932 <section id="class-RouterChain">
1933 <title><literal>mp::RouterChain</literal>
1934 (<filename>router_chain.cpp</filename>)</title>
1940 <section id="class-RouterFleXML">
1941 <title><literal>mp::RouterFleXML</literal>
1942 (<filename>router_flexml.cpp</filename>)</title>
1948 <section id="class-Session">
1949 <title><literal>mp::Session</literal>
1950 (<filename>session.cpp</filename>)</title>
1956 <section id="class-ThreadPoolSocketObserver">
1957 <title><literal>mp::ThreadPoolSocketObserver</literal>
1958 (<filename>thread_pool_observer.cpp</filename>)</title>
1964 <section id="class-util">
1965 <title><literal>mp::util</literal>
1966 (<filename>util.cpp</filename>)</title>
1968 A namespace of various small utility functions and classes,
1969 collected together for convenience. Most importantly, includes
1970 the <literal>mp::util::odr</literal> class, a wrapper for YAZ's
1975 <section id="class-xml">
1976 <title><literal>mp::xml</literal>
1977 (<filename>xmlutil.cpp</filename>)</title>
1979 A namespace of various XML utility functions and classes,
1980 collected together for convenience.
1986 <section id="other.source.files">
1987 <title>Other Source Files</title>
1989 In addition to the Metaproxy source files that define the classes
1990 described above, there are a few additional files which are
1991 briefly described here:
1995 <term><literal>metaproxy_prog.cpp</literal></term>
1998 The main function of the <command>metaproxy</command> program.
2003 <term><literal>ex_router_flexml.cpp</literal></term>
2006 Identical to <literal>metaproxy_prog.cpp</literal>: it's not clear why.
2011 <term><literal>test_*.cpp</literal></term>
2014 Unit-tests for various modules.
2020 ### Still to be described:
2021 <literal>ex_filter_frontend_net.cpp</literal>,
2022 <literal>filter_dl.cpp</literal>,
2023 <literal>plainfile.cpp</literal>,
2024 <literal>tstdl.cpp</literal>.
2030 <reference id="reference">
2031 <title>Reference</title>
2032 <partintro id="reference-introduction">
2034 The material in this chapter is drawn directly from the individual
2035 manual entries. In particular, the Metaproxy invocation section is
2036 available using <command>man metaproxy</command>, and the section
2037 on each individual filter is available using the name of the filter
2038 as the argument to the <command>man</command> command.
2044 <appendix id="license">
2045 <title>License</title>
2048 Metaproxy, Copyright © 1995-2007 Index Data ApS.
2052 Metaproxy is free software; you can redistribute it and/or modify it under
2053 the terms of the GNU General Public License as published by the Free
2054 Software Foundation; either version 2, or (at your option) any later
2059 Metaproxy is distributed in the hope that it will be useful, but WITHOUT ANY
2060 WARRANTY; without even the implied warranty of MERCHANTABILITY or
2061 FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License
2066 You should have received a copy of the GNU General Public License
2067 along with Metaproxy; see the file LICENSE. If not, write to the
2068 Free Software Foundation,
2069 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA
2073 <title>GNU General Public License</title>
2075 GNU GENERAL PUBLIC LICENSE
2076 Version 2, June 1991
2078 Copyright (C) 1989, 1991 Free Software Foundation, Inc.
2079 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA
2080 w Everyone is permitted to copy and distribute verbatim copies
2081 of this license document, but changing it is not allowed.
2085 The licenses for most software are designed to take away your
2086 freedom to share and change it. By contrast, the GNU General Public
2087 License is intended to guarantee your freedom to share and change free
2088 software--to make sure the software is free for all its users. This
2089 General Public License applies to most of the Free Software
2090 Foundation's software and to any other program whose authors commit to
2091 using it. (Some other Free Software Foundation software is covered by
2092 the GNU Library General Public License instead.) You can apply it to
2095 When we speak of free software, we are referring to freedom, not
2096 price. Our General Public Licenses are designed to make sure that you
2097 have the freedom to distribute copies of free software (and charge for
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2099 if you want it, that you can change the software or use pieces of it
2100 in new free programs; and that you know you can do these things.
2102 To protect your rights, we need to make restrictions that forbid
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2104 These restrictions translate to certain responsibilities for you if you
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