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23 <title>Metaproxy - User's Guide and Reference</title>
26 <firstname>Adam</firstname><surname>Dickmeiss</surname>
29 <firstname>Marc</firstname><surname>Cromme</surname>
32 <firstname>Mike</firstname><surname>Taylor</surname>
35 <releaseinfo>&version;</releaseinfo>
37 <year>2005-2007</year>
38 <holder>Index Data ApS</holder>
42 This manual is part of Metaproxy version &version;.
45 Metaproxy is a universal router, proxy and encapsulated
46 metasearcher for information retrieval protocols. It accepts,
47 processes, interprets and redirects requests from IR clients using
48 standard protocols such as the binary
49 <ulink url="&url.z39.50;">ANSI/NISO Z39.50</ulink>
50 and the information search and retrieval
51 web service <ulink url="&url.sru;">SRU</ulink>
52 as well as functioning as a limited
53 <ulink url="&url.http;">HTTP</ulink> server.
56 Metaproxy is configured by an XML file which
57 specifies how the software should function in terms of routes that
58 the request packets can take through the proxy, each step on a
59 route being an instantiation of a filter. Filters come in many
60 types, one for each operation: accepting Z39.50 packets, logging,
61 query transformation, multiplexing, etc. Further filter-types can
62 be added as loadable modules to extend Metaproxy functionality,
66 Metaproxy is covered by the GNU General Public License version 2.
71 <imagedata fileref="common/id.png" format="PNG"/>
74 <imagedata fileref="common/id.eps" format="EPS"/>
81 <chapter id="introduction">
82 <title>Introduction</title>
85 <ulink url="&url.metaproxy;">Metaproxy</ulink>
86 is a stand alone program that acts as a universal router, proxy and
87 encapsulated metasearcher for information retrieval protocols such
88 as <ulink url="&url.z39.50;">Z39.50</ulink> and
89 <ulink url="&url.sru;">SRU</ulink>.
90 To clients, it acts as a server of these protocols: it can be searched,
91 records can be retrieved from it, etc.
92 To servers, it acts as a client: it searches in them,
93 retrieves records from them, etc. it satisfies its clients'
94 requests by transforming them, multiplexing them, forwarding them
95 on to zero or more servers, merging the results, transforming
96 them, and delivering them back to the client. In addition, it
97 acts as a simple <ulink url="&url.http;">HTTP</ulink> server; support
98 for further protocols can be added in a modular fashion, through the
99 creation of new filters.
104 Fish, bananas, cold pyjamas,
105 Mutton, beef and trout!
106 - attributed to Cole Porter.
109 Metaproxy is a more capable alternative to
110 <ulink url="&url.yazproxy;">YAZ Proxy</ulink>,
111 being more powerful, flexible, configurable and extensible. Among
112 its many advantages over the older, more pedestrian work are
113 support for multiplexing (encapsulated metasearching), routing by
114 database name, authentication and authorization and serving local
115 files via HTTP. Equally significant, its modular architecture
116 facilitites the creation of pluggable modules implementing further
120 This manual will describe how to install Metaproxy
121 before giving an overview of its architecture, then discussing the
122 key concept of a filter in some depth and giving an overview of
123 the various filter types, then discussing the configuration file
124 format. After this come several optional chapters which may be
125 freely skipped: a detailed discussion of virtual databases and
126 multi-database searching, some notes on writing extensions
127 (additional filter types) and a high-level description of the
128 source code. Finally comes the reference guide, which contains
129 instructions for invoking the <command>metaproxy</command>
130 program, and detailed information on each type of filter,
135 <chapter id="installation">
136 <title>Installation</title>
138 Metaproxy depends on the following tools/libraries:
140 <varlistentry><term><ulink url="&url.yazplusplus;">YAZ++</ulink></term>
143 This is a C++ library based on <ulink url="&url.yaz;">YAZ</ulink>.
147 <varlistentry><term><ulink url="&url.libxslt;">Libxslt</ulink></term>
149 <para>This is an XSLT processor - based on
150 <ulink url="&url.libxml2;">Libxml2</ulink>. Both Libxml2 and
151 Libxslt must be installed with the development components
152 (header files, etc.) as well as the run-time libraries.
156 <varlistentry><term><ulink url="&url.boost;">Boost</ulink></term>
159 The popular C++ library. Initial versions of Metaproxy
160 was built with 1.32 but this is no longer supported.
161 Metaproxy is known to work with Boost version 1.33 / 1.34.
168 In order to compile Metaproxy a modern C++ compiler is
169 required. Boost, in particular, requires the C++ compiler
170 to facilitate the newest features. Refer to Boost
171 <ulink url="&url.boost.compilers.status;">Compiler Status</ulink>
172 for more information.
175 We have successfully built Metaproxy using the compilers
176 <ulink url="&url.gcc;">GCC</ulink> version 4.0 and
177 <ulink url="&url.vstudio;">Microsoft Visual Studio</ulink> 2003/2005.
180 <section id="installation.unix">
181 <title>Installation on Unix (from Source)</title>
183 Here is a quick step-by-step guide on how to compile all the
184 tools that Metaproxy uses. Only few systems have none of the required
185 tools binary packages. If, for example, Libxml2/libxslt are already
186 installed as development packages use those (and omit compilation).
193 gunzip -c libxml2-version.tar.gz|tar xf -
201 gunzip -c libxslt-version.tar.gz|tar xf -
212 gunzip -c yaz-version.tar.gz|tar xf -
220 gunzip -c yazpp-version.tar.gz|tar xf -
231 gunzip -c boost-version.tar.gz|tar xf -
242 gunzip -c metaproxy-version.tar.gz|tar xf -
251 <section id="installation.debian">
252 <title>Installation on Debian GNU/Linux</title>
254 All dependencies for Metaproxy are available as
255 <ulink url="&url.debian;">Debian</ulink>
256 packages for the sarge (stable in 2005) and etch (testing in 2005)
260 The procedures for Debian based systems, such as
261 <ulink url="&url.ubuntu;">Ubuntu</ulink> is probably similar
264 There is currently no official Debian package for YAZ++.
265 And the official Debian package for YAZ is probably too old.
266 But Index Data bulds "new" versions of those for Debian (i386 only).
269 Update the <filename>/etc/apt/sources.list</filename>
270 to include the Index Data repository.
271 See YAZ' <ulink url="&url.yaz.download.debian;">Download Debian</ulink>
272 for more information.
275 apt-get install libxslt1-dev
276 apt-get install libyazpp2-dev
277 apt-get install libboost-dev
278 apt-get install libboost-thread-dev
279 apt-get install libboost-test-dev
282 With these packages installed, the usual configure + make
283 procedure can be used for Metaproxy as outlined in
284 <xref linkend="installation.unix"/>.
288 <section id="installation.rpm">
289 <title>Installation on RPM based Linux Systems</title>
291 All external dependencies for Metaproxy are available as
292 RPM packages, either from your distribution site, or from the
293 <ulink url="http://fr.rpmfind.net/">RPMfind</ulink> site.
296 For example, an installation of the requires Boost C++ development
297 libraries on RedHat Fedora C4 and C5 can be done like this:
299 wget ftp://fr.rpmfind.net/wlinux/fedora/core/updates/testing/4/SRPMS/boost-1.33.0-3.fc4.src.rpm
300 sudo rpmbuild --buildroot src/ --rebuild -p fc4/boost-1.33.0-3.fc4.src.rpm
301 sudo rpm -U /usr/src/redhat/RPMS/i386/boost-*rpm
305 The <ulink url="&url.yaz;">YAZ</ulink> library is needed to
306 compile &metaproxy;, see there
307 for more information on available RPM packages.
310 There is currently no official RPM package for YAZ++.
311 See the <ulink url="&url.yazplusplus;">YAZ++</ulink> pages
312 for more information on a Unix tarball install.
315 With these packages installed, the usual configure + make
316 procedure can be used for Metaproxy as outlined in
317 <xref linkend="installation.unix"/>.
321 <section id="installation.windows">
322 <title>Installation on Windows</title>
324 Metaproxy can be compiled with Microsoft
325 <ulink url="&url.vstudio;">Visual Studio</ulink>.
326 Version 2003 (C 7.1) and 2005 (C 8.0) is known to work.
328 <section id="installation.windows.boost">
331 For Windows, it's easiest to get the precompiled Boost
332 package from <ulink url="&url.boost.windows.download;">here</ulink>.
333 Several versions of the Boost libraries may be selected when
334 installing Boost for windows. Please choose at least the
335 <emphasis>multithreaded</emphasis> (non-DLL) version because
336 the Metaproxy makefile uses that.
339 For more information about installing Boost refer to the
340 <ulink url="&url.boost.getting.started;">getting started</ulink>
345 <section id="installation.windows.libxslt">
346 <title>Libxslt</title>
348 <ulink url="&url.libxslt;">Libxslt</ulink> can be downloaded
350 <ulink url="&url.libxml2.download.win32;">here</ulink>.
353 Libxslt has other dependencies, but these can all be downloaded
354 from the same site. Get the following package:
355 iconv, zlib, libxml2, libxslt.
359 <section id="installation.windows.yaz">
362 <ulink url="&url.yaz;">YAZ</ulink> can be downloaded
364 <ulink url="&url.yaz.download.win32;">here</ulink>.
368 <section id="installation.windows.yazplusplus">
371 Get <ulink url="&url.yazplusplus;">YAZ++</ulink> as well.
372 Version 1.0.3 or later is required.
375 YAZ++ includes NMAKE makefiles, similar to those found in the
380 <section id="installation.windows.metaproxy">
381 <title>Metaproxy</title>
383 Metaproxy is shipped with NMAKE makefiles as well - similar
384 to those found in the YAZ++/YAZ packages. Adjust this Makefile
385 to point to the proper locations of Boost, Libxslt, Libxml2,
386 zlib, iconv, yaz and yazpp.
390 <varlistentry><term><literal>DEBUG</literal></term>
392 If set to 1, the software is
393 compiled with debugging libraries (code generation is
394 multi-threaded debug DLL).
395 If set to 0, the software is compiled with release libraries
396 (code generation is multi-threaded DLL).
401 <term><literal>BOOST</literal></term>
404 Boost install location
410 <term><literal>BOOST_VERSION</literal></term>
413 Boost version (replace . with _).
419 <term><literal>BOOST_TOOLSET</literal></term>
428 <term><literal>LIBXSLT_DIR</literal>,
429 <literal>LIBXML2_DIR</literal> ..</term>
432 Specify the locations of Libxslt, libiconv, libxml2 and
441 After successful compilation you'll find
442 <literal>metaproxy.exe</literal> in the
443 <literal>bin</literal> directory.
451 <chapter id="yazproxy-comparison">
452 <title>YAZ Proxy Comparison</title>
454 The table below lists facilities either supported by either
455 <ulink url="&url.yazproxy;">YAZ Proxy</ulink> or Metaproxy.
457 <table id="yazproxy-comparison-table">
458 <title>Metaproxy / YAZ Proxy comparison</title>
462 <entry>Facility</entry>
463 <entry>Metaproxy</entry>
464 <entry>YAZ Proxy</entry>
469 <entry>Z39.50 server</entry>
470 <entry>Using filter <literal>frontend_net</literal></entry>
471 <entry>Supported</entry>
474 <entry>SRU server</entry>
475 <entry>Supported with filter <literal>sru_z3950</literal></entry>
476 <entry>Supported</entry>
479 <entry>Z39.50 client</entry>
480 <entry>Supported with filter <literal>z3950_client</literal></entry>
481 <entry>Supported</entry>
484 <entry>SRU client</entry>
485 <entry>Unsupported</entry>
486 <entry>Unsupported</entry>
489 <entry>Connection reuse</entry>
490 <entry>Supported with filter <literal>session_shared</literal></entry>
491 <entry>Supported</entry>
494 <entry>Connection share</entry>
495 <entry>Supported with filter <literal>session_shared</literal></entry>
496 <entry>Unsupported</entry>
499 <entry>Result set reuse</entry>
500 <entry>Supported with filter <literal>session_shared</literal></entry>
501 <entry>Within one Z39.50 session / HTTP keep-alive</entry>
504 <entry>Record cache</entry>
505 <entry>Unsupported</entry>
506 <entry>Supported for last result set within one Z39.50/HTTP-keep alive session</entry>
509 <entry>Z39.50 Virtual database, i.e. select any Z39.50 target for database</entry>
510 <entry>Supported with filter <literal>virt_db</literal></entry>
511 <entry>Unsupported</entry>
514 <entry>SRU Virtual database, i.e. select any Z39.50 target for path</entry>
515 <entry>Supported with filter <literal>virt_db</literal>,
516 <literal>sru_z3950</literal></entry>
517 <entry>Supported</entry>
520 <entry>Multi target search</entry>
521 <entry>Supported with filter <literal>multi</literal> (round-robin)</entry>
522 <entry>Unsupported</entry>
525 <entry>Retrieval and search limits</entry>
526 <entry>Unsupported</entry>
527 <entry>Supported</entry>
530 <entry>Bandwidth limits</entry>
531 <entry>Unsupported</entry>
532 <entry>Supported</entry>
535 <entry>Connect limits</entry>
536 <entry>Unsupported</entry>
537 <entry>Supported</entry>
540 <entry>Retrieval sanity check and conversions</entry>
541 <entry>Supported using filter <literal>record_transform</literal></entry>
542 <entry>Supported</entry>
545 <entry>Query check</entry>
547 Supported in a limited way using <literal>query_rewrite</literal>
549 <entry>Supported</entry>
552 <entry>Query rewrite</entry>
553 <entry>Supported with <literal>query_rewrite</literal></entry>
554 <entry>Unsupported</entry>
557 <entry>Session invalidate for -1 hits</entry>
558 <entry>Unsupported</entry>
559 <entry>Supported</entry>
562 <entry>Architecture</entry>
563 <entry>Multi-threaded + select for networked modules such as
564 <literal>frontend_net</literal>)</entry>
565 <entry>Single-threaded using select</entry>
569 <entry>Extensability</entry>
570 <entry>Most functionality implemented as loadable modules</entry>
571 <entry>Unsupported and experimental</entry>
575 <entry><ulink url="&url.usemarcon;">USEMARCON</ulink></entry>
576 <entry>Unsupported</entry>
577 <entry>Supported</entry>
581 <entry>Portability</entry>
583 Requires YAZ, YAZ++ and modern C++ compiler supporting
584 <ulink url="&url.boost;">Boost</ulink>.
587 Requires YAZ and YAZ++.
588 STL is not required so pretty much any C++ compiler out there should work.
597 <chapter id="architecture">
598 <title>The Metaproxy Architecture</title>
600 The Metaproxy architecture is based on three concepts:
601 the <emphasis>package</emphasis>,
602 the <emphasis>route</emphasis>
603 and the <emphasis>filter</emphasis>.
607 <term>Packages</term>
610 A package is request or response, encoded in some protocol,
611 issued by a client, making its way through Metaproxy, send to or
612 received from a server, or sent back to the client.
615 The core of a package is the protocol unit - for example, a
616 Z39.50 Init Request or Search Response, or an SRU searchRetrieve
617 URL or Explain Response. In addition to this core, a package
618 also carries some extra information added and used by Metaproxy
622 In general, packages are doctored as they pass through
623 Metaproxy. For example, when the proxy performs authentication
624 and authorization on a Z39.50 Init request, it removes the
625 authentication credentials from the package so that they are not
626 passed onto the back-end server; and when search-response
627 packages are obtained from multiple servers, they are merged
628 into a single unified package that makes its way back to the
637 Packages make their way through routes, which can be thought of
638 as programs that operate on the package data-type. Each
639 incoming package initially makes its way through a default
640 route, but may be switched to a different route based on various
641 considerations. Routes are made up of sequences of filters (see
650 Filters provide the individual instructions within a route, and
651 effect the necessary transformations on packages. A particular
652 configuration of Metaproxy is essentially a set of filters,
653 described by configuration details and arranged in order in one
654 or more routes. There are many kinds of filter - about a dozen
655 at the time of writing with more appearing all the time - each
656 performing a specific function and configured by different
660 The word ``filter'' is sometimes used rather loosely, in two
661 different ways: it may be used to mean a particular
662 <emphasis>type</emphasis> of filter, as when we speak of ``the
663 auth_simple filter'' or ``the multi filter''; or it may be used
664 to be a specific <emphasis>instance</emphasis> of a filter
665 within a Metaproxy configuration. For example, a single
666 configuration will often contain multiple instances of the
667 <literal>z3950_client</literal> filter. In
668 operational terms, of these is a separate filter. In practice,
669 context always make it clear which sense of the word ``filter''
673 Extensibility of Metaproxy is primarily through the creation of
674 plugins that provide new filters. The filter API is small and
675 conceptually simple, but there are many details to master. See
677 <link linkend="filters">Filters</link>.
683 Since packages are created and handled by the system itself, and
684 routes are conceptually simple, most of the remainder of this
685 document concentrates on filters. After a brief overview of the
686 filter types follows, along with some thoughts on possible future
693 <chapter id="filters">
694 <title>Filters</title>
697 <section id="filters-introductory-notes">
698 <title>Introductory notes</title>
700 It's useful to think of Metaproxy as an interpreter providing a small
701 number of primitives and operations, but operating on a very
702 complex data type, namely the ``package''.
705 A package represents a Z39.50 or SRU/W request (whether for Init,
706 Search, Scan, etc.) together with information about where it came
707 from. Packages are created by front-end filters such as
708 <literal>frontend_net</literal> (see below), which reads them from
709 the network; other front-end filters are possible. They then pass
710 along a route consisting of a sequence of filters, each of which
711 transforms the package and may also have side-effects such as
712 generating logging. Eventually, the route will yield a response,
713 which is sent back to the origin.
716 There are many kinds of filter: some that are defined statically
717 as part of Metaproxy, and others may be provided by third parties
718 and dynamically loaded. They all conform to the same simple API
719 of essentially two methods: <function>configure()</function> is
720 called at startup time, and is passed an XML DOM tree representing that
721 part of the configuration file that pertains to this filter
722 instance: it is expected to walk that tree extracting relevant
723 information; and <function>process()</function> is called every
724 time the filter has to processes a package.
727 While all filters provide the same API, there are different modes
728 of functionality. Some filters are sources: they create
730 (<literal>frontend_net</literal>);
731 others are sinks: they consume packages and return a result
732 (<literal>backend_test</literal>,
733 <literal>bounce</literal>,
734 <literal>http_file</literal>,
735 <literal>z3950_client</literal>);
736 the others are true filters, that read, process and pass on the
737 packages they are fed
738 (<literal>auth_simple</literal>,
739 <literal>log</literal>,
740 <literal>multi</literal>,
741 <literal>query_rewrite</literal>,
742 <literal>record_transform</literal>,
743 <literal>session_shared</literal>,
744 <literal>sru_z3950</literal>,
745 <literal>template</literal>,
746 <literal>virt_db</literal>).
751 <section id="overview.filter.types">
752 <title>Overview of filter types</title>
754 We now briefly consider each of the types of filter supported by
755 the core Metaproxy binary. This overview is intended to give a
756 flavor of the available functionality; more detailed information
757 about each type of filter is included below in
758 <xref linkend="reference"/>.
761 The filters are here named by the string that is used as the
762 <literal>type</literal> attribute of a
763 <literal><filter></literal> element in the configuration
764 file to request them, with the name of the class that implements
765 them in parentheses. (The classname is not needed for normal
766 configuration and use of Metaproxy; it is useful only to
770 The filters are here listed in alphabetical order:
777 New virt_db-alike that does inteligent peer choice, explain merging,
778 adds FD&N to explain. Keeps init responses (like "virt_db Classic"),
779 makes routing choices based on local explain knowledge. Ref IDDI
782 Filter to convert Explain Classic to ZeeRex.
784 CQL2PQF (which needs augmented ZeeRex) - MARC for Talis.
788 Figure out what additional information we need in:
789 ZeeRex (check against D3.1)
790 Init request (e.g. loop detection)
791 Query package (e.g. number of hops)
792 Query response (e.g. record source)
796 <section id="auth_simple">
797 <title><literal>auth_simple</literal>
798 (mp::filter::AuthSimple)</title>
800 Simple authentication and authorization. The configuration
801 specifies the name of a file that is the user register, which
802 lists <varname>username</varname>:<varname>password</varname>
803 pairs, one per line, colon separated. When a session begins, it
804 is rejected unless username and passsword are supplied, and match
805 a pair in the register. The configuration file may also specific
806 the name of another file that is the target register: this lists
807 lists <varname>username</varname>:<varname>dbname</varname>,<varname>dbname</varname>...
808 sets, one per line, with multiple database names separated by
809 commas. When a search is processed, it is rejected unless the
810 database to be searched is one of those listed as available to
815 <section id="backend_test">
816 <title><literal>backend_test</literal>
817 (mp::filter::Backend_test)</title>
819 A partial sink that provides dummy responses in the manner of the
820 <literal>yaz-ztest</literal> Z39.50 server. This is useful only
821 for testing. Seriously, you don't need this. Pretend you didn't
822 even read this section.
826 <section id="bounce">
827 <title><literal>bounce</literal>
828 (mp::filter::Bounce)</title>
830 A sink that swallows <emphasis>all packages</emphasis>,
831 and returns them almost unprocessed.
832 It never sends any package of any type further down the row, but
833 sets Z39.50 packages to Z_Close, and HTTP_Request packages to
834 HTTP_Response err code 400 packages, and adds a suitable bounce
836 The bounce filter is usually added at end of each filter chain route
837 to prevent infinite hanging of for example HTTP
838 requests packages when only the Z39.50 client partial sink
839 filter is found in the
844 <section id="cql_rpn">
845 <title><literal>cql_rpn</literal>
846 (mp::filter::CQLtoRPN)</title>
848 A query language transforming filter which catches Z39.50
849 <literal>searchRequest</literal>
850 packages containing <literal>CQL</literal> queries, transforms
851 those to <literal>RPN</literal> queries,
852 and sends the <literal>searchRequests</literal> on to the next
853 filters. It is among other things useful in a SRU context.
857 <section id="frontend_net">
858 <title><literal>frontend_net</literal>
859 (mp::filter::FrontendNet)</title>
861 A source that accepts Z39.50 connections from a port
862 specified in the configuration, reads protocol units, and
863 feeds them into the next filter in the route. When the result is
864 received, it is returned to the original origin.
868 <section id="http_file">
869 <title><literal>http_file</literal>
870 (mp::filter::HttpFile)</title>
872 A partial sink which swallows only
873 <literal>HTTP_Request</literal> packages, and
874 returns the contents of files from the local
875 filesystem in response to HTTP requests.
876 It lets Z39.50 packages and all other forthcoming package types
879 does mean that Metaproxy is also a Web-server in its spare time. So
880 far it does not contain either an email-reader or a Lisp
881 interpreter, but that day is surely coming.)
885 <section id="load_balance">
886 <title><literal>load_balance</literal>
887 (mp::filter::LoadBalance)</title>
889 Performs load balancing for incoming Z39.50 init requests.
890 It is used together with the <literal>virt_db</literal> filter,
891 but unlike the <literal>multi</literal> filter it does send an
892 entire session to only one of the virtual backends. The
893 <literal>load_balance</literal> filter is assuming that
894 all backend targets have equal content, and chooses the backend
895 with least load cost for a new session.
898 This filter is experimental and yet not mature for heavy load
906 <title><literal>log</literal>
907 (mp::filter::Log)</title>
909 Writes logging information to standard output, and passes on
910 the package unchanged. A log file name can be specified, as well
911 as multiple different logging formats.
916 <title><literal>multi</literal>
917 (mp::filter::Multi)</title>
919 Performs multi-database searching.
921 <link linkend="multidb">the extended discussion</link>
922 of virtual databases and multi-database searching below.
926 <section id="query_rewrite">
927 <title><literal>query_rewrite</literal>
928 (mp::filter::QueryRewrite)</title>
930 Rewrites Z39.50 <literal>Type-1</literal>
931 and <literal>Type-101</literal> (``<literal>RPN</literal>'')
933 three-step process: the query is transliterated from Z39.50
934 packet structures into an XML representation; that XML
935 representation is transformed by an XSLT stylesheet; and the
936 resulting XML is transliterated back into the Z39.50 packet
942 <section id="record_transform">
943 <title><literal>record_transform</literal>
944 (mp::filter::RecordTransform)</title>
946 This filter acts only on Z3950 present requests, and let all
947 other types of packages and requests pass untouched. It's use is
948 twofold: blocking Z3950 present requests, which the backend
949 server does not understand and can not honor, and transforming
950 the present syntax and elementset name according to the rules
951 specified, to fetch only existing record formats, and transform
952 them on the fly to requested record syntaxes.
956 <section id="session_shared">
957 <title><literal>session_shared</literal>
958 (mp::filter::SessionShared)</title>
960 This filter implements global sharing of
961 result sets (i.e. between threads and therefore between
962 clients), yielding performance improvements by clever resource
967 <section id="sru_z3950">
968 <title><literal>sru_z3950</literal>
969 (mp::filter::SRUtoZ3950)</title>
971 This filter transforms valid
972 SRU GET/POST/SOAP searchRetrieve requests to Z3950 init, search,
973 and present requests, and wraps the
974 received hit counts and XML records into suitable SRU response
976 The <literal>sru_z3950</literal> filter processes also SRU
977 GET/POST/SOAP explain requests, returning
978 either the absolute minimum required by the standard, or a full
979 pre-defined ZeeReX explain record.
981 <ulink url="&url.zeerex.explain;">ZeeReX Explain</ulink>
982 standard pages and the
983 <ulink url="&url.sru.explain;">SRU Explain</ulink> pages
984 for more information on the correct explain syntax.
985 SRU scan requests are not supported yet.
989 <section id="template">
990 <title><literal>template</literal>
991 (mp::filter::Template)</title>
993 Does nothing at all, merely passing the packet on. (Maybe it
994 should be called <literal>nop</literal> or
995 <literal>passthrough</literal>?) This exists not to be used, but
996 to be copied - to become the skeleton of new filters as they are
997 written. As with <literal>backend_test</literal>, this is not
998 intended for civilians.
1002 <section id="virt_db">
1003 <title><literal>virt_db</literal>
1004 (mp::filter::VirtualDB)</title>
1006 Performs virtual database selection: based on the name of the
1007 database in the search request, a server is selected, and its
1008 address added to the request in a <literal>VAL_PROXY</literal>
1009 otherInfo packet. It will subsequently be used by a
1010 <literal>z3950_client</literal> filter.
1012 <link linkend="multidb">the extended discussion</link>
1013 of virtual databases and multi-database searching below.
1017 <section id="z3950_client">
1018 <title><literal>z3950_client</literal>
1019 (mp::filter::Z3950Client)</title>
1021 A partial sink which swallows only Z39.50 packages.
1022 It performs Z39.50 searching and retrieval by proxying the
1023 packages that are passed to it. Init requests are sent to the
1024 address specified in the <literal>VAL_PROXY</literal> otherInfo
1025 attached to the request: this may have been specified by client,
1026 or generated by a <literal>virt_db</literal> filter earlier in
1027 the route. Subsequent requests are sent to the same address,
1028 which is remembered at Init time in a Session object.
1029 HTTP_Request packages and all other forthcoming package types
1030 are passed untouched.
1035 <section id="zeerex_explain">
1036 <title><literal>zeerex_explain</literal>
1037 (mp::filter::ZeerexExplain)</title>
1039 This filter acts as a sink for
1040 Z39.50 explain requests, returning a static ZeeReX
1041 Explain XML record from the config section. All other packages
1044 <ulink url="&url.zeerex.explain;">ZeeReX Explain</ulink>
1046 for more information on the correct explain syntax.
1050 This filter is not yet completed.
1059 <section id="future.directions">
1060 <title>Future directions</title>
1062 Some other filters that do not yet exist, but which would be
1063 useful, are briefly described. These may be added in future
1064 releases (or may be created by third parties, as loadable
1070 <term><literal>frontend_cli</literal> (source)</term>
1073 Command-line interface for generating requests.
1078 <term><literal>sru_client</literal> (sink)</term>
1081 SRU/GET and SRU/SOAP searching and retrieval.
1086 <term><literal>opensearch_client</literal> (sink)</term>
1089 A9 OpenSearch searching and retrieval.
1099 <chapter id="configuration">
1100 <title>Configuration: the Metaproxy configuration file format</title>
1103 <section id="configuration-introductory-notes">
1104 <title>Introductory notes</title>
1106 If Metaproxy is an interpreter providing operations on packages, then
1107 its configuration file can be thought of as a program for that
1108 interpreter. Configuration is by means of a single XML file, the name
1109 of which is supplied as the sole command-line argument to the
1110 <command>metaproxy</command> program. (See
1111 <xref linkend="reference"/> below for more information on invoking
1116 <section id="overview.xml.structure">
1117 <title>Overview of the config file XML structure</title>
1119 All elements and attributes are in the namespace
1120 <ulink url="http://indexdata.com/metaproxy"/>.
1121 This is most easily achieved by setting the default namespace on
1122 the top-level element, as here:
1125 <metaproxy xmlns="http://indexdata.com/metaproxy" version="1.0">
1128 The top-level element is <metaproxy>. This contains a
1129 <start> element, a <filters> element and a
1130 <routes> element, in that order. <filters> is
1131 optional; the other two are mandatory. All three are
1135 The <start> element is empty, but carries a
1136 <literal>route</literal> attribute, whose value is the name of
1137 route at which to start running - analogous to the name of the
1138 start production in a formal grammar.
1141 If present, <filters> contains zero or more <filter>
1142 elements. Each filter carries a <literal>type</literal> attribute
1143 which specifies what kind of filter is being defined
1144 (<literal>frontend_net</literal>, <literal>log</literal>, etc.)
1145 and contain various elements that provide suitable configuration
1146 for a filter of its type. The filter-specific elements are
1148 <xref linkend="reference"/>.
1149 Filters defined in this part of the file must carry an
1150 <literal>id</literal> attribute so that they can be referenced
1154 <routes> contains one or more <route> elements, each
1155 of which must carry an <literal>id</literal> element. One of the
1156 routes must have the ID value that was specified as the start
1157 route in the <start> element's <literal>route</literal>
1158 attribute. Each route contains zero or more <filter>
1159 elements. These are of two types. They may be empty, but carry a
1160 <literal>refid</literal> attribute whose value is the same as the
1161 <literal>id</literal> of a filter previously defined in the
1162 <filters> section. Alternatively, a route within a filter
1163 may omit the <literal>refid</literal> attribute, but contain
1164 configuration elements similar to those used for filters defined
1165 in the <filters> section. (In other words, each filter in a
1166 route may be included either by reference or by physical
1172 <section id="example.configuration">
1173 <title>An example configuration</title>
1175 The following is a small, but complete, Metaproxy configuration
1176 file (included in the distribution as
1177 <literal>metaproxy/etc/config1.xml</literal>).
1178 This file defines a very simple configuration that simply proxies
1179 to whatever back-end server the client requests, but logs each
1180 request and response. This can be useful for debugging complex
1181 client-server dialogues.
1183 <screen><![CDATA[<?xml version="1.0"?>
1184 <metaproxy xmlns="http://indexdata.com/metaproxy" version="1.0">
1185 <start route="start"/>
1187 <filter id="frontend" type="frontend_net">
1190 <filter id="backend" type="z3950_client">
1195 <filter refid="frontend"/>
1196 <filter type="log"/>
1197 <filter refid="backend"/>
1198 <filter type="bounce"/>
1204 It works by defining a single route, called
1205 <literal>start</literal>, which consists of a sequence of four
1206 filters. The first and last of these are included by reference:
1207 their <literal><filter></literal> elements have
1208 <literal>refid</literal> attributes that refer to filters defined
1209 within the prior <literal><filters></literal> section. The
1210 middle filter is included inline in the route.
1213 The four filters in the route are as follows: first, a
1214 <literal>frontend_net</literal> filter accepts Z39.50 requests
1215 from any host on port 9000; then these requests are passed through
1216 a <literal>log</literal> filter that emits a message for each
1217 request; they are then fed into a <literal>z3950_client</literal>
1218 filter, which forwards all Z39.50 requests to the client-specified
1219 back-end Z39.509 server. Those Z39.50 packages are returned by the
1220 <literal>z3950_client</literal> filter, with the response data
1221 filled by the external Z39.50 server targeted.
1222 All non-Z39.50 packages are passed through to the
1223 <literal>bounce</literal> filter, which definitely bounces
1224 everything, including fish, bananas, cold pyjamas,
1225 mutton, beef and trout packages.
1226 When the response arrives, it is handed
1227 back to the <literal>log</literal> filter, which emits another
1228 message; and then to the <literal>frontend_net</literal> filter,
1229 which returns the response to the client.
1233 <section id="config-file-modularity">
1234 <title>Config file modularity</title>
1236 Metaproxy XML configuration snippets can be reused by other
1237 filters using the <literal>XInclude</literal> standard, as seen in
1238 the <literal>/etc/config-sru-to-z3950.xml</literal> example SRU
1241 <filter id="sru" type="sru_z3950">
1242 <database name="Default">
1243 <xi:include xmlns:xi="http://www.w3.org/2001/XInclude"
1244 href="explain.xml"/>
1251 <section id="config-file-syntax-check">
1252 <title>Config file syntax checking</title>
1254 The distribution contains RelaxNG Compact and XML syntax checking
1255 files, as well as XML Schema files. These are found in the
1258 xml/schema/metaproxy.rnc
1259 xml/schema/metaproxy.rng
1260 xml/schema/metaproxy.xsd
1262 and can be used to verify or debug the XML structure of
1263 configuration files. For example, using the utility
1264 <filename>xmllint</filename>, syntax checking is done like this:
1266 xmllint --noout --schema xml/schema/metaproxy.xsd etc/config-local.xml
1267 xmllint --noout --relaxng xml/schema/metaproxy.rng etc/config-local.xml
1269 (A recent version of <literal>libxml2</literal> is required, as
1270 support for XML Schemas is a relatively recent addition.)
1273 You can of course use any other RelaxNG or XML Schema compliant tool
1281 <chapter id="multidb">
1282 <title>Virtual databases and multi-database searching</title>
1285 <section id="multidb-introductory-notes">
1286 <title>Introductory notes</title>
1288 Two of Metaproxy's filters are concerned with multiple-database
1289 operations. Of these, <literal>virt_db</literal> can work alone
1290 to control the routing of searches to one of a number of servers,
1291 while <literal>multi</literal> can work together with
1292 <literal>virt_db</literal> to perform multi-database searching, merging
1293 the results into a unified result-set - ``metasearch in a box''.
1296 The interaction between
1297 these two filters is necessarily complex: it reflects the real,
1298 irreducible complexity of multi-database searching in a protocol such
1299 as Z39.50 that separates initialization from searching, and in
1300 which the database to be searched is not known at initialization
1304 It's possible to use these filters without understanding the
1305 details of their functioning and the interaction between them; the
1306 next two sections of this chapter are ``HOW-TO'' guides for doing
1307 just that. However, debugging complex configurations will require
1308 a deeper understanding, which the last two sections of this
1309 chapters attempt to provide.
1314 <section id="multidb.virt_db">
1315 <title>Virtual databases with the <literal>virt_db</literal> filter</title>
1317 Working alone, the purpose of the
1318 <literal>virt_db</literal>
1319 filter is to route search requests to one of a selection of
1320 back-end databases. In this way, a single Z39.50 endpoint
1321 (running Metaproxy) can provide access to several different
1322 underlying services, including those that would otherwise be
1323 inaccessible due to firewalls. In many useful configurations, the
1324 back-end databases are local to the Metaproxy installation, but
1325 the software does not enforce this, and any valid Z39.50 servers
1326 may be used as back-ends.
1329 For example, a <literal>virt_db</literal>
1330 filter could be set up so that searches in the virtual database
1331 ``lc'' are forwarded to the Library of Congress bibliographic
1332 catalogue server, and searches in the virtual database ``marc''
1333 are forwarded to the toy database of MARC records that Index Data
1334 hosts for testing purposes. A <literal>virt_db</literal>
1335 configuration to make this switch would look like this:
1337 <screen><![CDATA[<filter type="virt_db">
1339 <database>lc</database>
1340 <target>z3950.loc.gov:7090/voyager</target>
1343 <database>marc</database>
1344 <target>indexdata.com/marc</target>
1346 </filter>]]></screen>
1348 As well as being useful in it own right, this filter also provides
1349 the foundation for multi-database searching.
1354 <section id="multidb.multi">
1355 <title>Multi-database search with the <literal>multi</literal> filter</title>
1357 To arrange for Metaproxy to broadcast searches to multiple back-end
1358 servers, the configuration needs to include two components: a
1359 <literal>virt_db</literal>
1360 filter that specifies multiple
1361 <literal><target></literal>
1362 elements, and a subsequent
1363 <literal>multi</literal>
1364 filter. Here, for example, is a complete configuration that
1365 broadcasts searches to both the Library of Congress catalogue and
1366 Index Data's tiny testing database of MARC records:
1368 <screen><![CDATA[<?xml version="1.0"?>
1369 <metaproxy xmlns="http://indexdata.com/metaproxy" version="1.0">
1370 <start route="start"/>
1373 <filter type="frontend_net">
1374 <threads>10</threads>
1377 <filter type="virt_db">
1379 <database>lc</database>
1380 <target>z3950.loc.gov:7090/voyager</target>
1383 <database>marc</database>
1384 <target>indexdata.com/marc</target>
1387 <database>all</database>
1388 <target>z3950.loc.gov:7090/voyager</target>
1389 <target>indexdata.com/marc</target>
1392 <filter type="multi"/>
1393 <filter type="z3950_client">
1394 <timeout>30</timeout>
1396 <filter type="bounce"/>
1399 </metaproxy>]]></screen>
1402 <literal>virt_db</literal>
1403 filter that specifies multiple
1404 <literal><target></literal>
1405 elements but without a subsequent
1406 <literal>multi</literal>
1407 filter yields surprising and undesirable results, as will be
1408 described below. Don't do that.)
1411 Metaproxy can be invoked with this configuration as follows:
1413 <screen>../src/metaproxy --config config-simple-multi.xml</screen>
1415 And thereafter, Z39.50 clients can connect to the running server
1416 (on port 9000, as specified in the configuration) and search in
1417 any of the databases
1418 <literal>lc</literal> (the Library of Congress catalogue),
1419 <literal>marc</literal> (Index Data's test database of MARC records)
1421 <literal>all</literal> (both of these). As an example, a session
1422 using the YAZ command-line client <literal>yaz-client</literal> is
1423 here included (edited for brevity and clarity):
1425 <screen><![CDATA[$ yaz-client @:9000
1429 Search was a success.
1430 Number of hits: 10000, setno 1
1434 Search was a success.
1435 Number of hits: 10, setno 3
1439 Search was a success.
1440 Number of hits: 10010, setno 4
1443 [marc]Record type: USmarc
1446 005 00000000000000.0
1447 008 910710c19910701nju 00010 eng
1451 100 10 $a Jack Collins
1452 245 10 $a How to program a computer
1458 [VOYAGER]Record type: USmarc
1460 005 20041229102447.0
1461 008 030910s2004 caua 000 0 eng
1462 035 $a (DLC) 2003112666
1463 906 $a 7 $b cbc $c orignew $d 4 $e epcn $f 20 $g y-gencatlg
1464 925 0 $a acquire $b 1 shelf copy $x policy default
1465 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.
1468 040 $a DLC $c DLC $d DLC
1469 050 00 $a MLCM 2004/03312 (G)
1470 245 10 $a 007, everything or nothing : $b Prima's official strategy guide / $c created by Kaizen Media Group.
1471 246 3 $a Double-O-seven, everything or nothing
1472 246 30 $a Prima's official strategy guide
1473 260 $a Roseville, CA : $b Prima Games, $c c2004.
1474 300 $a 161 p. : $b col. ill. ; $c 28 cm.
1475 500 $a "Platforms: Nintendo GameCube, Macintosh, PC, PlayStation 2 computer entertainment system, Xbox"--P. [4] of cover.
1476 650 0 $a Video games.
1477 710 2 $a Kaizen Media Group.
1478 856 42 $3 Publisher description $u http://www.loc.gov/catdir/description/random052/2003112666.html
1483 As can be seen, the first record in the result set is from the
1484 Index Data test database, and the second from the Library of
1485 Congress database. The result-set continues alternating records
1486 round-robin style until the point where one of the databases'
1487 records are exhausted.
1490 This example uses only two back-end databases; more may be used.
1491 There is no limitation imposed on the number of databases that may
1492 be metasearched in this way: issues of resource usage and
1493 administrative complexity dictate the practical limits.
1496 What happens when one of the databases doesn't respond? By default,
1497 the entire multi-database search fails, and the appropriate
1498 diagnostic is returned to the client. This is usually appropriate
1499 during development, when technicians need maximum information, but
1500 can be inconvenient in deployment, when users typically don't want
1501 to be bothered with problems of this kind and prefer just to get
1502 the records from the databases that are available. To obtain this
1503 latter behavior add an empty
1504 <literal><hideunavailable></literal>
1506 <literal>multi</literal> filter:
1508 <screen><![CDATA[ <filter type="multi">
1510 </filter>]]></screen>
1512 Under this regime, an error is reported to the client only if
1513 <emphasis>all</emphasis> the databases in a multi-database search
1519 <section id="multidb.what">
1520 <title>What's going on?</title>
1522 <title>Lark's vomit</title>
1524 This section goes into a level of technical detail that is
1525 probably not necessary in order to configure and use Metaproxy.
1526 It is provided only for those who like to know how things work.
1527 You should feel free to skip on to the next section if this one
1528 doesn't seem like fun.
1532 Hold on tight - this may get a little hairy.
1535 In the general course of things, a Z39.50 Init request may carry
1536 with it an otherInfo packet of type <literal>VAL_PROXY</literal>,
1537 whose value indicates the address of a Z39.50 server to which the
1538 ultimate connection is to be made. (This otherInfo packet is
1539 supported by YAZ-based Z39.50 clients and servers, but has not yet
1540 been ratified by the Maintenance Agency and so is not widely used
1541 in non-Index Data software. We're working on it.)
1542 The <literal>VAL_PROXY</literal> packet functions
1543 analogously to the absoluteURI-style Request-URI used with the GET
1544 method when a web browser asks a proxy to forward its request: see
1546 <ulink url="http://www.w3.org/Protocols/rfc2616/rfc2616-sec5.html#sec5.1.2"
1547 >Request-URI</ulink>
1549 <ulink url="http://www.w3.org/Protocols/rfc2616/rfc2616.html"
1550 >the HTTP 1.1 specification</ulink>.
1553 Within Metaproxy, Search requests that are part of the same
1554 session as an Init request that carries a
1555 <literal>VAL_PROXY</literal> otherInfo are also annotated with the
1556 same information. The role of the <literal>virt_db</literal>
1557 filter is to rewrite this otherInfo packet dependent on the
1558 virtual database that the client wants to search.
1561 When Metaproxy receives a Z39.50 Init request from a client, it
1562 doesn't immediately forward that request to the back-end server.
1563 Why not? Because it doesn't know <emphasis>which</emphasis>
1564 back-end server to forward it to until the client sends a Search
1565 request that specifies the database that it wants to search in.
1566 Instead, it just treasures the Init request up in its heart; and,
1567 later, the first time the client does a search on one of the
1568 specified virtual databases, a connection is forged to the
1569 appropriate server and the Init request is forwarded to it. If,
1570 later in the session, the same client searches in a different
1571 virtual database, then a connection is forged to the server that
1572 hosts it, and the same cached Init request is forwarded there,
1576 All of this clever Init-delaying is done by the
1577 <literal>frontend_net</literal> filter. The
1578 <literal>virt_db</literal> filter knows nothing about it; in
1579 fact, because the Init request that is received from the client
1580 doesn't get forwarded until a Search request is received, the
1581 <literal>virt_db</literal> filter (and the
1582 <literal>z3950_client</literal> filter behind it) doesn't even get
1583 invoked at Init time. The <emphasis>only</emphasis> thing that a
1584 <literal>virt_db</literal> filter ever does is rewrite the
1585 <literal>VAL_PROXY</literal> otherInfo in the requests that pass
1589 It is possible for a <literal>virt_db</literal> filter to contain
1591 <literal><target></literal>
1592 elements. What does this mean? Only that the filter will add
1593 multiple <literal>VAL_PROXY</literal> otherInfo packets to the
1594 Search requests that pass through it. That's because the virtual
1595 DB filter is dumb, and does exactly what it's told - no more, no
1597 If a Search request with multiple <literal>VAL_PROXY</literal>
1598 otherInfo packets reaches a <literal>z3950_client</literal>
1599 filter, this is an error. That filter doesn't know how to deal
1600 with multiple targets, so it will either just pick one and search
1601 in it, or (better) fail with an error message.
1604 The <literal>multi</literal> filter comes to the rescue! This is
1605 the only filter that knows how to deal with multiple
1606 <literal>VAL_PROXY</literal> otherInfo packets, and it does so by
1607 making multiple copies of the entire Search request: one for each
1608 <literal>VAL_PROXY</literal>. Each of these new copies is then
1609 passed down through the remaining filters in the route. (The
1610 copies are handled in parallel though the
1611 spawning of new threads.) Since the copies each have only one
1612 <literal>VAL_PROXY</literal> otherInfo, they can be handled by the
1613 <literal>z3950_client</literal> filter, which happily deals with
1614 each one individually. When the results of the individual
1615 searches come back up to the <literal>multi</literal> filter, it
1616 merges them into a single Search response, which is what
1617 eventually makes it back to the client.
1622 <section id="multidb.picture">
1623 <title>A picture is worth a thousand words (but only five hundred on 64-bit architectures)</title>
1627 <imagedata fileref="multi.pdf" format="PDF" scale="50"/>
1630 <imagedata fileref="multi.png" format="PNG"/>
1633 <!-- Fall back if none of the images can be used -->
1635 [Here there should be a diagram showing the progress of
1636 packages through the filters during a simple virtual-database
1637 search and a multi-database search, but is seems that your
1638 tool chain has not been able to include the diagram in this
1642 <!-- ### This used to work with an older version of DocBook
1644 <para>Caption: progress of packages through filters.</para>
1647 </inlinemediaobject>
1653 <chapter id="sru-server">
1654 <title>Combined SRU webservice and Z39.50 server configuration</title>
1656 Metaproxy can act as
1657 <ulink url="&url.sru;">SRU</ulink> and
1658 web service server, which translates web service requests to
1659 <ulink url="&url.z39.50;">ANSI/NISO Z39.50</ulink> packages and
1660 sends them off to common available targets.
1663 A typical setup for this operation needs a filter route including the
1667 <table id="sru-server-table-config" frame="top">
1668 <title>SRU/Z39.50 Server Filter Route Configuration</title>
1672 <entry>Filter</entry>
1673 <entry>Importance</entry>
1674 <entry>Purpose</entry>
1680 <entry><literal>frontend_net</literal></entry>
1681 <entry>required</entry>
1682 <entry>Accepting HTTP connections and passing them to following
1683 filters. Since this filter also accepts Z39.50 connections, the
1684 server works as SRU and Z39.50 server on the same port.</entry>
1687 <entry><literal>sru_z3950</literal></entry>
1688 <entry>required</entry>
1689 <entry>Accepting SRU GET/POST/SOAP explain and
1690 searchRetrieve requests for the the configured databases.
1691 Explain requests are directly served from the static XML configuration.
1692 SearchRetrieve requests are
1693 transformed to Z39.50 search and present packages.
1694 All other HTTP and Z39.50 packages are passed unaltered.</entry>
1697 <entry><literal>http_file</literal></entry>
1698 <entry>optional</entry>
1699 <entry>Serving HTTP requests from the filesystem. This is only
1700 needed if the server should serve XSLT stylesheets, static HTML
1701 files or Java Script for thin browser based clients.
1702 Z39.50 packages are passed unaltered.</entry>
1705 <entry><literal>cql_rpn</literal></entry>
1706 <entry>required</entry>
1707 <entry>Usually, Z39.50 servers do not talk CQL, hence the
1708 translation of the CQL query language to RPN is mandatory in
1709 most cases. Affects only Z39.50 search packages.</entry>
1712 <entry><literal>record_transform</literal></entry>
1713 <entry>optional</entry>
1714 <entry>Some Z39.50 backend targets can not present XML record
1715 syntaxes in common wanted element sets. using this filter, one
1716 can transform binary MARC records to MARCXML records, and
1717 further transform those to any needed XML schema/format by XSLT
1718 transformations. Changes only Z39.50 present packages.</entry>
1721 <entry><literal>session_shared</literal></entry>
1722 <entry>optional</entry>
1723 <entry>The stateless nature of web services requires frequent
1724 re-searching of the same targets for display of paged result set
1725 records. This might be an unacceptable burden for the accessed
1726 backend Z39.50 targets, and this mosule can be added for
1727 efficient backend target resource pooling.</entry>
1730 <entry><literal>z3950_client</literal></entry>
1731 <entry>required</entry>
1732 <entry>Finally, a Z39.50 package sink is needed in the filter
1733 chain to provide the response packages. The Z39.50 client module
1734 is used to access external targets over the network, but any
1735 coming local Z39.50 package sink could be used instead of.</entry>
1738 <entry><literal>bounce</literal></entry>
1739 <entry>required</entry>
1740 <entry>Any Metaproxy package arriving here did not do so by
1741 purpose, and is bounced back with connection closure. this
1742 prevents inifinite package hanging inside the SRU server.</entry>
1748 A typical minimal example <ulink url="&url.sru;">SRU</ulink>
1749 server configuration file is found in the tarball distribution at
1750 <literal>etc/config-sru-to-z3950.xml</literal>.
1753 Off course, any other metaproxy modules can be integrated into a
1754 SRU server solution, including, but not limited to, load balancing,
1755 multiple target querying
1756 (see <xref linkend="multidb"/>), and complex RPN query rewrites.
1763 <chapter id="extensions">
1764 <title>Writing extensions for Metaproxy</title>
1765 <para>### To be written</para>
1771 <chapter id="classes">
1772 <title>Classes in the Metaproxy source code</title>
1775 <section id="classes-introductory-notes">
1776 <title>Introductory notes</title>
1778 <emphasis>Stop! Do not read this!</emphasis>
1779 You won't enjoy it at all. You should just skip ahead to
1780 <xref linkend="reference"/>,
1782 <!-- The remainder of this paragraph is lifted verbatim from
1783 Douglas Adams' _Hitch Hiker's Guide to the Galaxy_, chapter 8 -->
1784 you things you really need to know, like the fact that the
1785 fabulously beautiful planet Bethselamin is now so worried about
1786 the cumulative erosion by ten billion visiting tourists a year
1787 that any net imbalance between the amount you eat and the amount
1788 you excrete whilst on the planet is surgically removed from your
1789 bodyweight when you leave: so every time you go to the lavatory it
1790 is vitally important to get a receipt.
1793 This chapter contains documentation of the Metaproxy source code, and is
1794 of interest only to maintainers and developers. If you need to
1795 change Metaproxy's behavior or write a new filter, then you will most
1796 likely find this chapter helpful. Otherwise it's a waste of your
1797 good time. Seriously: go and watch a film or something.
1798 <citetitle>This is Spinal Tap</citetitle> is particularly good.
1801 Still here? OK, let's continue.
1804 In general, classes seem to be named big-endianly, so that
1805 <literal>FactoryFilter</literal> is not a filter that filters
1806 factories, but a factory that produces filters; and
1807 <literal>FactoryStatic</literal> is a factory for the statically
1808 registered filters (as opposed to those that are dynamically
1813 <section id="individual.classes">
1814 <title>Individual classes</title>
1816 The classes making up the Metaproxy application are here listed by
1817 class-name, with the names of the source files that define them in
1821 <section id="class-FactoryFilter">
1822 <title><literal>mp::FactoryFilter</literal>
1823 (<filename>factory_filter.cpp</filename>)</title>
1825 A factory class that exists primarily to provide the
1826 <literal>create()</literal> method, which takes the name of a
1827 filter class as its argument and returns a new filter of that
1828 type. To enable this, the factory must first be populated by
1829 calling <literal>add_creator()</literal> for static filters (this
1830 is done by the <literal>FactoryStatic</literal> class, see below)
1831 and <literal>add_creator_dyn()</literal> for filters loaded
1836 <section id="class-FactoryStatic">
1837 <title><literal>mp::FactoryStatic</literal>
1838 (<filename>factory_static.cpp</filename>)</title>
1840 A subclass of <literal>FactoryFilter</literal> which is
1841 responsible for registering all the statically defined filter
1842 types. It does this by knowing about all those filters'
1843 structures, which are listed in its constructor. Merely
1844 instantiating this class registers all the static classes. It is
1845 for the benefit of this class that <literal>struct
1846 metaproxy_1_filter_struct</literal> exists, and that all the filter
1847 classes provide a static object of that type.
1851 <section id="class-filter-Base">
1852 <title><literal>mp::filter::Base</literal>
1853 (<filename>filter.cpp</filename>)</title>
1855 The virtual base class of all filters. The filter API is, on the
1856 surface at least, extremely simple: two methods.
1857 <literal>configure()</literal> is passed an XML DOM tree representing
1858 that part of the configuration file that pertains to this filter
1859 instance, and is expected to walk that tree extracting relevant
1860 information. And <literal>process()</literal> processes a
1861 package (see below). That surface simplicity is a bit
1862 misleading, as <literal>process()</literal> needs to know a lot
1863 about the <literal>Package</literal> class in order to do
1868 <section id="class-AuthSimple">
1869 <title><literal>mp::filter::AuthSimple</literal>,
1870 <literal>Backend_test</literal>, etc.
1871 (<filename>filter_auth_simple.cpp</filename>,
1872 <filename>filter_backend_test.cpp</filename>, etc.)</title>
1874 Individual filters. Each of these is implemented by a header and
1875 a source file, named <filename>filter_*.hpp</filename> and
1876 <filename>filter_*.cpp</filename> respectively. All the header
1877 files should be pretty much identical, in that they declare the
1878 class, including a private <literal>Rep</literal> class and a
1879 member pointer to it, and the two public methods.
1882 The source file for each filter needs to supply:
1887 A definition of the private <literal>Rep</literal> class.
1892 Some boilerplate constructors and destructors.
1897 A <literal>configure()</literal> method that uses the
1898 appropriate XML fragment.
1903 Most important, the <literal>process()</literal> method that
1904 does all the actual work.
1910 <section id="class-Package">
1911 <title><literal>mp::Package</literal>
1912 (<filename>package.cpp</filename>)</title>
1914 Represents a package on its way through the series of filters
1915 that make up a route. This is essentially a Z39.50 or SRU APDU
1916 together with information about where it came from, which is
1917 modified as it passes through the various filters.
1921 <section id="class-Pipe">
1922 <title><literal>mp::Pipe</literal>
1923 (<filename>pipe.cpp</filename>)</title>
1925 This class provides a compatibility layer so that we have an IPC
1926 mechanism that works the same under Unix and Windows. It's not
1927 particularly exciting.
1931 <section id="class-RouterChain">
1932 <title><literal>mp::RouterChain</literal>
1933 (<filename>router_chain.cpp</filename>)</title>
1939 <section id="class-RouterFleXML">
1940 <title><literal>mp::RouterFleXML</literal>
1941 (<filename>router_flexml.cpp</filename>)</title>
1947 <section id="class-Session">
1948 <title><literal>mp::Session</literal>
1949 (<filename>session.cpp</filename>)</title>
1955 <section id="class-ThreadPoolSocketObserver">
1956 <title><literal>mp::ThreadPoolSocketObserver</literal>
1957 (<filename>thread_pool_observer.cpp</filename>)</title>
1963 <section id="class-util">
1964 <title><literal>mp::util</literal>
1965 (<filename>util.cpp</filename>)</title>
1967 A namespace of various small utility functions and classes,
1968 collected together for convenience. Most importantly, includes
1969 the <literal>mp::util::odr</literal> class, a wrapper for YAZ's
1974 <section id="class-xml">
1975 <title><literal>mp::xml</literal>
1976 (<filename>xmlutil.cpp</filename>)</title>
1978 A namespace of various XML utility functions and classes,
1979 collected together for convenience.
1985 <section id="other.source.files">
1986 <title>Other Source Files</title>
1988 In addition to the Metaproxy source files that define the classes
1989 described above, there are a few additional files which are
1990 briefly described here:
1994 <term><literal>metaproxy_prog.cpp</literal></term>
1997 The main function of the <command>metaproxy</command> program.
2002 <term><literal>ex_router_flexml.cpp</literal></term>
2005 Identical to <literal>metaproxy_prog.cpp</literal>: it's not clear why.
2010 <term><literal>test_*.cpp</literal></term>
2013 Unit-tests for various modules.
2019 ### Still to be described:
2020 <literal>ex_filter_frontend_net.cpp</literal>,
2021 <literal>filter_dl.cpp</literal>,
2022 <literal>plainfile.cpp</literal>,
2023 <literal>tstdl.cpp</literal>.
2029 <reference id="reference">
2030 <title>Reference</title>
2031 <partintro id="reference-introduction">
2033 The material in this chapter is drawn directly from the individual
2034 manual entries. In particular, the Metaproxy invocation section is
2035 available using <command>man metaproxy</command>, and the section
2036 on each individual filter is available using the name of the filter
2037 as the argument to the <command>man</command> command.
2043 <appendix id="license">
2044 <title>License</title>
2047 Metaproxy, Copyright © 1995-2007 Index Data ApS.
2051 Metaproxy is free software; you can redistribute it and/or modify it under
2052 the terms of the GNU General Public License as published by the Free
2053 Software Foundation; either version 2, or (at your option) any later
2058 Metaproxy is distributed in the hope that it will be useful, but WITHOUT ANY
2059 WARRANTY; without even the implied warranty of MERCHANTABILITY or
2060 FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License
2065 You should have received a copy of the GNU General Public License
2066 along with Metaproxy; see the file LICENSE. If not, write to the
2067 Free Software Foundation,
2068 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA
2072 <title>GNU General Public License</title>
2074 GNU GENERAL PUBLIC LICENSE
2075 Version 2, June 1991
2077 Copyright (C) 1989, 1991 Free Software Foundation, Inc.
2078 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA
2079 w Everyone is permitted to copy and distribute verbatim copies
2080 of this license document, but changing it is not allowed.
2084 The licenses for most software are designed to take away your
2085 freedom to share and change it. By contrast, the GNU General Public
2086 License is intended to guarantee your freedom to share and change free
2087 software--to make sure the software is free for all its users. This
2088 General Public License applies to most of the Free Software
2089 Foundation's software and to any other program whose authors commit to
2090 using it. (Some other Free Software Foundation software is covered by
2091 the GNU Library General Public License instead.) You can apply it to
2094 When we speak of free software, we are referring to freedom, not
2095 price. Our General Public Licenses are designed to make sure that you
2096 have the freedom to distribute copies of free software (and charge for
2097 this service if you wish), that you receive source code or can get it
2098 if you want it, that you can change the software or use pieces of it
2099 in new free programs; and that you know you can do these things.
2101 To protect your rights, we need to make restrictions that forbid
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2103 These restrictions translate to certain responsibilities for you if you
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