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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>2005-2010</year>
39 <holder>Index Data</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 through 1.38.
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).
190 <section id="libxml2.fromsource">
191 <title>Libxml2/libxslt</title>
196 gunzip -c libxml2-version.tar.gz|tar xf -
204 gunzip -c libxslt-version.tar.gz|tar xf -
212 <section id="yaz.fromsource">
213 <title>YAZ/YAZ++</title>
215 gunzip -c yaz-version.tar.gz|tar xf -
223 gunzip -c yazpp-version.tar.gz|tar xf -
232 <title id="boost.fromsource">Boost</title>
234 Metaproxy needs components thread and test from
238 gunzip -c boost-version.tar.gz|tar xf -
240 ./configure --with-libraries=thread,test --with-toolset=gcc
246 However, under the hood bjam is used. You can invoke that with
249 ./bjam --toolset=gcc --with-thread --with-test stage
252 Replace <literal>stage</literal> with <literal>clean</literal> /
253 <literal>install</literal> to perform clean and install respectively.
256 Add <literal>--prefix=DIR</literal> to install Boost in other
257 prefix than <literal>/usr/local</literal>.
260 <section id="metaproxy.fromsource">
261 <title>Metaproxy</title>
263 gunzip -c metaproxy-version.tar.gz|tar xf -
271 You may have to tell configure where Boost is installed by supplying
272 options <literal>--with-boost</literal> and <literal>--with-boost-toolset</literal>.
273 The former sets the PREFIX for Boost (same as --prefix for Boost above).
274 The latter the compiler toolset (eg. gcc34).
277 Pass <literal>--help</literal> to configure to get a list of
283 <section id="installation.debian">
284 <title>Installation on Debian GNU/Linux</title>
286 All dependencies for Metaproxy are available as
287 <ulink url="&url.debian;">Debian</ulink>
288 packages for the sarge (stable in 2005) and etch (testing in 2005)
292 The procedures for Debian based systems, such as
293 <ulink url="&url.ubuntu;">Ubuntu</ulink> is probably similar
296 There is currently no official Debian package for YAZ++.
297 And the official Debian package for YAZ is probably too old.
298 But Index Data bulds "new" versions of those for Debian (i386 only).
301 Update the <filename>/etc/apt/sources.list</filename>
302 to include the Index Data repository.
303 See YAZ' <ulink url="&url.yaz.download.debian;">Download Debian</ulink>
304 for more information.
307 apt-get install libxslt1-dev
308 apt-get install libyazpp2-dev
309 apt-get install libboost-dev
310 apt-get install libboost-thread-dev
311 apt-get install libboost-test-dev
314 With these packages installed, the usual configure + make
315 procedure can be used for Metaproxy as outlined in
316 <xref linkend="installation.unix"/>.
320 <section id="installation.rpm">
321 <title>Installation on RPM based Linux Systems</title>
323 All external dependencies for Metaproxy are available as
324 RPM packages, either from your distribution site, or from the
325 <ulink url="http://fr.rpmfind.net/">RPMfind</ulink> site.
328 For example, an installation of the requires Boost C++ development
329 libraries on RedHat Fedora C4 and C5 can be done like this:
331 wget ftp://fr.rpmfind.net/wlinux/fedora/core/updates/testing/4/SRPMS/boost-1.33.0-3.fc4.src.rpm
332 sudo rpmbuild --buildroot src/ --rebuild -p fc4/boost-1.33.0-3.fc4.src.rpm
333 sudo rpm -U /usr/src/redhat/RPMS/i386/boost-*rpm
337 The <ulink url="&url.yaz;">YAZ</ulink> library is needed to
338 compile &metaproxy;, see there
339 for more information on available RPM packages.
342 There is currently no official RPM package for YAZ++.
343 See the <ulink url="&url.yazplusplus;">YAZ++</ulink> pages
344 for more information on a Unix tarball install.
347 With these packages installed, the usual configure + make
348 procedure can be used for Metaproxy as outlined in
349 <xref linkend="installation.unix"/>.
353 <section id="installation.windows">
354 <title>Installation on Windows</title>
356 Metaproxy can be compiled with Microsoft
357 <ulink url="&url.vstudio;">Visual Studio</ulink>.
358 Versions 2003 (C 7.1), 2005 (C 8.0) and 2008 (C 9.0) is known to work.
360 <section id="installation.windows.boost">
363 For Windows, it's easiest to get the precompiled Boost
364 package from <ulink url="&url.boost.windows.download;">here</ulink>.
365 Several versions of the Boost libraries may be selected when
366 installing Boost for windows. Please choose at least the
367 <emphasis>multithreaded</emphasis> (non-DLL) version because
368 the Metaproxy makefile uses that.
371 For more information about installing Boost refer to the
372 <ulink url="&url.boost.getting.started;">getting started</ulink>
377 <section id="installation.windows.libxslt">
378 <title>Libxslt</title>
380 <ulink url="&url.libxslt;">Libxslt</ulink> can be downloaded
382 <ulink url="&url.libxml2.download.win32;">here</ulink>.
385 Libxslt has other dependencies, but these can all be downloaded
386 from the same site. Get the following package:
387 iconv, zlib, libxml2, libxslt.
391 <section id="installation.windows.yaz">
394 <ulink url="&url.yaz;">YAZ</ulink> can be downloaded
396 <ulink url="&url.yaz.download.win32;">here</ulink>.
400 <section id="installation.windows.yazplusplus">
403 Get <ulink url="&url.yazplusplus;">YAZ++</ulink> as well.
404 Version 1.1.0 or later is required.
407 YAZ++ includes NMAKE makefiles, similar to those found in the
412 <section id="installation.windows.metaproxy">
413 <title>Metaproxy</title>
415 Metaproxy is shipped with NMAKE makefiles as well - similar
416 to those found in the YAZ++/YAZ packages. Adjust this Makefile
417 to point to the proper locations of Boost, Libxslt, Libxml2,
418 zlib, iconv, yaz and yazpp.
422 <varlistentry><term><literal>DEBUG</literal></term>
424 If set to 1, the software is
425 compiled with debugging libraries (code generation is
426 multi-threaded debug DLL).
427 If set to 0, the software is compiled with release libraries
428 (code generation is multi-threaded DLL).
433 <term><literal>BOOST</literal></term>
436 Boost install location
442 <term><literal>BOOST_VERSION</literal></term>
445 Boost version (replace . with _).
451 <term><literal>BOOST_TOOLSET</literal></term>
460 <term><literal>LIBXSLT_DIR</literal>,
461 <literal>LIBXML2_DIR</literal> ..</term>
464 Specify the locations of Libxslt, libiconv, libxml2 and
473 After successful compilation you'll find
474 <literal>metaproxy.exe</literal> in the
475 <literal>bin</literal> directory.
483 <chapter id="yazproxy-comparison">
484 <title>YAZ Proxy Comparison</title>
486 The table below lists facilities either supported by either
487 <ulink url="&url.yazproxy;">YAZ Proxy</ulink> or Metaproxy.
489 <table id="yazproxy-comparison-table">
490 <title>Metaproxy / YAZ Proxy comparison</title>
494 <entry>Facility</entry>
495 <entry>Metaproxy</entry>
496 <entry>YAZ Proxy</entry>
501 <entry>Z39.50 server</entry>
502 <entry>Using filter <literal>frontend_net</literal></entry>
503 <entry>Supported</entry>
506 <entry>SRU server</entry>
507 <entry>Supported with filter <literal>sru_z3950</literal></entry>
508 <entry>Supported</entry>
511 <entry>Z39.50 client</entry>
512 <entry>Supported with filter <literal>z3950_client</literal></entry>
513 <entry>Supported</entry>
516 <entry>SRU client</entry>
517 <entry>Unsupported</entry>
518 <entry>Unsupported</entry>
521 <entry>Connection reuse</entry>
522 <entry>Supported with filter <literal>session_shared</literal></entry>
523 <entry>Supported</entry>
526 <entry>Connection share</entry>
527 <entry>Supported with filter <literal>session_shared</literal></entry>
528 <entry>Unsupported</entry>
531 <entry>Result set reuse</entry>
532 <entry>Supported with filter <literal>session_shared</literal></entry>
533 <entry>Within one Z39.50 session / HTTP keep-alive</entry>
536 <entry>Record cache</entry>
537 <entry>Supported by filter <literal>session_shared</literal></entry>
538 <entry>Supported for last result set within one Z39.50/HTTP-keep alive session</entry>
541 <entry>Z39.50 Virtual database, i.e. select any Z39.50 target for database</entry>
542 <entry>Supported with filter <literal>virt_db</literal></entry>
543 <entry>Unsupported</entry>
546 <entry>SRU Virtual database, i.e. select any Z39.50 target for path</entry>
547 <entry>Supported with filter <literal>virt_db</literal>,
548 <literal>sru_z3950</literal></entry>
549 <entry>Supported</entry>
552 <entry>Multi target search</entry>
553 <entry>Supported with filter <literal>multi</literal> (round-robin)</entry>
554 <entry>Unsupported</entry>
557 <entry>Retrieval and search limits</entry>
558 <entry>Supported using filter <literal>limit</literal></entry>
559 <entry>Supported</entry>
562 <entry>Bandwidth limits</entry>
563 <entry>Supported using filter <literal>limit</literal></entry>
564 <entry>Supported</entry>
567 <entry>Connect limits</entry>
568 <entry>Supported by filter <literal>frontend_net</literal> (connect-max)</entry>
569 <entry>Supported</entry>
572 <entry>Retrieval sanity check and conversions</entry>
573 <entry>Supported using filter <literal>record_transform</literal></entry>
574 <entry>Supported</entry>
577 <entry>Query check</entry>
579 Supported by <literal>query_rewrite</literal> which may be check
580 a query and throw diagnostics (errors)
582 <entry>Supported</entry>
585 <entry>Query rewrite</entry>
586 <entry>Supported with <literal>query_rewrite</literal></entry>
587 <entry>Unsupported</entry>
590 <entry>Session invalidate for -1 hits</entry>
591 <entry>Unsupported</entry>
592 <entry>Supported</entry>
595 <entry>Architecture</entry>
596 <entry>Multi-threaded + select for networked modules such as
597 <literal>frontend_net</literal>)</entry>
598 <entry>Single-threaded using select</entry>
602 <entry>Extensability</entry>
603 <entry>Most functionality implemented as loadable modules</entry>
604 <entry>Unsupported and experimental</entry>
608 <entry><ulink url="&url.usemarcon;">USEMARCON</ulink></entry>
609 <entry>Unsupported</entry>
610 <entry>Supported</entry>
614 <entry>Portability</entry>
616 Requires YAZ, YAZ++ and modern C++ compiler supporting
617 <ulink url="&url.boost;">Boost</ulink>.
620 Requires YAZ and YAZ++.
621 STL is not required so pretty much any C++ compiler out there should work.
630 <chapter id="architecture">
631 <title>The Metaproxy Architecture</title>
633 The Metaproxy architecture is based on three concepts:
634 the <emphasis>package</emphasis>,
635 the <emphasis>route</emphasis>
636 and the <emphasis>filter</emphasis>.
640 <term>Packages</term>
643 A package is request or response, encoded in some protocol,
644 issued by a client, making its way through Metaproxy, send to or
645 received from a server, or sent back to the client.
648 The core of a package is the protocol unit - for example, a
649 Z39.50 Init Request or Search Response, or an SRU searchRetrieve
650 URL or Explain Response. In addition to this core, a package
651 also carries some extra information added and used by Metaproxy
655 In general, packages are doctored as they pass through
656 Metaproxy. For example, when the proxy performs authentication
657 and authorization on a Z39.50 Init request, it removes the
658 authentication credentials from the package so that they are not
659 passed onto the back-end server; and when search-response
660 packages are obtained from multiple servers, they are merged
661 into a single unified package that makes its way back to the
670 Packages make their way through routes, which can be thought of
671 as programs that operate on the package data-type. Each
672 incoming package initially makes its way through a default
673 route, but may be switched to a different route based on various
674 considerations. Routes are made up of sequences of filters (see
683 Filters provide the individual instructions within a route, and
684 effect the necessary transformations on packages. A particular
685 configuration of Metaproxy is essentially a set of filters,
686 described by configuration details and arranged in order in one
687 or more routes. There are many kinds of filter - about a dozen
688 at the time of writing with more appearing all the time - each
689 performing a specific function and configured by different
693 The word ``filter'' is sometimes used rather loosely, in two
694 different ways: it may be used to mean a particular
695 <emphasis>type</emphasis> of filter, as when we speak of ``the
696 auth_simple filter'' or ``the multi filter''; or it may be used
697 to be a specific <emphasis>instance</emphasis> of a filter
698 within a Metaproxy configuration. For example, a single
699 configuration will often contain multiple instances of the
700 <literal>z3950_client</literal> filter. In
701 operational terms, of these is a separate filter. In practice,
702 context always make it clear which sense of the word ``filter''
706 Extensibility of Metaproxy is primarily through the creation of
707 plugins that provide new filters. The filter API is small and
708 conceptually simple, but there are many details to master. See
710 <link linkend="filters">Filters</link>.
716 Since packages are created and handled by the system itself, and
717 routes are conceptually simple, most of the remainder of this
718 document concentrates on filters. After a brief overview of the
719 filter types follows, along with some thoughts on possible future
726 <chapter id="filters">
727 <title>Filters</title>
730 <section id="filters-introductory-notes">
731 <title>Introductory notes</title>
733 It's useful to think of Metaproxy as an interpreter providing a small
734 number of primitives and operations, but operating on a very
735 complex data type, namely the ``package''.
738 A package represents a Z39.50 or SRU/W request (whether for Init,
739 Search, Scan, etc.) together with information about where it came
740 from. Packages are created by front-end filters such as
741 <literal>frontend_net</literal> (see below), which reads them from
742 the network; other front-end filters are possible. They then pass
743 along a route consisting of a sequence of filters, each of which
744 transforms the package and may also have side-effects such as
745 generating logging. Eventually, the route will yield a response,
746 which is sent back to the origin.
749 There are many kinds of filter: some that are defined statically
750 as part of Metaproxy, and others may be provided by third parties
751 and dynamically loaded. They all conform to the same simple API
752 of essentially two methods: <function>configure()</function> is
753 called at startup time, and is passed an XML DOM tree representing that
754 part of the configuration file that pertains to this filter
755 instance: it is expected to walk that tree extracting relevant
756 information; and <function>process()</function> is called every
757 time the filter has to processes a package.
760 While all filters provide the same API, there are different modes
761 of functionality. Some filters are sources: they create
763 (<literal>frontend_net</literal>);
764 others are sinks: they consume packages and return a result
765 (<literal>backend_test</literal>,
766 <literal>bounce</literal>,
767 <literal>http_file</literal>,
768 <literal>z3950_client</literal>);
769 the others are true filters, that read, process and pass on the
770 packages they are fed
771 (<literal>auth_simple</literal>,
772 <literal>log</literal>,
773 <literal>multi</literal>,
774 <literal>query_rewrite</literal>,
775 <literal>record_transform</literal>,
776 <literal>session_shared</literal>,
777 <literal>sru_z3950</literal>,
778 <literal>template</literal>,
779 <literal>virt_db</literal>).
784 <section id="overview.filter.types">
785 <title>Overview of filter types</title>
787 We now briefly consider each of the types of filter supported by
788 the core Metaproxy binary. This overview is intended to give a
789 flavor of the available functionality; more detailed information
790 about each type of filter is included below in
791 <xref linkend="reference"/>.
794 The filters are here named by the string that is used as the
795 <literal>type</literal> attribute of a
796 <literal><filter></literal> element in the configuration
797 file to request them, with the name of the class that implements
798 them in parentheses. (The classname is not needed for normal
799 configuration and use of Metaproxy; it is useful only to
803 The filters are here listed in alphabetical order:
810 New virt_db-alike that does inteligent peer choice, explain merging,
811 adds FD&N to explain. Keeps init responses (like "virt_db Classic"),
812 makes routing choices based on local explain knowledge. Ref IDDI
815 Filter to convert Explain Classic to ZeeRex.
817 CQL2PQF (which needs augmented ZeeRex) - MARC for Talis.
821 Figure out what additional information we need in:
822 ZeeRex (check against D3.1)
823 Init request (e.g. loop detection)
824 Query package (e.g. number of hops)
825 Query response (e.g. record source)
829 <section id="auth_simple">
830 <title><literal>auth_simple</literal>
831 (mp::filter::AuthSimple)</title>
833 Simple authentication and authorization. The configuration
834 specifies the name of a file that is the user register, which
835 lists <varname>username</varname>:<varname>password</varname>
836 pairs, one per line, colon separated. When a session begins, it
837 is rejected unless username and passsword are supplied, and match
838 a pair in the register. The configuration file may also specific
839 the name of another file that is the target register: this lists
840 lists <varname>username</varname>:<varname>dbname</varname>,<varname>dbname</varname>...
841 sets, one per line, with multiple database names separated by
842 commas. When a search is processed, it is rejected unless the
843 database to be searched is one of those listed as available to
848 <section id="backend_test">
849 <title><literal>backend_test</literal>
850 (mp::filter::Backend_test)</title>
852 A partial sink that provides dummy responses in the manner of the
853 <literal>yaz-ztest</literal> Z39.50 server. This is useful only
854 for testing. Seriously, you don't need this. Pretend you didn't
855 even read this section.
859 <section id="bounce">
860 <title><literal>bounce</literal>
861 (mp::filter::Bounce)</title>
863 A sink that swallows <emphasis>all packages</emphasis>,
864 and returns them almost unprocessed.
865 It never sends any package of any type further down the row, but
866 sets Z39.50 packages to Z_Close, and HTTP_Request packages to
867 HTTP_Response err code 400 packages, and adds a suitable bounce
869 The bounce filter is usually added at end of each filter chain route
870 to prevent infinite hanging of for example HTTP
871 requests packages when only the Z39.50 client partial sink
872 filter is found in the
877 <section id="cql_rpn">
878 <title><literal>cql_rpn</literal>
879 (mp::filter::CQLtoRPN)</title>
881 A query language transforming filter which catches Z39.50
882 <literal>searchRequest</literal>
883 packages containing <literal>CQL</literal> queries, transforms
884 those to <literal>RPN</literal> queries,
885 and sends the <literal>searchRequests</literal> on to the next
886 filters. It is among other things useful in a SRU context.
890 <section id="frontend_net">
891 <title><literal>frontend_net</literal>
892 (mp::filter::FrontendNet)</title>
894 A source that accepts Z39.50 connections from a port
895 specified in the configuration, reads protocol units, and
896 feeds them into the next filter in the route. When the result is
897 received, it is returned to the original origin.
901 <section id="http_file">
902 <title><literal>http_file</literal>
903 (mp::filter::HttpFile)</title>
905 A partial sink which swallows only
906 <literal>HTTP_Request</literal> packages, and
907 returns the contents of files from the local
908 filesystem in response to HTTP requests.
909 It lets Z39.50 packages and all other forthcoming package types
912 does mean that Metaproxy is also a Web-server in its spare time. So
913 far it does not contain either an email-reader or a Lisp
914 interpreter, but that day is surely coming.)
918 <section id="load_balance">
919 <title><literal>load_balance</literal>
920 (mp::filter::LoadBalance)</title>
922 Performs load balancing for incoming Z39.50 init requests.
923 It is used together with the <literal>virt_db</literal> filter,
924 but unlike the <literal>multi</literal> filter it does send an
925 entire session to only one of the virtual backends. The
926 <literal>load_balance</literal> filter is assuming that
927 all backend targets have equal content, and chooses the backend
928 with least load cost for a new session.
931 This filter is experimental and yet not mature for heavy load
939 <title><literal>log</literal>
940 (mp::filter::Log)</title>
942 Writes logging information to standard output, and passes on
943 the package unchanged. A log file name can be specified, as well
944 as multiple different logging formats.
949 <title><literal>multi</literal>
950 (mp::filter::Multi)</title>
952 Performs multi-database searching.
954 <link linkend="multidb">the extended discussion</link>
955 of virtual databases and multi-database searching below.
959 <section id="query_rewrite">
960 <title><literal>query_rewrite</literal>
961 (mp::filter::QueryRewrite)</title>
963 Rewrites Z39.50 <literal>Type-1</literal>
964 and <literal>Type-101</literal> (``<literal>RPN</literal>'')
966 three-step process: the query is transliterated from Z39.50
967 packet structures into an XML representation; that XML
968 representation is transformed by an XSLT stylesheet; and the
969 resulting XML is transliterated back into the Z39.50 packet
975 <section id="record_transform">
976 <title><literal>record_transform</literal>
977 (mp::filter::RecordTransform)</title>
979 This filter acts only on Z3950 present requests, and let all
980 other types of packages and requests pass untouched. It's use is
981 twofold: blocking Z3950 present requests, which the backend
982 server does not understand and can not honor, and transforming
983 the present syntax and elementset name according to the rules
984 specified, to fetch only existing record formats, and transform
985 them on the fly to requested record syntaxes.
989 <section id="session_shared">
990 <title><literal>session_shared</literal>
991 (mp::filter::SessionShared)</title>
993 This filter implements global sharing of
994 result sets (i.e. between threads and therefore between
995 clients), yielding performance improvements by clever resource
1000 <section id="sru_z3950">
1001 <title><literal>sru_z3950</literal>
1002 (mp::filter::SRUtoZ3950)</title>
1004 This filter transforms valid
1005 SRU GET/POST/SOAP searchRetrieve requests to Z3950 init, search,
1006 and present requests, and wraps the
1007 received hit counts and XML records into suitable SRU response
1009 The <literal>sru_z3950</literal> filter processes also SRU
1010 GET/POST/SOAP explain requests, returning
1011 either the absolute minimum required by the standard, or a full
1012 pre-defined ZeeReX explain record.
1014 <ulink url="&url.zeerex.explain;">ZeeReX Explain</ulink>
1015 standard pages and the
1016 <ulink url="&url.sru.explain;">SRU Explain</ulink> pages
1017 for more information on the correct explain syntax.
1018 SRU scan requests are not supported yet.
1022 <section id="template">
1023 <title><literal>template</literal>
1024 (mp::filter::Template)</title>
1026 Does nothing at all, merely passing the packet on. (Maybe it
1027 should be called <literal>nop</literal> or
1028 <literal>passthrough</literal>?) This exists not to be used, but
1029 to be copied - to become the skeleton of new filters as they are
1030 written. As with <literal>backend_test</literal>, this is not
1031 intended for civilians.
1035 <section id="virt_db">
1036 <title><literal>virt_db</literal>
1037 (mp::filter::VirtualDB)</title>
1039 Performs virtual database selection: based on the name of the
1040 database in the search request, a server is selected, and its
1041 address added to the request in a <literal>VAL_PROXY</literal>
1042 otherInfo packet. It will subsequently be used by a
1043 <literal>z3950_client</literal> filter.
1045 <link linkend="multidb">the extended discussion</link>
1046 of virtual databases and multi-database searching below.
1050 <section id="z3950_client">
1051 <title><literal>z3950_client</literal>
1052 (mp::filter::Z3950Client)</title>
1054 A partial sink which swallows only Z39.50 packages.
1055 It performs Z39.50 searching and retrieval by proxying the
1056 packages that are passed to it. Init requests are sent to the
1057 address specified in the <literal>VAL_PROXY</literal> otherInfo
1058 attached to the request: this may have been specified by client,
1059 or generated by a <literal>virt_db</literal> filter earlier in
1060 the route. Subsequent requests are sent to the same address,
1061 which is remembered at Init time in a Session object.
1062 HTTP_Request packages and all other forthcoming package types
1063 are passed untouched.
1068 <section id="zeerex_explain">
1069 <title><literal>zeerex_explain</literal>
1070 (mp::filter::ZeerexExplain)</title>
1072 This filter acts as a sink for
1073 Z39.50 explain requests, returning a static ZeeReX
1074 Explain XML record from the config section. All other packages
1077 <ulink url="&url.zeerex.explain;">ZeeReX Explain</ulink>
1079 for more information on the correct explain syntax.
1083 This filter is not yet completed.
1092 <section id="future.directions">
1093 <title>Future directions</title>
1095 Some other filters that do not yet exist, but which would be
1096 useful, are briefly described. These may be added in future
1097 releases (or may be created by third parties, as loadable
1103 <term><literal>frontend_cli</literal> (source)</term>
1106 Command-line interface for generating requests.
1111 <term><literal>sru_client</literal> (sink)</term>
1114 SRU/GET and SRU/SOAP searching and retrieval.
1119 <term><literal>opensearch_client</literal> (sink)</term>
1122 A9 OpenSearch searching and retrieval.
1132 <chapter id="configuration">
1133 <title>Configuration: the Metaproxy configuration file format</title>
1136 <section id="configuration-introductory-notes">
1137 <title>Introductory notes</title>
1139 If Metaproxy is an interpreter providing operations on packages, then
1140 its configuration file can be thought of as a program for that
1141 interpreter. Configuration is by means of a single XML file, the name
1142 of which is supplied as the sole command-line argument to the
1143 <command>metaproxy</command> program. (See
1144 <xref linkend="reference"/> below for more information on invoking
1149 <section id="overview.xml.structure">
1150 <title>Overview of the config file XML structure</title>
1152 All elements and attributes are in the namespace
1153 <ulink url="http://indexdata.com/metaproxy"/>.
1154 This is most easily achieved by setting the default namespace on
1155 the top-level element, as here:
1158 <metaproxy xmlns="http://indexdata.com/metaproxy" version="1.0">
1161 The top-level element is <metaproxy>. This contains
1162 a <dlpath> element,
1163 a <start> element,
1164 a <filters> element and
1165 a <routes> element, in that order. <dlpath> and
1166 <filters> are optional; the other two are mandatory.
1167 All four are non-repeatable.
1170 The <dlpath;> element contains a text element which
1171 specifies the location of filter modules. This is only needed
1172 if Metaproxy must load 3rd party filters (most filters with Metaproxy
1173 are built into the Metaproxy application).
1176 The <start> element is empty, but carries a
1177 <literal>route</literal> attribute, whose value is the name of
1178 route at which to start running - analogous to the name of the
1179 start production in a formal grammar.
1182 If present, <filters> contains zero or more <filter>
1183 elements. Each filter carries a <literal>type</literal> attribute
1184 which specifies what kind of filter is being defined
1185 (<literal>frontend_net</literal>, <literal>log</literal>, etc.)
1186 and contain various elements that provide suitable configuration
1187 for a filter of its type. The filter-specific elements are
1189 <xref linkend="reference"/>.
1190 Filters defined in this part of the file must carry an
1191 <literal>id</literal> attribute so that they can be referenced
1195 <routes> contains one or more <route> elements, each
1196 of which must carry an <literal>id</literal> element. One of the
1197 routes must have the ID value that was specified as the start
1198 route in the <start> element's <literal>route</literal>
1199 attribute. Each route contains zero or more <filter>
1200 elements. These are of two types. They may be empty, but carry a
1201 <literal>refid</literal> attribute whose value is the same as the
1202 <literal>id</literal> of a filter previously defined in the
1203 <filters> section. Alternatively, a route within a filter
1204 may omit the <literal>refid</literal> attribute, but contain
1205 configuration elements similar to those used for filters defined
1206 in the <filters> section. (In other words, each filter in a
1207 route may be included either by reference or by physical
1213 <section id="example.configuration">
1214 <title>An example configuration</title>
1216 The following is a small, but complete, Metaproxy configuration
1217 file (included in the distribution as
1218 <literal>metaproxy/etc/config1.xml</literal>).
1219 This file defines a very simple configuration that simply proxies
1220 to whatever back-end server the client requests, but logs each
1221 request and response. This can be useful for debugging complex
1222 client-server dialogues.
1224 <screen><![CDATA[<?xml version="1.0"?>
1225 <metaproxy xmlns="http://indexdata.com/metaproxy" version="1.0">
1226 <dlpath>/usr/lib/metaproxy/modules</dlpath>
1227 <start route="start"/>
1229 <filter id="frontend" type="frontend_net">
1232 <filter id="backend" type="z3950_client">
1237 <filter refid="frontend"/>
1238 <filter type="log"/>
1239 <filter refid="backend"/>
1240 <filter type="bounce"/>
1246 It works by defining a single route, called
1247 <literal>start</literal>, which consists of a sequence of four
1248 filters. The first and last of these are included by reference:
1249 their <literal><filter></literal> elements have
1250 <literal>refid</literal> attributes that refer to filters defined
1251 within the prior <literal><filters></literal> section. The
1252 middle filter is included inline in the route.
1255 The four filters in the route are as follows: first, a
1256 <literal>frontend_net</literal> filter accepts Z39.50 requests
1257 from any host on port 9000; then these requests are passed through
1258 a <literal>log</literal> filter that emits a message for each
1259 request; they are then fed into a <literal>z3950_client</literal>
1260 filter, which forwards all Z39.50 requests to the client-specified
1261 back-end Z39.509 server. Those Z39.50 packages are returned by the
1262 <literal>z3950_client</literal> filter, with the response data
1263 filled by the external Z39.50 server targeted.
1264 All non-Z39.50 packages are passed through to the
1265 <literal>bounce</literal> filter, which definitely bounces
1266 everything, including fish, bananas, cold pyjamas,
1267 mutton, beef and trout packages.
1268 When the response arrives, it is handed
1269 back to the <literal>log</literal> filter, which emits another
1270 message; and then to the <literal>frontend_net</literal> filter,
1271 which returns the response to the client.
1275 <section id="config-file-modularity">
1276 <title>Config file modularity</title>
1278 Metaproxy XML configuration snippets can be reused by other
1279 filters using the <literal>XInclude</literal> standard, as seen in
1280 the <literal>/etc/config-sru-to-z3950.xml</literal> example SRU
1283 <filter id="sru" type="sru_z3950">
1284 <database name="Default">
1285 <xi:include xmlns:xi="http://www.w3.org/2001/XInclude"
1286 href="explain.xml"/>
1293 <section id="config-file-syntax-check">
1294 <title>Config file syntax checking</title>
1296 The distribution contains RelaxNG Compact and XML syntax checking
1297 files, as well as XML Schema files. These are found in the
1300 xml/schema/metaproxy.rnc
1301 xml/schema/metaproxy.rng
1302 xml/schema/metaproxy.xsd
1304 and can be used to verify or debug the XML structure of
1305 configuration files. For example, using the utility
1306 <filename>xmllint</filename>, syntax checking is done like this:
1308 xmllint --noout --schema xml/schema/metaproxy.xsd etc/config-local.xml
1309 xmllint --noout --relaxng xml/schema/metaproxy.rng etc/config-local.xml
1311 (A recent version of <literal>libxml2</literal> is required, as
1312 support for XML Schemas is a relatively recent addition.)
1315 You can of course use any other RelaxNG or XML Schema compliant tool
1323 <chapter id="multidb">
1324 <title>Virtual databases and multi-database searching</title>
1327 <section id="multidb-introductory-notes">
1328 <title>Introductory notes</title>
1330 Two of Metaproxy's filters are concerned with multiple-database
1331 operations. Of these, <literal>virt_db</literal> can work alone
1332 to control the routing of searches to one of a number of servers,
1333 while <literal>multi</literal> can work together with
1334 <literal>virt_db</literal> to perform multi-database searching, merging
1335 the results into a unified result-set - ``metasearch in a box''.
1338 The interaction between
1339 these two filters is necessarily complex: it reflects the real,
1340 irreducible complexity of multi-database searching in a protocol such
1341 as Z39.50 that separates initialization from searching, and in
1342 which the database to be searched is not known at initialization
1346 It's possible to use these filters without understanding the
1347 details of their functioning and the interaction between them; the
1348 next two sections of this chapter are ``HOW-TO'' guides for doing
1349 just that. However, debugging complex configurations will require
1350 a deeper understanding, which the last two sections of this
1351 chapters attempt to provide.
1356 <section id="multidb.virt_db">
1357 <title>Virtual databases with the <literal>virt_db</literal> filter</title>
1359 Working alone, the purpose of the
1360 <literal>virt_db</literal>
1361 filter is to route search requests to one of a selection of
1362 back-end databases. In this way, a single Z39.50 endpoint
1363 (running Metaproxy) can provide access to several different
1364 underlying services, including those that would otherwise be
1365 inaccessible due to firewalls. In many useful configurations, the
1366 back-end databases are local to the Metaproxy installation, but
1367 the software does not enforce this, and any valid Z39.50 servers
1368 may be used as back-ends.
1371 For example, a <literal>virt_db</literal>
1372 filter could be set up so that searches in the virtual database
1373 ``lc'' are forwarded to the Library of Congress bibliographic
1374 catalogue server, and searches in the virtual database ``marc''
1375 are forwarded to the toy database of MARC records that Index Data
1376 hosts for testing purposes. A <literal>virt_db</literal>
1377 configuration to make this switch would look like this:
1379 <screen><![CDATA[<filter type="virt_db">
1381 <database>lc</database>
1382 <target>z3950.loc.gov:7090/voyager</target>
1385 <database>marc</database>
1386 <target>indexdata.com/marc</target>
1388 </filter>]]></screen>
1390 As well as being useful in it own right, this filter also provides
1391 the foundation for multi-database searching.
1396 <section id="multidb.multi">
1397 <title>Multi-database search with the <literal>multi</literal> filter</title>
1399 To arrange for Metaproxy to broadcast searches to multiple back-end
1400 servers, the configuration needs to include two components: a
1401 <literal>virt_db</literal>
1402 filter that specifies multiple
1403 <literal><target></literal>
1404 elements, and a subsequent
1405 <literal>multi</literal>
1406 filter. Here, for example, is a complete configuration that
1407 broadcasts searches to both the Library of Congress catalogue and
1408 Index Data's tiny testing database of MARC records:
1410 <screen><![CDATA[<?xml version="1.0"?>
1411 <metaproxy xmlns="http://indexdata.com/metaproxy" version="1.0">
1412 <start route="start"/>
1415 <filter type="frontend_net">
1416 <threads>10</threads>
1419 <filter type="virt_db">
1421 <database>lc</database>
1422 <target>z3950.loc.gov:7090/voyager</target>
1425 <database>marc</database>
1426 <target>indexdata.com/marc</target>
1429 <database>all</database>
1430 <target>z3950.loc.gov:7090/voyager</target>
1431 <target>indexdata.com/marc</target>
1434 <filter type="multi"/>
1435 <filter type="z3950_client">
1436 <timeout>30</timeout>
1438 <filter type="bounce"/>
1441 </metaproxy>]]></screen>
1444 <literal>virt_db</literal>
1445 filter that specifies multiple
1446 <literal><target></literal>
1447 elements but without a subsequent
1448 <literal>multi</literal>
1449 filter yields surprising and undesirable results, as will be
1450 described below. Don't do that.)
1453 Metaproxy can be invoked with this configuration as follows:
1455 <screen>../src/metaproxy --config config-simple-multi.xml</screen>
1457 And thereafter, Z39.50 clients can connect to the running server
1458 (on port 9000, as specified in the configuration) and search in
1459 any of the databases
1460 <literal>lc</literal> (the Library of Congress catalogue),
1461 <literal>marc</literal> (Index Data's test database of MARC records)
1463 <literal>all</literal> (both of these). As an example, a session
1464 using the YAZ command-line client <literal>yaz-client</literal> is
1465 here included (edited for brevity and clarity):
1467 <screen><![CDATA[$ yaz-client @:9000
1471 Search was a success.
1472 Number of hits: 10000, setno 1
1476 Search was a success.
1477 Number of hits: 10, setno 3
1481 Search was a success.
1482 Number of hits: 10010, setno 4
1485 [marc]Record type: USmarc
1488 005 00000000000000.0
1489 008 910710c19910701nju 00010 eng
1493 100 10 $a Jack Collins
1494 245 10 $a How to program a computer
1500 [VOYAGER]Record type: USmarc
1502 005 20041229102447.0
1503 008 030910s2004 caua 000 0 eng
1504 035 $a (DLC) 2003112666
1505 906 $a 7 $b cbc $c orignew $d 4 $e epcn $f 20 $g y-gencatlg
1506 925 0 $a acquire $b 1 shelf copy $x policy default
1507 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.
1510 040 $a DLC $c DLC $d DLC
1511 050 00 $a MLCM 2004/03312 (G)
1512 245 10 $a 007, everything or nothing : $b Prima's official strategy guide / $c created by Kaizen Media Group.
1513 246 3 $a Double-O-seven, everything or nothing
1514 246 30 $a Prima's official strategy guide
1515 260 $a Roseville, CA : $b Prima Games, $c c2004.
1516 300 $a 161 p. : $b col. ill. ; $c 28 cm.
1517 500 $a "Platforms: Nintendo GameCube, Macintosh, PC, PlayStation 2 computer entertainment system, Xbox"--P. [4] of cover.
1518 650 0 $a Video games.
1519 710 2 $a Kaizen Media Group.
1520 856 42 $3 Publisher description $u http://www.loc.gov/catdir/description/random052/2003112666.html
1525 As can be seen, the first record in the result set is from the
1526 Index Data test database, and the second from the Library of
1527 Congress database. The result-set continues alternating records
1528 round-robin style until the point where one of the databases'
1529 records are exhausted.
1532 This example uses only two back-end databases; more may be used.
1533 There is no limitation imposed on the number of databases that may
1534 be metasearched in this way: issues of resource usage and
1535 administrative complexity dictate the practical limits.
1538 What happens when one of the databases doesn't respond? By default,
1539 the entire multi-database search fails, and the appropriate
1540 diagnostic is returned to the client. This is usually appropriate
1541 during development, when technicians need maximum information, but
1542 can be inconvenient in deployment, when users typically don't want
1543 to be bothered with problems of this kind and prefer just to get
1544 the records from the databases that are available. To obtain this
1545 latter behavior add an empty
1546 <literal><hideunavailable></literal>
1548 <literal>multi</literal> filter:
1550 <screen><![CDATA[ <filter type="multi">
1552 </filter>]]></screen>
1554 Under this regime, an error is reported to the client only if
1555 <emphasis>all</emphasis> the databases in a multi-database search
1561 <section id="multidb.what">
1562 <title>What's going on?</title>
1564 <title>Lark's vomit</title>
1566 This section goes into a level of technical detail that is
1567 probably not necessary in order to configure and use Metaproxy.
1568 It is provided only for those who like to know how things work.
1569 You should feel free to skip on to the next section if this one
1570 doesn't seem like fun.
1574 Hold on tight - this may get a little hairy.
1577 In the general course of things, a Z39.50 Init request may carry
1578 with it an otherInfo packet of type <literal>VAL_PROXY</literal>,
1579 whose value indicates the address of a Z39.50 server to which the
1580 ultimate connection is to be made. (This otherInfo packet is
1581 supported by YAZ-based Z39.50 clients and servers, but has not yet
1582 been ratified by the Maintenance Agency and so is not widely used
1583 in non-Index Data software. We're working on it.)
1584 The <literal>VAL_PROXY</literal> packet functions
1585 analogously to the absoluteURI-style Request-URI used with the GET
1586 method when a web browser asks a proxy to forward its request: see
1588 <ulink url="http://www.w3.org/Protocols/rfc2616/rfc2616-sec5.html#sec5.1.2"
1589 >Request-URI</ulink>
1591 <ulink url="http://www.w3.org/Protocols/rfc2616/rfc2616.html"
1592 >the HTTP 1.1 specification</ulink>.
1595 Within Metaproxy, Search requests that are part of the same
1596 session as an Init request that carries a
1597 <literal>VAL_PROXY</literal> otherInfo are also annotated with the
1598 same information. The role of the <literal>virt_db</literal>
1599 filter is to rewrite this otherInfo packet dependent on the
1600 virtual database that the client wants to search.
1603 When Metaproxy receives a Z39.50 Init request from a client, it
1604 doesn't immediately forward that request to the back-end server.
1605 Why not? Because it doesn't know <emphasis>which</emphasis>
1606 back-end server to forward it to until the client sends a Search
1607 request that specifies the database that it wants to search in.
1608 Instead, it just treasures the Init request up in its heart; and,
1609 later, the first time the client does a search on one of the
1610 specified virtual databases, a connection is forged to the
1611 appropriate server and the Init request is forwarded to it. If,
1612 later in the session, the same client searches in a different
1613 virtual database, then a connection is forged to the server that
1614 hosts it, and the same cached Init request is forwarded there,
1618 All of this clever Init-delaying is done by the
1619 <literal>frontend_net</literal> filter. The
1620 <literal>virt_db</literal> filter knows nothing about it; in
1621 fact, because the Init request that is received from the client
1622 doesn't get forwarded until a Search request is received, the
1623 <literal>virt_db</literal> filter (and the
1624 <literal>z3950_client</literal> filter behind it) doesn't even get
1625 invoked at Init time. The <emphasis>only</emphasis> thing that a
1626 <literal>virt_db</literal> filter ever does is rewrite the
1627 <literal>VAL_PROXY</literal> otherInfo in the requests that pass
1631 It is possible for a <literal>virt_db</literal> filter to contain
1633 <literal><target></literal>
1634 elements. What does this mean? Only that the filter will add
1635 multiple <literal>VAL_PROXY</literal> otherInfo packets to the
1636 Search requests that pass through it. That's because the virtual
1637 DB filter is dumb, and does exactly what it's told - no more, no
1639 If a Search request with multiple <literal>VAL_PROXY</literal>
1640 otherInfo packets reaches a <literal>z3950_client</literal>
1641 filter, this is an error. That filter doesn't know how to deal
1642 with multiple targets, so it will either just pick one and search
1643 in it, or (better) fail with an error message.
1646 The <literal>multi</literal> filter comes to the rescue! This is
1647 the only filter that knows how to deal with multiple
1648 <literal>VAL_PROXY</literal> otherInfo packets, and it does so by
1649 making multiple copies of the entire Search request: one for each
1650 <literal>VAL_PROXY</literal>. Each of these new copies is then
1651 passed down through the remaining filters in the route. (The
1652 copies are handled in parallel though the
1653 spawning of new threads.) Since the copies each have only one
1654 <literal>VAL_PROXY</literal> otherInfo, they can be handled by the
1655 <literal>z3950_client</literal> filter, which happily deals with
1656 each one individually. When the results of the individual
1657 searches come back up to the <literal>multi</literal> filter, it
1658 merges them into a single Search response, which is what
1659 eventually makes it back to the client.
1664 <section id="multidb.picture">
1665 <title>A picture is worth a thousand words (but only five hundred on 64-bit architectures)</title>
1669 <imagedata fileref="multi.pdf" format="PDF" scale="50"/>
1672 <imagedata fileref="multi.png" format="PNG"/>
1675 <!-- Fall back if none of the images can be used -->
1677 [Here there should be a diagram showing the progress of
1678 packages through the filters during a simple virtual-database
1679 search and a multi-database search, but is seems that your
1680 tool chain has not been able to include the diagram in this
1684 <!-- ### This used to work with an older version of DocBook
1686 <para>Caption: progress of packages through filters.</para>
1689 </inlinemediaobject>
1695 <chapter id="sru-server">
1696 <title>Combined SRU webservice and Z39.50 server configuration</title>
1698 Metaproxy can act as
1699 <ulink url="&url.sru;">SRU</ulink> and
1700 web service server, which translates web service requests to
1701 <ulink url="&url.z39.50;">ANSI/NISO Z39.50</ulink> packages and
1702 sends them off to common available targets.
1705 A typical setup for this operation needs a filter route including the
1709 <table id="sru-server-table-config" frame="top">
1710 <title>SRU/Z39.50 Server Filter Route Configuration</title>
1714 <entry>Filter</entry>
1715 <entry>Importance</entry>
1716 <entry>Purpose</entry>
1722 <entry><literal>frontend_net</literal></entry>
1723 <entry>required</entry>
1724 <entry>Accepting HTTP connections and passing them to following
1725 filters. Since this filter also accepts Z39.50 connections, the
1726 server works as SRU and Z39.50 server on the same port.</entry>
1729 <entry><literal>sru_z3950</literal></entry>
1730 <entry>required</entry>
1731 <entry>Accepting SRU GET/POST/SOAP explain and
1732 searchRetrieve requests for the the configured databases.
1733 Explain requests are directly served from the static XML configuration.
1734 SearchRetrieve requests are
1735 transformed to Z39.50 search and present packages.
1736 All other HTTP and Z39.50 packages are passed unaltered.</entry>
1739 <entry><literal>http_file</literal></entry>
1740 <entry>optional</entry>
1741 <entry>Serving HTTP requests from the filesystem. This is only
1742 needed if the server should serve XSLT stylesheets, static HTML
1743 files or Java Script for thin browser based clients.
1744 Z39.50 packages are passed unaltered.</entry>
1747 <entry><literal>cql_rpn</literal></entry>
1748 <entry>required</entry>
1749 <entry>Usually, Z39.50 servers do not talk CQL, hence the
1750 translation of the CQL query language to RPN is mandatory in
1751 most cases. Affects only Z39.50 search packages.</entry>
1754 <entry><literal>record_transform</literal></entry>
1755 <entry>optional</entry>
1756 <entry>Some Z39.50 backend targets can not present XML record
1757 syntaxes in common wanted element sets. using this filter, one
1758 can transform binary MARC records to MARCXML records, and
1759 further transform those to any needed XML schema/format by XSLT
1760 transformations. Changes only Z39.50 present packages.</entry>
1763 <entry><literal>session_shared</literal></entry>
1764 <entry>optional</entry>
1765 <entry>The stateless nature of web services requires frequent
1766 re-searching of the same targets for display of paged result set
1767 records. This might be an unacceptable burden for the accessed
1768 backend Z39.50 targets, and this mosule can be added for
1769 efficient backend target resource pooling.</entry>
1772 <entry><literal>z3950_client</literal></entry>
1773 <entry>required</entry>
1774 <entry>Finally, a Z39.50 package sink is needed in the filter
1775 chain to provide the response packages. The Z39.50 client module
1776 is used to access external targets over the network, but any
1777 coming local Z39.50 package sink could be used instead of.</entry>
1780 <entry><literal>bounce</literal></entry>
1781 <entry>required</entry>
1782 <entry>Any Metaproxy package arriving here did not do so by
1783 purpose, and is bounced back with connection closure. this
1784 prevents inifinite package hanging inside the SRU server.</entry>
1790 A typical minimal example <ulink url="&url.sru;">SRU</ulink>
1791 server configuration file is found in the tarball distribution at
1792 <literal>etc/config-sru-to-z3950.xml</literal>.
1795 Off course, any other metaproxy modules can be integrated into a
1796 SRU server solution, including, but not limited to, load balancing,
1797 multiple target querying
1798 (see <xref linkend="multidb"/>), and complex RPN query rewrites.
1805 <chapter id="extensions">
1806 <title>Writing extensions for Metaproxy</title>
1807 <para>### To be written</para>
1813 <chapter id="classes">
1814 <title>Classes in the Metaproxy source code</title>
1817 <section id="classes-introductory-notes">
1818 <title>Introductory notes</title>
1820 <emphasis>Stop! Do not read this!</emphasis>
1821 You won't enjoy it at all. You should just skip ahead to
1822 <xref linkend="reference"/>,
1824 <!-- The remainder of this paragraph is lifted verbatim from
1825 Douglas Adams' _Hitch Hiker's Guide to the Galaxy_, chapter 8 -->
1826 you things you really need to know, like the fact that the
1827 fabulously beautiful planet Bethselamin is now so worried about
1828 the cumulative erosion by ten billion visiting tourists a year
1829 that any net imbalance between the amount you eat and the amount
1830 you excrete whilst on the planet is surgically removed from your
1831 bodyweight when you leave: so every time you go to the lavatory it
1832 is vitally important to get a receipt.
1835 This chapter contains documentation of the Metaproxy source code, and is
1836 of interest only to maintainers and developers. If you need to
1837 change Metaproxy's behavior or write a new filter, then you will most
1838 likely find this chapter helpful. Otherwise it's a waste of your
1839 good time. Seriously: go and watch a film or something.
1840 <citetitle>This is Spinal Tap</citetitle> is particularly good.
1843 Still here? OK, let's continue.
1846 In general, classes seem to be named big-endianly, so that
1847 <literal>FactoryFilter</literal> is not a filter that filters
1848 factories, but a factory that produces filters; and
1849 <literal>FactoryStatic</literal> is a factory for the statically
1850 registered filters (as opposed to those that are dynamically
1855 <section id="individual.classes">
1856 <title>Individual classes</title>
1858 The classes making up the Metaproxy application are here listed by
1859 class-name, with the names of the source files that define them in
1863 <section id="class-FactoryFilter">
1864 <title><literal>mp::FactoryFilter</literal>
1865 (<filename>factory_filter.cpp</filename>)</title>
1867 A factory class that exists primarily to provide the
1868 <literal>create()</literal> method, which takes the name of a
1869 filter class as its argument and returns a new filter of that
1870 type. To enable this, the factory must first be populated by
1871 calling <literal>add_creator()</literal> for static filters (this
1872 is done by the <literal>FactoryStatic</literal> class, see below)
1873 and <literal>add_creator_dyn()</literal> for filters loaded
1878 <section id="class-FactoryStatic">
1879 <title><literal>mp::FactoryStatic</literal>
1880 (<filename>factory_static.cpp</filename>)</title>
1882 A subclass of <literal>FactoryFilter</literal> which is
1883 responsible for registering all the statically defined filter
1884 types. It does this by knowing about all those filters'
1885 structures, which are listed in its constructor. Merely
1886 instantiating this class registers all the static classes. It is
1887 for the benefit of this class that <literal>struct
1888 metaproxy_1_filter_struct</literal> exists, and that all the filter
1889 classes provide a static object of that type.
1893 <section id="class-filter-Base">
1894 <title><literal>mp::filter::Base</literal>
1895 (<filename>filter.cpp</filename>)</title>
1897 The virtual base class of all filters. The filter API is, on the
1898 surface at least, extremely simple: two methods.
1899 <literal>configure()</literal> is passed an XML DOM tree representing
1900 that part of the configuration file that pertains to this filter
1901 instance, and is expected to walk that tree extracting relevant
1902 information. And <literal>process()</literal> processes a
1903 package (see below). That surface simplicity is a bit
1904 misleading, as <literal>process()</literal> needs to know a lot
1905 about the <literal>Package</literal> class in order to do
1910 <section id="class-AuthSimple">
1911 <title><literal>mp::filter::AuthSimple</literal>,
1912 <literal>Backend_test</literal>, etc.
1913 (<filename>filter_auth_simple.cpp</filename>,
1914 <filename>filter_backend_test.cpp</filename>, etc.)</title>
1916 Individual filters. Each of these is implemented by a header and
1917 a source file, named <filename>filter_*.hpp</filename> and
1918 <filename>filter_*.cpp</filename> respectively. All the header
1919 files should be pretty much identical, in that they declare the
1920 class, including a private <literal>Rep</literal> class and a
1921 member pointer to it, and the two public methods.
1924 The source file for each filter needs to supply:
1929 A definition of the private <literal>Rep</literal> class.
1934 Some boilerplate constructors and destructors.
1939 A <literal>configure()</literal> method that uses the
1940 appropriate XML fragment.
1945 Most important, the <literal>process()</literal> method that
1946 does all the actual work.
1952 <section id="class-Package">
1953 <title><literal>mp::Package</literal>
1954 (<filename>package.cpp</filename>)</title>
1956 Represents a package on its way through the series of filters
1957 that make up a route. This is essentially a Z39.50 or SRU APDU
1958 together with information about where it came from, which is
1959 modified as it passes through the various filters.
1963 <section id="class-Pipe">
1964 <title><literal>mp::Pipe</literal>
1965 (<filename>pipe.cpp</filename>)</title>
1967 This class provides a compatibility layer so that we have an IPC
1968 mechanism that works the same under Unix and Windows. It's not
1969 particularly exciting.
1973 <section id="class-RouterChain">
1974 <title><literal>mp::RouterChain</literal>
1975 (<filename>router_chain.cpp</filename>)</title>
1981 <section id="class-RouterFleXML">
1982 <title><literal>mp::RouterFleXML</literal>
1983 (<filename>router_flexml.cpp</filename>)</title>
1989 <section id="class-Session">
1990 <title><literal>mp::Session</literal>
1991 (<filename>session.cpp</filename>)</title>
1997 <section id="class-ThreadPoolSocketObserver">
1998 <title><literal>mp::ThreadPoolSocketObserver</literal>
1999 (<filename>thread_pool_observer.cpp</filename>)</title>
2005 <section id="class-util">
2006 <title><literal>mp::util</literal>
2007 (<filename>util.cpp</filename>)</title>
2009 A namespace of various small utility functions and classes,
2010 collected together for convenience. Most importantly, includes
2011 the <literal>mp::util::odr</literal> class, a wrapper for YAZ's
2016 <section id="class-xml">
2017 <title><literal>mp::xml</literal>
2018 (<filename>xmlutil.cpp</filename>)</title>
2020 A namespace of various XML utility functions and classes,
2021 collected together for convenience.
2027 <section id="other.source.files">
2028 <title>Other Source Files</title>
2030 In addition to the Metaproxy source files that define the classes
2031 described above, there are a few additional files which are
2032 briefly described here:
2036 <term><literal>metaproxy_prog.cpp</literal></term>
2039 The main function of the <command>metaproxy</command> program.
2044 <term><literal>ex_router_flexml.cpp</literal></term>
2047 Identical to <literal>metaproxy_prog.cpp</literal>: it's not clear why.
2052 <term><literal>test_*.cpp</literal></term>
2055 Unit-tests for various modules.
2061 ### Still to be described:
2062 <literal>ex_filter_frontend_net.cpp</literal>,
2063 <literal>filter_dl.cpp</literal>,
2064 <literal>plainfile.cpp</literal>,
2065 <literal>tstdl.cpp</literal>.
2071 <reference id="reference">
2072 <title>Reference</title>
2073 <partintro id="reference-introduction">
2075 The material in this chapter is drawn directly from the individual
2076 manual entries. In particular, the Metaproxy invocation section is
2077 available using <command>man metaproxy</command>, and the section
2078 on each individual filter is available using the name of the filter
2079 as the argument to the <command>man</command> command.
2085 <appendix id="license">
2086 <title>License</title>
2089 Metaproxy, Copyright © 1995-2010 Index Data.
2093 Metaproxy is free software; you can redistribute it and/or modify it under
2094 the terms of the GNU General Public License as published by the Free
2095 Software Foundation; either version 2, or (at your option) any later
2100 Metaproxy is distributed in the hope that it will be useful, but WITHOUT ANY
2101 WARRANTY; without even the implied warranty of MERCHANTABILITY or
2102 FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License
2107 You should have received a copy of the GNU General Public License
2108 along with Metaproxy; see the file LICENSE. If not, write to the
2109 Free Software Foundation,
2110 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA
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