1 <?xml version="1.0" standalone="no"?>
2 <!DOCTYPE book PUBLIC "-//OASIS//DTD DocBook XML V4.1//EN"
3 "http://www.oasis-open.org/docbook/xml/4.1/docbookx.dtd"
5 <!ENTITY % local SYSTEM "local.ent">
7 <!ENTITY manref SYSTEM "manref.xml">
8 <!ENTITY progref SYSTEM "progref.xml">
9 <!ENTITY % common SYSTEM "common/common.ent">
11 <!-- Next line allows imagedata/@format="PDF" and is taken from
12 http://lists.oasis-open.org/archives/docbook/200303/msg00163.html
14 <!ENTITY % local.notation.class "| PDF">
15 <!-- Next line is necessary for some XML parsers, for reasons I
16 don't understand. I got this from
17 http://lists.oasis-open.org/archives/docbook/200303/msg00180.html
19 <!NOTATION PDF SYSTEM "PDF">
21 <!-- $Id: book.xml,v 1.59 2007-05-09 21:23:08 adam Exp $ -->
24 <title>Metaproxy - User's Guide and Reference</title>
27 <firstname>Adam</firstname><surname>Dickmeiss</surname>
30 <firstname>Marc</firstname><surname>Cromme</surname>
33 <firstname>Mike</firstname><surname>Taylor</surname>
36 <releaseinfo>&version;</releaseinfo>
38 <year>2005-2007</year>
39 <holder>Index Data ApS</holder>
43 This manual is part of Metaproxy version &version;.
46 Metaproxy is a universal router, proxy and encapsulated
47 metasearcher for information retrieval protocols. It accepts,
48 processes, interprets and redirects requests from IR clients using
49 standard protocols such as the binary
50 <ulink url="&url.z39.50;">ANSI/NISO Z39.50</ulink>
51 and the information search and retireval
52 web services <ulink url="&url.sru;">SRU</ulink>
53 and <ulink url="&url.srw;">SRW</ulink>, as
54 well as functioning as a limited
55 <ulink url="&url.http;">HTTP</ulink> server.
58 Metaproxy is configured by an XML file which
59 specifies how the software should function in terms of routes that
60 the request packets can take through the proxy, each step on a
61 route being an instantiation of a filter. Filters come in many
62 types, one for each operation: accepting Z39.50 packets, logging,
63 query transformation, multiplexing, etc. Further filter-types can
64 be added as loadable modules to extend Metaproxy functionality,
68 Metaproxy is covered by the GNU General Public License version 2.
73 <imagedata fileref="common/id.png" format="PNG"/>
76 <imagedata fileref="common/id.eps" format="EPS"/>
83 <chapter id="introduction">
84 <title>Introduction</title>
87 <ulink url="&url.metaproxy;">Metaproxy</ulink>
88 is a stand alone program that acts as a universal router, proxy and
89 encapsulated metasearcher for information retrieval protocols such
90 as <ulink url="&url.z39.50;">Z39.50</ulink>, and in the future
91 <ulink url="&url.sru;">SRU</ulink> and <ulink url="&url.srw;">SRW</ulink>.
92 To clients, it acts as a server of these protocols: it can be searched,
93 records can be retrieved from it, etc.
94 To servers, it acts as a client: it searches in them,
95 retrieves records from them, etc. it satisfies its clients'
96 requests by transforming them, multiplexing them, forwarding them
97 on to zero or more servers, merging the results, transforming
98 them, and delivering them back to the client. In addition, it
99 acts as a simple <ulink url="&url.http;">HTTP</ulink> server; support
100 for further protocols can be added in a modular fashion, through the
101 creation of new filters.
106 Fish, bananas, cold pyjamas,
107 Mutton, beef and trout!
108 - attributed to Cole Porter.
111 Metaproxy is a more capable alternative to
112 <ulink url="&url.yazproxy;">YAZ Proxy</ulink>,
113 being more powerful, flexible, configurable and extensible. Among
114 its many advantages over the older, more pedestrian work are
115 support for multiplexing (encapsulated metasearching), routing by
116 database name, authentication and authorization and serving local
117 files via HTTP. Equally significant, its modular architecture
118 facilitites the creation of pluggable modules implementing further
122 This manual will describe how to install Metaproxy
123 before giving an overview of its architecture, then discussing the
124 key concept of a filter in some depth and giving an overview of
125 the various filter types, then discussing the configuration file
126 format. After this come several optional chapters which may be
127 freely skipped: a detailed discussion of virtual databases and
128 multi-database searching, some notes on writing extensions
129 (additional filter types) and a high-level description of the
130 source code. Finally comes the reference guide, which contains
131 instructions for invoking the <command>metaproxy</command>
132 program, and detailed information on each type of filter,
137 <chapter id="installation">
138 <title>Installation</title>
140 Metaproxy depends on the following tools/libraries:
142 <varlistentry><term><ulink url="&url.yazplusplus;">YAZ++</ulink></term>
145 This is a C++ library based on <ulink url="&url.yaz;">YAZ</ulink>.
149 <varlistentry><term><ulink url="&url.libxslt;">Libxslt</ulink></term>
151 <para>This is an XSLT processor - based on
152 <ulink url="&url.libxml2;">Libxml2</ulink>. Both Libxml2 and
153 Libxslt must be installed with the development components
154 (header files, etc.) as well as the run-time libraries.
158 <varlistentry><term><ulink url="&url.boost;">Boost</ulink></term>
161 The popular C++ library. Initial versions of Metaproxy
162 was built with 1.33.0. Version 1.33.1 works too.
169 In order to compile Metaproxy a modern C++ compiler is
170 required. Boost, in particular, requires the C++ compiler
171 to facilitate the newest features. Refer to Boost
172 <ulink url="&url.boost.compilers.status;">Compiler Status</ulink>
173 for more information.
176 We have successfully built Metaproxy using the compilers
177 <ulink url="&url.gcc;">GCC</ulink> version 4.0 and
178 <ulink url="&url.vstudio;">Microsoft Visual Studio</ulink> 2003/2005.
181 <section id="installation.unix">
182 <title>Installation on Unix (from Source)</title>
184 Here is a quick step-by-step guide on how to compile all the
185 tools that Metaproxy uses. Only few systems have none of the required
186 tools binary packages. If, for example, Libxml2/libxslt are already
187 installed as development packages use those (and omit compilation).
194 gunzip -c libxml2-version.tar.gz|tar xf -
202 gunzip -c libxslt-version.tar.gz|tar xf -
213 gunzip -c yaz-version.tar.gz|tar xf -
221 gunzip -c yazpp-version.tar.gz|tar xf -
232 gunzip -c boost-version.tar.gz|tar xf -
243 gunzip -c metaproxy-version.tar.gz|tar xf -
252 <section id="installation.debian">
253 <title>Installation on Debian GNU/Linux</title>
255 All dependencies for Metaproxy are available as
256 <ulink url="&url.debian;">Debian</ulink>
257 packages for the sarge (stable in 2005) and etch (testing in 2005)
261 The procedures for Debian based systems, such as
262 <ulink url="&url.ubuntu;">Ubuntu</ulink> is probably similar
265 There is currently no official Debian package for YAZ++.
266 And the Debian package for YAZ is probably too old.
267 Update the <filename>/etc/apt/sources.list</filename>
268 to include the Index Data repository.
269 See YAZ' <ulink url="&url.yaz.download.debian;">Download Debian</ulink>
270 for more information.
273 apt-get install libxslt1-dev
274 apt-get install libyazpp-dev
275 apt-get install libboost-dev
276 apt-get install libboost-thread-dev
277 apt-get install libboost-date-time-dev
278 apt-get install libboost-program-options-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 Get Boost from its <ulink url="&url.boost;">home page</ulink>.
332 You also need Boost Jam (an alternative to make).
333 That's also available from the Boost home page.
334 The files to be downloaded are called something like:
335 <filename>boost_1_33-1.exe</filename>
337 <filename>boost-jam-3.1.12-1-ntx86.zip</filename>.
338 Unpack Boost Jam first. Put <filename>bjam.exe</filename>
339 in your system path. Make a command prompt and ensure
340 it can be found automatically. If not check the PATH.
341 The Boost .exe is a self-extracting exe with
342 complete source for Boost. Compile that source with
343 Boost Jam (An alternative to Make).
344 The compilation takes a while.
345 For Visual Studio 2003, use
347 bjam "-sTOOLS=vc-7_1"
349 Here <literal>vc-7_1</literal> refers to a "Toolset" (compiler system).
350 For Visual Studio 2005, use
352 bjam "-sTOOLS=vc-8_0"
354 To install the libraries in a common place, use
356 bjam "-sTOOLS=vc-7_1" install
358 (or vc-8_0 for VS 2005).
361 By default, the Boost build process installs the resulting
362 libraries + header files in
363 <literal>\boost\lib</literal>, <literal>\boost\include</literal>.
366 For more information about installing Boost refer to the
367 <ulink url="&url.boost.getting.started;">getting started</ulink>
372 <section id="installation.windows.libxslt">
373 <title>Libxslt</title>
375 <ulink url="&url.libxslt;">Libxslt</ulink> can be downloaded
377 <ulink url="&url.libxml2.download.win32;">here</ulink>.
380 Libxslt has other dependencies, but these can all be downloaded
381 from the same site. Get the following:
382 iconv, zlib, libxml2, libxslt.
386 <section id="installation.windows.yaz">
389 <ulink url="&url.yaz;">YAZ</ulink> can be downloaded
391 <ulink url="&url.yaz.download.win32;">here</ulink>.
395 <section id="installation.windows.yazplusplus">
398 Get <ulink url="&url.yazplusplus;">YAZ++</ulink> as well.
399 Version 1.0 or later is required. For now get it from
401 <ulink url="&url.snapshot.download;">Snapshot area</ulink>.
404 YAZ++ includes NMAKE makefiles, similar to those found in the
409 <section id="installation.windows.metaproxy">
410 <title>Metaproxy</title>
412 Metaproxy is shipped with NMAKE makefiles as well - similar
413 to those found in the YAZ++/YAZ packages. Adjust this Makefile
414 to point to the proper locations of Boost, Libxslt, Libxml2,
415 zlib, iconv, yaz and yazpp.
419 <varlistentry><term><literal>DEBUG</literal></term>
421 If set to 1, the software is
422 compiled with debugging libraries (code generation is
423 multi-threaded debug DLL).
424 If set to 0, the software is compiled with release libraries
425 (code generation is multi-threaded DLL).
430 <term><literal>BOOST</literal></term>
433 Boost install location
439 <term><literal>BOOST_VERSION</literal></term>
442 Boost version (replace . with _).
448 <term><literal>BOOST_TOOLSET</literal></term>
457 <term><literal>LIBXSLT_DIR</literal>,
458 <literal>LIBXML2_DIR</literal> ..</term>
461 Specify the locations of Libxslt, libiconv, libxml2 and
470 After successful compilation you'll find
471 <literal>metaproxy.exe</literal> in the
472 <literal>bin</literal> directory.
480 <chapter id="yazproxy-comparison">
481 <title>YAZ Proxy Comparison</title>
483 The table below lists facilities either supported by either
484 <ulink url="&url.yazproxy;">YAZ Proxy</ulink> or Metaproxy.
486 <table id="yazproxy-comparison-table">
487 <title>Metaproxy / YAZ Proxy comparison</title>
491 <entry>Facility</entry>
492 <entry>Metaproxy</entry>
493 <entry>YAZ Proxy</entry>
498 <entry>Z39.50 server</entry>
499 <entry>Using filter <literal>frontend_net</literal></entry>
500 <entry>Supported</entry>
503 <entry>SRU server</entry>
504 <entry>Supported with filter <literal>sru_z3950</literal></entry>
505 <entry>Supported</entry>
508 <entry>Z39.50 client</entry>
509 <entry>Supported with filter <literal>z3950_client</literal></entry>
510 <entry>Supported</entry>
513 <entry>SRU client</entry>
514 <entry>Unsupported</entry>
515 <entry>Unsupported</entry>
518 <entry>Connection reuse</entry>
519 <entry>Supported with filter <literal>session_shared</literal></entry>
520 <entry>Supported</entry>
523 <entry>Connection share</entry>
524 <entry>Supported with filter <literal>session_shared</literal></entry>
525 <entry>Unsupported</entry>
528 <entry>Result set reuse</entry>
529 <entry>Supported with filter <literal>session_shared</literal></entry>
530 <entry>Within one Z39.50 session / HTTP keep-alive</entry>
533 <entry>Record cache</entry>
534 <entry>Unsupported</entry>
535 <entry>Supported for last result set within one Z39.50/HTTP-keep alive session</entry>
538 <entry>Z39.50 Virtual database, i.e. select any Z39.50 target for database</entry>
539 <entry>Supported with filter <literal>virt_db</literal></entry>
540 <entry>Unsupported</entry>
543 <entry>SRU Virtual database, i.e. select any Z39.50 target for path</entry>
544 <entry>Supported with filter <literal>virt_db</literal>,
545 <literal>sru_z3950</literal></entry>
546 <entry>Supported</entry>
549 <entry>Multi target search</entry>
550 <entry>Supported with filter <literal>multi</literal> (round-robin)</entry>
551 <entry>Unsupported</entry>
554 <entry>Retrieval and search limits</entry>
555 <entry>Unsupported</entry>
556 <entry>Supported</entry>
559 <entry>Bandwidth limits</entry>
560 <entry>Unsupported</entry>
561 <entry>Supported</entry>
564 <entry>Connect limits</entry>
565 <entry>Unsupported</entry>
566 <entry>Supported</entry>
569 <entry>Retrieval sanity check and conversions</entry>
570 <entry>Supported using filter <literal>record_transform</literal></entry>
571 <entry>Supported</entry>
574 <entry>Query check</entry>
576 Supported in a limited way using <literal>query_rewrite</literal>
578 <entry>Supported</entry>
581 <entry>Query rewrite</entry>
582 <entry>Supported with <literal>query_rewrite</literal></entry>
583 <entry>Unsupported</entry>
586 <entry>Session invalidate for -1 hits</entry>
587 <entry>Unsupported</entry>
588 <entry>Supported</entry>
591 <entry>Architecture</entry>
592 <entry>Multi-threaded + select for networked modules such as
593 <literal>frontend_net</literal>)</entry>
594 <entry>Single-threaded using select</entry>
598 <entry>Extensability</entry>
599 <entry>Most functionality implemented as loadable modules</entry>
600 <entry>Unsupported and experimental</entry>
604 <entry><ulink url="&url.usemarcon;">USEMARCON</ulink></entry>
605 <entry>Unsupported</entry>
606 <entry>Supported</entry>
610 <entry>Portability</entry>
612 Requires YAZ, YAZ++ and modern C++ compiler supporting
613 <ulink url="&url.boost;">Boost</ulink>.
616 Requires YAZ and YAZ++.
617 STL is not required so pretty much any C++ compiler out there should work.
626 <chapter id="architecture">
627 <title>The Metaproxy Architecture</title>
629 The Metaproxy architecture is based on three concepts:
630 the <emphasis>package</emphasis>,
631 the <emphasis>route</emphasis>
632 and the <emphasis>filter</emphasis>.
636 <term>Packages</term>
639 A package is request or response, encoded in some protocol,
640 issued by a client, making its way through Metaproxy, send to or
641 received from a server, or sent back to the client.
644 The core of a package is the protocol unit - for example, a
645 Z39.50 Init Request or Search Response, or an SRU searchRetrieve
646 URL or Explain Response. In addition to this core, a package
647 also carries some extra information added and used by Metaproxy
651 In general, packages are doctored as they pass through
652 Metaproxy. For example, when the proxy performs authentication
653 and authorization on a Z39.50 Init request, it removes the
654 authentication credentials from the package so that they are not
655 passed onto the back-end server; and when search-response
656 packages are obtained from multiple servers, they are merged
657 into a single unified package that makes its way back to the
666 Packages make their way through routes, which can be thought of
667 as programs that operate on the package data-type. Each
668 incoming package initially makes its way through a default
669 route, but may be switched to a different route based on various
670 considerations. Routes are made up of sequences of filters (see
679 Filters provide the individual instructions within a route, and
680 effect the necessary transformations on packages. A particular
681 configuration of Metaproxy is essentially a set of filters,
682 described by configuration details and arranged in order in one
683 or more routes. There are many kinds of filter - about a dozen
684 at the time of writing with more appearing all the time - each
685 performing a specific function and configured by different
689 The word ``filter'' is sometimes used rather loosely, in two
690 different ways: it may be used to mean a particular
691 <emphasis>type</emphasis> of filter, as when we speak of ``the
692 auth_simple filter'' or ``the multi filter''; or it may be used
693 to be a specific <emphasis>instance</emphasis> of a filter
694 within a Metaproxy configuration. For example, a single
695 configuration will often contain multiple instances of the
696 <literal>z3950_client</literal> filter. In
697 operational terms, of these is a separate filter. In practice,
698 context always make it clear which sense of the word ``filter''
702 Extensibility of Metaproxy is primarily through the creation of
703 plugins that provide new filters. The filter API is small and
704 conceptually simple, but there are many details to master. See
706 <link linkend="filters">Filters</link>.
712 Since packages are created and handled by the system itself, and
713 routes are conceptually simple, most of the remainder of this
714 document concentrates on filters. After a brief overview of the
715 filter types follows, along with some thoughts on possible future
722 <chapter id="filters">
723 <title>Filters</title>
726 <section id="filters-introductory-notes">
727 <title>Introductory notes</title>
729 It's useful to think of Metaproxy as an interpreter providing a small
730 number of primitives and operations, but operating on a very
731 complex data type, namely the ``package''.
734 A package represents a Z39.50 or SRU/W request (whether for Init,
735 Search, Scan, etc.) together with information about where it came
736 from. Packages are created by front-end filters such as
737 <literal>frontend_net</literal> (see below), which reads them from
738 the network; other front-end filters are possible. They then pass
739 along a route consisting of a sequence of filters, each of which
740 transforms the package and may also have side-effects such as
741 generating logging. Eventually, the route will yield a response,
742 which is sent back to the origin.
745 There are many kinds of filter: some that are defined statically
746 as part of Metaproxy, and others may be provided by third parties
747 and dynamically loaded. They all conform to the same simple API
748 of essentially two methods: <function>configure()</function> is
749 called at startup time, and is passed an XML DOM tree representing that
750 part of the configuration file that pertains to this filter
751 instance: it is expected to walk that tree extracting relevant
752 information; and <function>process()</function> is called every
753 time the filter has to processes a package.
756 While all filters provide the same API, there are different modes
757 of functionality. Some filters are sources: they create
759 (<literal>frontend_net</literal>);
760 others are sinks: they consume packages and return a result
761 (<literal>backend_test</literal>,
762 <literal>bounce</literal>,
763 <literal>http_file</literal>,
764 <literal>z3950_client</literal>);
765 the others are true filters, that read, process and pass on the
766 packages they are fed
767 (<literal>auth_simple</literal>,
768 <literal>log</literal>,
769 <literal>multi</literal>,
770 <literal>query_rewrite</literal>,
771 <literal>record_transform</literal>,
772 <literal>session_shared</literal>,
773 <literal>sru_z3950</literal>,
774 <literal>template</literal>,
775 <literal>virt_db</literal>).
780 <section id="overview.filter.types">
781 <title>Overview of filter types</title>
783 We now briefly consider each of the types of filter supported by
784 the core Metaproxy binary. This overview is intended to give a
785 flavor of the available functionality; more detailed information
786 about each type of filter is included below in
787 <xref linkend="reference"/>.
790 The filters are here named by the string that is used as the
791 <literal>type</literal> attribute of a
792 <literal><filter></literal> element in the configuration
793 file to request them, with the name of the class that implements
794 them in parentheses. (The classname is not needed for normal
795 configuration and use of Metaproxy; it is useful only to
799 The filters are here listed in alphabetical order:
806 New virt_db-alike that does inteligent peer choice, explain merging,
807 adds FD&N to explain. Keeps init responses (like "virt_db Classic"),
808 makes routing choices based on local explain knowledge. Ref IDDI
811 Filter to convert Explain Classic to ZeeRex.
813 CQL2PQF (which needs augmented ZeeRex) - MARC for Talis.
817 Figure out what additional information we need in:
818 ZeeRex (check against D3.1)
819 Init request (e.g. loop detection)
820 Query package (e.g. number of hops)
821 Query response (e.g. record source)
825 <section id="auth_simple">
826 <title><literal>auth_simple</literal>
827 (mp::filter::AuthSimple)</title>
829 Simple authentication and authorization. The configuration
830 specifies the name of a file that is the user register, which
831 lists <varname>username</varname>:<varname>password</varname>
832 pairs, one per line, colon separated. When a session begins, it
833 is rejected unless username and passsword are supplied, and match
834 a pair in the register. The configuration file may also specific
835 the name of another file that is the target register: this lists
836 lists <varname>username</varname>:<varname>dbname</varname>,<varname>dbname</varname>...
837 sets, one per line, with multiple database names separated by
838 commas. When a search is processed, it is rejected unless the
839 database to be searched is one of those listed as available to
844 <section id="backend_test">
845 <title><literal>backend_test</literal>
846 (mp::filter::Backend_test)</title>
848 A partial sink that provides dummy responses in the manner of the
849 <literal>yaz-ztest</literal> Z39.50 server. This is useful only
850 for testing. Seriously, you don't need this. Pretend you didn't
851 even read this section.
855 <section id="bounce">
856 <title><literal>bounce</literal>
857 (mp::filter::Bounce)</title>
859 A sink that swallows <emphasis>all packages</emphasis>,
860 and returns them almost unprocessed.
861 It never sends any package of any type further down the row, but
862 sets Z39.50 packages to Z_Close, and HTTP_Request packages to
863 HTTP_Response err code 400 packages, and adds a suitable bounce
865 The bounce filter is usually added at end of each filter chain route
866 to prevent infinite hanging of for example HTTP
867 requests packages when only the Z39.50 client partial sink
868 filter is found in the
873 <section id="cql_rpn">
874 <title><literal>cql_rpn</literal>
875 (mp::filter::CQLtoRPN)</title>
877 A query language transforming filter which catches Z39.50
878 <literal>searchRequest</literal>
879 packages containing <literal>CQL</literal> queries, transforms
880 those to <literal>RPN</literal> queries,
881 and sends the <literal>searchRequests</literal> on to the next
882 filters. It is among other things useful in a SRU context.
886 <section id="frontend_net">
887 <title><literal>frontend_net</literal>
888 (mp::filter::FrontendNet)</title>
890 A source that accepts Z39.50 connections from a port
891 specified in the configuration, reads protocol units, and
892 feeds them into the next filter in the route. When the result is
893 received, it is returned to the original origin.
897 <section id="http_file">
898 <title><literal>http_file</literal>
899 (mp::filter::HttpFile)</title>
901 A partial sink which swallows only
902 <literal>HTTP_Request</literal> packages, and
903 returns the contents of files from the local
904 filesystem in response to HTTP requests.
905 It lets Z39.50 packages and all other forthcoming package types
908 does mean that Metaproxy is also a Web-server in its spare time. So
909 far it does not contain either an email-reader or a Lisp
910 interpreter, but that day is surely coming.)
914 <section id="load_balance">
915 <title><literal>load_balance</literal>
916 (mp::filter::LoadBalance)</title>
918 Performs load balancing for incoming Z39.50 init requests.
919 It is used together with the <literal>virt_db</literal> filter,
920 but unlike the <literal>multi</literal> filter it does send an
921 entire session to only one of the virtual backends. The
922 <literal>load_balance</literal> filter is assuming that
923 all backend targets have equal content, and chooses the backend
924 with least load cost for a new session.
927 This filter is experimental and yet not mature for heavy load
935 <title><literal>log</literal>
936 (mp::filter::Log)</title>
938 Writes logging information to standard output, and passes on
939 the package unchanged. A log file name can be specified, as well
940 as multiple different logging formats.
945 <title><literal>multi</literal>
946 (mp::filter::Multi)</title>
948 Performs multi-database searching.
950 <link linkend="multidb">the extended discussion</link>
951 of virtual databases and multi-database searching below.
955 <section id="query_rewrite">
956 <title><literal>query_rewrite</literal>
957 (mp::filter::QueryRewrite)</title>
959 Rewrites Z39.50 <literal>Type-1</literal>
960 and <literal>Type-101</literal> (``<literal>RPN</literal>'')
962 three-step process: the query is transliterated from Z39.50
963 packet structures into an XML representation; that XML
964 representation is transformed by an XSLT stylesheet; and the
965 resulting XML is transliterated back into the Z39.50 packet
971 <section id="record_transform">
972 <title><literal>record_transform</literal>
973 (mp::filter::RecordTransform)</title>
975 This filter acts only on Z3950 present requests, and let all
976 other types of packages and requests pass untouched. It's use is
977 twofold: blocking Z3950 present requests, which the backend
978 server does not understand and can not honor, and transforming
979 the present syntax and elementset name according to the rules
980 specified, to fetch only existing record formats, and transform
981 them on the fly to requested record syntaxes.
985 <section id="session_shared">
986 <title><literal>session_shared</literal>
987 (mp::filter::SessionShared)</title>
989 This filter implements global sharing of
990 result sets (i.e. between threads and therefore between
991 clients), yielding performance improvements by clever resource
996 <section id="sru_z3950">
997 <title><literal>sru_z3950</literal>
998 (mp::filter::SRUtoZ3950)</title>
1000 This filter transforms valid
1001 SRU GET/POST/SOAP searchRetrieve requests to Z3950 init, search,
1002 and present requests, and wraps the
1003 received hit counts and XML records into suitable SRU response
1005 The <literal>sru_z3950</literal> filter processes also SRU
1006 GET/POST/SOAP explain requests, returning
1007 either the absolute minimum required by the standard, or a full
1008 pre-defined ZeeReX explain record.
1010 <ulink url="&url.zeerex.explain;">ZeeReX Explain</ulink>
1011 standard pages and the
1012 <ulink url="&url.sru.explain;">SRU Explain</ulink> pages
1013 for more information on the correct explain syntax.
1014 SRU scan requests are not supported yet.
1018 <section id="template">
1019 <title><literal>template</literal>
1020 (mp::filter::Template)</title>
1022 Does nothing at all, merely passing the packet on. (Maybe it
1023 should be called <literal>nop</literal> or
1024 <literal>passthrough</literal>?) This exists not to be used, but
1025 to be copied - to become the skeleton of new filters as they are
1026 written. As with <literal>backend_test</literal>, this is not
1027 intended for civilians.
1031 <section id="virt_db">
1032 <title><literal>virt_db</literal>
1033 (mp::filter::VirtualDB)</title>
1035 Performs virtual database selection: based on the name of the
1036 database in the search request, a server is selected, and its
1037 address added to the request in a <literal>VAL_PROXY</literal>
1038 otherInfo packet. It will subsequently be used by a
1039 <literal>z3950_client</literal> filter.
1041 <link linkend="multidb">the extended discussion</link>
1042 of virtual databases and multi-database searching below.
1046 <section id="z3950_client">
1047 <title><literal>z3950_client</literal>
1048 (mp::filter::Z3950Client)</title>
1050 A partial sink which swallows only Z39.50 packages.
1051 It performs Z39.50 searching and retrieval by proxying the
1052 packages that are passed to it. Init requests are sent to the
1053 address specified in the <literal>VAL_PROXY</literal> otherInfo
1054 attached to the request: this may have been specified by client,
1055 or generated by a <literal>virt_db</literal> filter earlier in
1056 the route. Subsequent requests are sent to the same address,
1057 which is remembered at Init time in a Session object.
1058 HTTP_Request packages and all other forthcoming package types
1059 are passed untouched.
1064 <section id="zeerex_explain">
1065 <title><literal>zeerex_explain</literal>
1066 (mp::filter::ZeerexExplain)</title>
1068 This filter acts as a sink for
1069 Z39.50 explain requests, returning a static ZeeReX
1070 Explain XML record from the config section. All other packages
1073 <ulink url="&url.zeerex.explain;">ZeeReX Explain</ulink>
1075 for more information on the correct explain syntax.
1079 This filter is not yet completed.
1088 <section id="future.directions">
1089 <title>Future directions</title>
1091 Some other filters that do not yet exist, but which would be
1092 useful, are briefly described. These may be added in future
1093 releases (or may be created by third parties, as loadable
1099 <term><literal>frontend_cli</literal> (source)</term>
1102 Command-line interface for generating requests.
1107 <term><literal>sru_client</literal> (sink)</term>
1110 SRU/GET and SRU/SOAP searching and retrieval.
1115 <term><literal>opensearch_client</literal> (sink)</term>
1118 A9 OpenSearch searching and retrieval.
1128 <chapter id="configuration">
1129 <title>Configuration: the Metaproxy configuration file format</title>
1132 <section id="configuration-introductory-notes">
1133 <title>Introductory notes</title>
1135 If Metaproxy is an interpreter providing operations on packages, then
1136 its configuration file can be thought of as a program for that
1137 interpreter. Configuration is by means of a single XML file, the name
1138 of which is supplied as the sole command-line argument to the
1139 <command>metaproxy</command> program. (See
1140 <xref linkend="reference"/> below for more information on invoking
1145 <section id="overview.xml.structure">
1146 <title>Overview of the config file XML structure</title>
1148 All elements and attributes are in the namespace
1149 <ulink url="http://indexdata.com/metaproxy"/>.
1150 This is most easily achieved by setting the default namespace on
1151 the top-level element, as here:
1154 <metaproxy xmlns="http://indexdata.com/metaproxy" version="1.0">
1157 The top-level element is <metaproxy>. This contains a
1158 <start> element, a <filters> element and a
1159 <routes> element, in that order. <filters> is
1160 optional; the other two are mandatory. All three are
1164 The <start> element is empty, but carries a
1165 <literal>route</literal> attribute, whose value is the name of
1166 route at which to start running - analogous to the name of the
1167 start production in a formal grammar.
1170 If present, <filters> contains zero or more <filter>
1171 elements. Each filter carries a <literal>type</literal> attribute
1172 which specifies what kind of filter is being defined
1173 (<literal>frontend_net</literal>, <literal>log</literal>, etc.)
1174 and contain various elements that provide suitable configuration
1175 for a filter of its type. The filter-specific elements are
1177 <xref linkend="reference"/>.
1178 Filters defined in this part of the file must carry an
1179 <literal>id</literal> attribute so that they can be referenced
1183 <routes> contains one or more <route> elements, each
1184 of which must carry an <literal>id</literal> element. One of the
1185 routes must have the ID value that was specified as the start
1186 route in the <start> element's <literal>route</literal>
1187 attribute. Each route contains zero or more <filter>
1188 elements. These are of two types. They may be empty, but carry a
1189 <literal>refid</literal> attribute whose value is the same as the
1190 <literal>id</literal> of a filter previously defined in the
1191 <filters> section. Alternatively, a route within a filter
1192 may omit the <literal>refid</literal> attribute, but contain
1193 configuration elements similar to those used for filters defined
1194 in the <filters> section. (In other words, each filter in a
1195 route may be included either by reference or by physical
1201 <section id="example.configuration">
1202 <title>An example configuration</title>
1204 The following is a small, but complete, Metaproxy configuration
1205 file (included in the distribution as
1206 <literal>metaproxy/etc/config1.xml</literal>).
1207 This file defines a very simple configuration that simply proxies
1208 to whatever back-end server the client requests, but logs each
1209 request and response. This can be useful for debugging complex
1210 client-server dialogues.
1212 <screen><![CDATA[<?xml version="1.0"?>
1213 <metaproxy xmlns="http://indexdata.com/metaproxy" version="1.0">
1214 <start route="start"/>
1216 <filter id="frontend" type="frontend_net">
1219 <filter id="backend" type="z3950_client">
1224 <filter refid="frontend"/>
1225 <filter type="log"/>
1226 <filter refid="backend"/>
1227 <filter type="bounce"/>
1233 It works by defining a single route, called
1234 <literal>start</literal>, which consists of a sequence of four
1235 filters. The first and last of these are included by reference:
1236 their <literal><filter></literal> elements have
1237 <literal>refid</literal> attributes that refer to filters defined
1238 within the prior <literal><filters></literal> section. The
1239 middle filter is included inline in the route.
1242 The four filters in the route are as follows: first, a
1243 <literal>frontend_net</literal> filter accepts Z39.50 requests
1244 from any host on port 9000; then these requests are passed through
1245 a <literal>log</literal> filter that emits a message for each
1246 request; they are then fed into a <literal>z3950_client</literal>
1247 filter, which forwards all Z39.50 requests to the client-specified
1248 back-end Z39.509 server. Those Z39.50 packages are returned by the
1249 <literal>z3950_client</literal> filter, with the response data
1250 filled by the external Z39.50 server targeted.
1251 All non-Z39.50 packages are passed through to the
1252 <literal>bounce</literal> filter, which definitely bounces
1253 everything, including fish, bananas, cold pyjamas,
1254 mutton, beef and trout packages.
1255 When the response arrives, it is handed
1256 back to the <literal>log</literal> filter, which emits another
1257 message; and then to the <literal>frontend_net</literal> filter,
1258 which returns the response to the client.
1262 <section id="config-file-modularity">
1263 <title>Config file modularity</title>
1265 Metaproxy XML configuration snippets can be reused by other
1266 filters using the <literal>XInclude</literal> standard, as seen in
1267 the <literal>/etc/config-sru-to-z3950.xml</literal> example SRU
1270 <filter id="sru" type="sru_z3950">
1271 <database name="Default">
1272 <xi:include xmlns:xi="http://www.w3.org/2001/XInclude"
1273 href="explain.xml"/>
1280 <section id="config-file-syntax-check">
1281 <title>Config file syntax checking</title>
1283 The distribution contains RelaxNG Compact and XML syntax checking
1284 files, as well as XML Schema files. These are found in the
1287 xml/schema/metaproxy.rnc
1288 xml/schema/metaproxy.rng
1289 xml/schema/metaproxy.xsd
1291 and can be used to verify or debug the XML structure of
1292 configuration files. For example, using the utility
1293 <filename>xmllint</filename>, syntax checking is done like this:
1295 xmllint --noout --schema xml/schema/metaproxy.xsd etc/config-local.xml
1296 xmllint --noout --relaxng xml/schema/metaproxy.rng etc/config-local.xml
1298 (A recent version of <literal>libxml2</literal> is required, as
1299 support for XML Schemas is a relatively recent addition.)
1302 You can of course use any other RelaxNG or XML Schema compliant tool
1310 <chapter id="multidb">
1311 <title>Virtual databases and multi-database searching</title>
1314 <section id="multidb-introductory-notes">
1315 <title>Introductory notes</title>
1317 Two of Metaproxy's filters are concerned with multiple-database
1318 operations. Of these, <literal>virt_db</literal> can work alone
1319 to control the routing of searches to one of a number of servers,
1320 while <literal>multi</literal> can work together with
1321 <literal>virt_db</literal> to perform multi-database searching, merging
1322 the results into a unified result-set - ``metasearch in a box''.
1325 The interaction between
1326 these two filters is necessarily complex: it reflects the real,
1327 irreducible complexity of multi-database searching in a protocol such
1328 as Z39.50 that separates initialization from searching, and in
1329 which the database to be searched is not known at initialization
1333 It's possible to use these filters without understanding the
1334 details of their functioning and the interaction between them; the
1335 next two sections of this chapter are ``HOW-TO'' guides for doing
1336 just that. However, debugging complex configurations will require
1337 a deeper understanding, which the last two sections of this
1338 chapters attempt to provide.
1343 <section id="multidb.virt_db">
1344 <title>Virtual databases with the <literal>virt_db</literal> filter</title>
1346 Working alone, the purpose of the
1347 <literal>virt_db</literal>
1348 filter is to route search requests to one of a selection of
1349 back-end databases. In this way, a single Z39.50 endpoint
1350 (running Metaproxy) can provide access to several different
1351 underlying services, including those that would otherwise be
1352 inaccessible due to firewalls. In many useful configurations, the
1353 back-end databases are local to the Metaproxy installation, but
1354 the software does not enforce this, and any valid Z39.50 servers
1355 may be used as back-ends.
1358 For example, a <literal>virt_db</literal>
1359 filter could be set up so that searches in the virtual database
1360 ``lc'' are forwarded to the Library of Congress bibliographic
1361 catalogue server, and searches in the virtual database ``marc''
1362 are forwarded to the toy database of MARC records that Index Data
1363 hosts for testing purposes. A <literal>virt_db</literal>
1364 configuration to make this switch would look like this:
1366 <screen><![CDATA[<filter type="virt_db">
1368 <database>lc</database>
1369 <target>z3950.loc.gov:7090/voyager</target>
1372 <database>marc</database>
1373 <target>indexdata.com/marc</target>
1375 </filter>]]></screen>
1377 As well as being useful in it own right, this filter also provides
1378 the foundation for multi-database searching.
1383 <section id="multidb.multi">
1384 <title>Multi-database search with the <literal>multi</literal> filter</title>
1386 To arrange for Metaproxy to broadcast searches to multiple back-end
1387 servers, the configuration needs to include two components: a
1388 <literal>virt_db</literal>
1389 filter that specifies multiple
1390 <literal><target></literal>
1391 elements, and a subsequent
1392 <literal>multi</literal>
1393 filter. Here, for example, is a complete configuration that
1394 broadcasts searches to both the Library of Congress catalogue and
1395 Index Data's tiny testing database of MARC records:
1397 <screen><![CDATA[<?xml version="1.0"?>
1398 <metaproxy xmlns="http://indexdata.com/metaproxy" version="1.0">
1399 <start route="start"/>
1402 <filter type="frontend_net">
1403 <threads>10</threads>
1406 <filter type="virt_db">
1408 <database>lc</database>
1409 <target>z3950.loc.gov:7090/voyager</target>
1412 <database>marc</database>
1413 <target>indexdata.com/marc</target>
1416 <database>all</database>
1417 <target>z3950.loc.gov:7090/voyager</target>
1418 <target>indexdata.com/marc</target>
1421 <filter type="multi"/>
1422 <filter type="z3950_client">
1423 <timeout>30</timeout>
1425 <filter type="bounce"/>
1428 </metaproxy>]]></screen>
1431 <literal>virt_db</literal>
1432 filter that specifies multiple
1433 <literal><target></literal>
1434 elements but without a subsequent
1435 <literal>multi</literal>
1436 filter yields surprising and undesirable results, as will be
1437 described below. Don't do that.)
1440 Metaproxy can be invoked with this configuration as follows:
1442 <screen>../src/metaproxy --config config-simple-multi.xml</screen>
1444 And thereafter, Z39.50 clients can connect to the running server
1445 (on port 9000, as specified in the configuration) and search in
1446 any of the databases
1447 <literal>lc</literal> (the Library of Congress catalogue),
1448 <literal>marc</literal> (Index Data's test database of MARC records)
1450 <literal>all</literal> (both of these). As an example, a session
1451 using the YAZ command-line client <literal>yaz-client</literal> is
1452 here included (edited for brevity and clarity):
1454 <screen><![CDATA[$ yaz-client @:9000
1458 Search was a success.
1459 Number of hits: 10000, setno 1
1463 Search was a success.
1464 Number of hits: 10, setno 3
1468 Search was a success.
1469 Number of hits: 10010, setno 4
1472 [marc]Record type: USmarc
1475 005 00000000000000.0
1476 008 910710c19910701nju 00010 eng
1480 100 10 $a Jack Collins
1481 245 10 $a How to program a computer
1487 [VOYAGER]Record type: USmarc
1489 005 20041229102447.0
1490 008 030910s2004 caua 000 0 eng
1491 035 $a (DLC) 2003112666
1492 906 $a 7 $b cbc $c orignew $d 4 $e epcn $f 20 $g y-gencatlg
1493 925 0 $a acquire $b 1 shelf copy $x policy default
1494 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.
1497 040 $a DLC $c DLC $d DLC
1498 050 00 $a MLCM 2004/03312 (G)
1499 245 10 $a 007, everything or nothing : $b Prima's official strategy guide / $c created by Kaizen Media Group.
1500 246 3 $a Double-O-seven, everything or nothing
1501 246 30 $a Prima's official strategy guide
1502 260 $a Roseville, CA : $b Prima Games, $c c2004.
1503 300 $a 161 p. : $b col. ill. ; $c 28 cm.
1504 500 $a "Platforms: Nintendo GameCube, Macintosh, PC, PlayStation 2 computer entertainment system, Xbox"--P. [4] of cover.
1505 650 0 $a Video games.
1506 710 2 $a Kaizen Media Group.
1507 856 42 $3 Publisher description $u http://www.loc.gov/catdir/description/random052/2003112666.html
1512 As can be seen, the first record in the result set is from the
1513 Index Data test database, and the second from the Library of
1514 Congress database. The result-set continues alternating records
1515 round-robin style until the point where one of the databases'
1516 records are exhausted.
1519 This example uses only two back-end databases; more may be used.
1520 There is no limitation imposed on the number of databases that may
1521 be metasearched in this way: issues of resource usage and
1522 administrative complexity dictate the practical limits.
1525 What happens when one of the databases doesn't respond? By default,
1526 the entire multi-database search fails, and the appropriate
1527 diagnostic is returned to the client. This is usually appropriate
1528 during development, when technicians need maximum information, but
1529 can be inconvenient in deployment, when users typically don't want
1530 to be bothered with problems of this kind and prefer just to get
1531 the records from the databases that are available. To obtain this
1532 latter behavior add an empty
1533 <literal><hideunavailable></literal>
1535 <literal>multi</literal> filter:
1537 <screen><![CDATA[ <filter type="multi">
1539 </filter>]]></screen>
1541 Under this regime, an error is reported to the client only if
1542 <emphasis>all</emphasis> the databases in a multi-database search
1548 <section id="multidb.what">
1549 <title>What's going on?</title>
1551 <title>Lark's vomit</title>
1553 This section goes into a level of technical detail that is
1554 probably not necessary in order to configure and use Metaproxy.
1555 It is provided only for those who like to know how things work.
1556 You should feel free to skip on to the next section if this one
1557 doesn't seem like fun.
1561 Hold on tight - this may get a little hairy.
1564 In the general course of things, a Z39.50 Init request may carry
1565 with it an otherInfo packet of type <literal>VAL_PROXY</literal>,
1566 whose value indicates the address of a Z39.50 server to which the
1567 ultimate connection is to be made. (This otherInfo packet is
1568 supported by YAZ-based Z39.50 clients and servers, but has not yet
1569 been ratified by the Maintenance Agency and so is not widely used
1570 in non-Index Data software. We're working on it.)
1571 The <literal>VAL_PROXY</literal> packet functions
1572 analogously to the absoluteURI-style Request-URI used with the GET
1573 method when a web browser asks a proxy to forward its request: see
1575 <ulink url="http://www.w3.org/Protocols/rfc2616/rfc2616-sec5.html#sec5.1.2"
1576 >Request-URI</ulink>
1578 <ulink url="http://www.w3.org/Protocols/rfc2616/rfc2616.html"
1579 >the HTTP 1.1 specification</ulink>.
1582 Within Metaproxy, Search requests that are part of the same
1583 session as an Init request that carries a
1584 <literal>VAL_PROXY</literal> otherInfo are also annotated with the
1585 same information. The role of the <literal>virt_db</literal>
1586 filter is to rewrite this otherInfo packet dependent on the
1587 virtual database that the client wants to search.
1590 When Metaproxy receives a Z39.50 Init request from a client, it
1591 doesn't immediately forward that request to the back-end server.
1592 Why not? Because it doesn't know <emphasis>which</emphasis>
1593 back-end server to forward it to until the client sends a Search
1594 request that specifies the database that it wants to search in.
1595 Instead, it just treasures the Init request up in its heart; and,
1596 later, the first time the client does a search on one of the
1597 specified virtual databases, a connection is forged to the
1598 appropriate server and the Init request is forwarded to it. If,
1599 later in the session, the same client searches in a different
1600 virtual database, then a connection is forged to the server that
1601 hosts it, and the same cached Init request is forwarded there,
1605 All of this clever Init-delaying is done by the
1606 <literal>frontend_net</literal> filter. The
1607 <literal>virt_db</literal> filter knows nothing about it; in
1608 fact, because the Init request that is received from the client
1609 doesn't get forwarded until a Search request is received, the
1610 <literal>virt_db</literal> filter (and the
1611 <literal>z3950_client</literal> filter behind it) doesn't even get
1612 invoked at Init time. The <emphasis>only</emphasis> thing that a
1613 <literal>virt_db</literal> filter ever does is rewrite the
1614 <literal>VAL_PROXY</literal> otherInfo in the requests that pass
1618 It is possible for a <literal>virt_db</literal> filter to contain
1620 <literal><target></literal>
1621 elements. What does this mean? Only that the filter will add
1622 multiple <literal>VAL_PROXY</literal> otherInfo packets to the
1623 Search requests that pass through it. That's because the virtual
1624 DB filter is dumb, and does exactly what it's told - no more, no
1626 If a Search request with multiple <literal>VAL_PROXY</literal>
1627 otherInfo packets reaches a <literal>z3950_client</literal>
1628 filter, this is an error. That filter doesn't know how to deal
1629 with multiple targets, so it will either just pick one and search
1630 in it, or (better) fail with an error message.
1633 The <literal>multi</literal> filter comes to the rescue! This is
1634 the only filter that knows how to deal with multiple
1635 <literal>VAL_PROXY</literal> otherInfo packets, and it does so by
1636 making multiple copies of the entire Search request: one for each
1637 <literal>VAL_PROXY</literal>. Each of these new copies is then
1638 passed down through the remaining filters in the route. (The
1639 copies are handled in parallel though the
1640 spawning of new threads.) Since the copies each have only one
1641 <literal>VAL_PROXY</literal> otherInfo, they can be handled by the
1642 <literal>z3950_client</literal> filter, which happily deals with
1643 each one individually. When the results of the individual
1644 searches come back up to the <literal>multi</literal> filter, it
1645 merges them into a single Search response, which is what
1646 eventually makes it back to the client.
1651 <section id="multidb.picture">
1652 <title>A picture is worth a thousand words (but only five hundred on 64-bit architectures)</title>
1656 <imagedata fileref="multi.pdf" format="PDF" scale="50"/>
1659 <imagedata fileref="multi.png" format="PNG"/>
1662 <!-- Fall back if none of the images can be used -->
1664 [Here there should be a diagram showing the progress of
1665 packages through the filters during a simple virtual-database
1666 search and a multi-database search, but is seems that your
1667 tool chain has not been able to include the diagram in this
1671 <!-- ### This used to work with an older version of DocBook
1673 <para>Caption: progress of packages through filters.</para>
1676 </inlinemediaobject>
1682 <chapter id="sru-server">
1683 <title>Combined SRU webservice and Z39.50 server configuration</title>
1685 Metaproxy can act as
1686 <ulink url="&url.sru;">SRU</ulink> and
1687 <ulink url="&url.srw;">SRW</ulink>
1688 web service server, which translates web service requests to
1689 <ulink url="&url.z39.50;">ANSI/NISO Z39.50</ulink> packages and
1690 sends them off to common available targets.
1693 A typical setup for this operation needs a filter route including the
1697 <table id="sru-server-table-config" frame="top">
1698 <title>SRU/Z39.50 Server Filter Route Configuration</title>
1702 <entry>Filter</entry>
1703 <entry>Importance</entry>
1704 <entry>Purpose</entry>
1710 <entry><literal>frontend_net</literal></entry>
1711 <entry>required</entry>
1712 <entry>Accepting HTTP connections and passing them to following
1713 filters. Since this filter also accepts Z39.50 connections, the
1714 server works as SRU and Z39.50 server on the same port.</entry>
1717 <entry><literal>sru_z3950</literal></entry>
1718 <entry>required</entry>
1719 <entry>Accepting SRU GET/POST/SOAP explain and
1720 searchRetrieve requests for the the configured databases.
1721 Explain requests are directly served from the static XML configuration.
1722 SearchRetrieve requests are
1723 transformed to Z39.50 search and present packages.
1724 All other HTTP and Z39.50 packages are passed unaltered.</entry>
1727 <entry><literal>http_file</literal></entry>
1728 <entry>optional</entry>
1729 <entry>Serving HTTP requests from the filesystem. This is only
1730 needed if the server should serve XSLT stylesheets, static HTML
1731 files or Java Script for thin browser based clients.
1732 Z39.50 packages are passed unaltered.</entry>
1735 <entry><literal>cql_rpn</literal></entry>
1736 <entry>required</entry>
1737 <entry>Usually, Z39.50 servers do not talk CQL, hence the
1738 translation of the CQL query language to RPN is mandatory in
1739 most cases. Affects only Z39.50 search packages.</entry>
1742 <entry><literal>record_transform</literal></entry>
1743 <entry>optional</entry>
1744 <entry>Some Z39.50 backend targets can not present XML record
1745 syntaxes in common wanted element sets. using this filter, one
1746 can transform binary MARC records to MARCXML records, and
1747 further transform those to any needed XML schema/format by XSLT
1748 transformations. Changes only Z39.50 present packages.</entry>
1751 <entry><literal>session_shared</literal></entry>
1752 <entry>optional</entry>
1753 <entry>The stateless nature of web services requires frequent
1754 re-searching of the same targets for display of paged result set
1755 records. This might be an unacceptable burden for the accessed
1756 backend Z39.50 targets, and this mosule can be added for
1757 efficient backend target resource pooling.</entry>
1760 <entry><literal>z3950_client</literal></entry>
1761 <entry>required</entry>
1762 <entry>Finally, a Z39.50 package sink is needed in the filter
1763 chain to provide the response packages. The Z39.50 client module
1764 is used to access external targets over the network, but any
1765 coming local Z39.50 package sink could be used instead of.</entry>
1768 <entry><literal>bounce</literal></entry>
1769 <entry>required</entry>
1770 <entry>Any Metaproxy package arriving here did not do so by
1771 purpose, and is bounced back with connection closure. this
1772 prevents inifinite package hanging inside the SRU server.</entry>
1778 A typical minimal example <ulink url="&url.sru;">SRU</ulink> and
1779 <ulink url="&url.srw;">SRW</ulink> server configuration file is found
1780 in the tarball distribution at
1781 <literal>etc/config-sru-to-z3950.xml</literal>.
1784 Off course, any other metaproxy modules can be integrated into a
1785 SRU server solution, including, but not limited to, load balancing,
1786 multiple target querying
1787 (see <xref linkend="multidb"/>), and complex RPN query rewrites.
1794 <chapter id="extensions">
1795 <title>Writing extensions for Metaproxy</title>
1796 <para>### To be written</para>
1802 <chapter id="classes">
1803 <title>Classes in the Metaproxy source code</title>
1806 <section id="classes-introductory-notes">
1807 <title>Introductory notes</title>
1809 <emphasis>Stop! Do not read this!</emphasis>
1810 You won't enjoy it at all. You should just skip ahead to
1811 <xref linkend="reference"/>,
1813 <!-- The remainder of this paragraph is lifted verbatim from
1814 Douglas Adams' _Hitch Hiker's Guide to the Galaxy_, chapter 8 -->
1815 you things you really need to know, like the fact that the
1816 fabulously beautiful planet Bethselamin is now so worried about
1817 the cumulative erosion by ten billion visiting tourists a year
1818 that any net imbalance between the amount you eat and the amount
1819 you excrete whilst on the planet is surgically removed from your
1820 bodyweight when you leave: so every time you go to the lavatory it
1821 is vitally important to get a receipt.
1824 This chapter contains documentation of the Metaproxy source code, and is
1825 of interest only to maintainers and developers. If you need to
1826 change Metaproxy's behavior or write a new filter, then you will most
1827 likely find this chapter helpful. Otherwise it's a waste of your
1828 good time. Seriously: go and watch a film or something.
1829 <citetitle>This is Spinal Tap</citetitle> is particularly good.
1832 Still here? OK, let's continue.
1835 In general, classes seem to be named big-endianly, so that
1836 <literal>FactoryFilter</literal> is not a filter that filters
1837 factories, but a factory that produces filters; and
1838 <literal>FactoryStatic</literal> is a factory for the statically
1839 registered filters (as opposed to those that are dynamically
1844 <section id="individual.classes">
1845 <title>Individual classes</title>
1847 The classes making up the Metaproxy application are here listed by
1848 class-name, with the names of the source files that define them in
1852 <section id="class-FactoryFilter">
1853 <title><literal>mp::FactoryFilter</literal>
1854 (<filename>factory_filter.cpp</filename>)</title>
1856 A factory class that exists primarily to provide the
1857 <literal>create()</literal> method, which takes the name of a
1858 filter class as its argument and returns a new filter of that
1859 type. To enable this, the factory must first be populated by
1860 calling <literal>add_creator()</literal> for static filters (this
1861 is done by the <literal>FactoryStatic</literal> class, see below)
1862 and <literal>add_creator_dyn()</literal> for filters loaded
1867 <section id="class-FactoryStatic">
1868 <title><literal>mp::FactoryStatic</literal>
1869 (<filename>factory_static.cpp</filename>)</title>
1871 A subclass of <literal>FactoryFilter</literal> which is
1872 responsible for registering all the statically defined filter
1873 types. It does this by knowing about all those filters'
1874 structures, which are listed in its constructor. Merely
1875 instantiating this class registers all the static classes. It is
1876 for the benefit of this class that <literal>struct
1877 metaproxy_1_filter_struct</literal> exists, and that all the filter
1878 classes provide a static object of that type.
1882 <section id="class-filter-Base">
1883 <title><literal>mp::filter::Base</literal>
1884 (<filename>filter.cpp</filename>)</title>
1886 The virtual base class of all filters. The filter API is, on the
1887 surface at least, extremely simple: two methods.
1888 <literal>configure()</literal> is passed an XML DOM tree representing
1889 that part of the configuration file that pertains to this filter
1890 instance, and is expected to walk that tree extracting relevant
1891 information. And <literal>process()</literal> processes a
1892 package (see below). That surface simplicity is a bit
1893 misleading, as <literal>process()</literal> needs to know a lot
1894 about the <literal>Package</literal> class in order to do
1899 <section id="class-AuthSimple">
1900 <title><literal>mp::filter::AuthSimple</literal>,
1901 <literal>Backend_test</literal>, etc.
1902 (<filename>filter_auth_simple.cpp</filename>,
1903 <filename>filter_backend_test.cpp</filename>, etc.)</title>
1905 Individual filters. Each of these is implemented by a header and
1906 a source file, named <filename>filter_*.hpp</filename> and
1907 <filename>filter_*.cpp</filename> respectively. All the header
1908 files should be pretty much identical, in that they declare the
1909 class, including a private <literal>Rep</literal> class and a
1910 member pointer to it, and the two public methods.
1913 The source file for each filter needs to supply:
1918 A definition of the private <literal>Rep</literal> class.
1923 Some boilerplate constructors and destructors.
1928 A <literal>configure()</literal> method that uses the
1929 appropriate XML fragment.
1934 Most important, the <literal>process()</literal> method that
1935 does all the actual work.
1941 <section id="class-Package">
1942 <title><literal>mp::Package</literal>
1943 (<filename>package.cpp</filename>)</title>
1945 Represents a package on its way through the series of filters
1946 that make up a route. This is essentially a Z39.50 or SRU APDU
1947 together with information about where it came from, which is
1948 modified as it passes through the various filters.
1952 <section id="class-Pipe">
1953 <title><literal>mp::Pipe</literal>
1954 (<filename>pipe.cpp</filename>)</title>
1956 This class provides a compatibility layer so that we have an IPC
1957 mechanism that works the same under Unix and Windows. It's not
1958 particularly exciting.
1962 <section id="class-RouterChain">
1963 <title><literal>mp::RouterChain</literal>
1964 (<filename>router_chain.cpp</filename>)</title>
1970 <section id="class-RouterFleXML">
1971 <title><literal>mp::RouterFleXML</literal>
1972 (<filename>router_flexml.cpp</filename>)</title>
1978 <section id="class-Session">
1979 <title><literal>mp::Session</literal>
1980 (<filename>session.cpp</filename>)</title>
1986 <section id="class-ThreadPoolSocketObserver">
1987 <title><literal>mp::ThreadPoolSocketObserver</literal>
1988 (<filename>thread_pool_observer.cpp</filename>)</title>
1994 <section id="class-util">
1995 <title><literal>mp::util</literal>
1996 (<filename>util.cpp</filename>)</title>
1998 A namespace of various small utility functions and classes,
1999 collected together for convenience. Most importantly, includes
2000 the <literal>mp::util::odr</literal> class, a wrapper for YAZ's
2005 <section id="class-xml">
2006 <title><literal>mp::xml</literal>
2007 (<filename>xmlutil.cpp</filename>)</title>
2009 A namespace of various XML utility functions and classes,
2010 collected together for convenience.
2016 <section id="other.source.files">
2017 <title>Other Source Files</title>
2019 In addition to the Metaproxy source files that define the classes
2020 described above, there are a few additional files which are
2021 briefly described here:
2025 <term><literal>metaproxy_prog.cpp</literal></term>
2028 The main function of the <command>metaproxy</command> program.
2033 <term><literal>ex_router_flexml.cpp</literal></term>
2036 Identical to <literal>metaproxy_prog.cpp</literal>: it's not clear why.
2041 <term><literal>test_*.cpp</literal></term>
2044 Unit-tests for various modules.
2050 ### Still to be described:
2051 <literal>ex_filter_frontend_net.cpp</literal>,
2052 <literal>filter_dl.cpp</literal>,
2053 <literal>plainfile.cpp</literal>,
2054 <literal>tstdl.cpp</literal>.
2060 <reference id="reference">
2061 <title>Reference</title>
2062 <partintro id="reference-introduction">
2064 The material in this chapter is drawn directly from the individual
2065 manual entries. In particular, the Metaproxy invocation section is
2066 available using <command>man metaproxy</command>, and the section
2067 on each individual filter is available using the name of the filter
2068 as the argument to the <command>man</command> command.
2074 <appendix id="license">
2075 <title>License</title>
2078 Metaproxy, Copyright © 1995-2007 Index Data ApS.
2082 Metaproxy is free software; you can redistribute it and/or modify it under
2083 the terms of the GNU General Public License as published by the Free
2084 Software Foundation; either version 2, or (at your option) any later
2089 Metaproxy is distributed in the hope that it will be useful, but WITHOUT ANY
2090 WARRANTY; without even the implied warranty of MERCHANTABILITY or
2091 FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License
2096 You should have received a copy of the GNU General Public License
2097 along with Metaproxy; see the file LICENSE. If not, write to the
2098 Free Software Foundation,
2099 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA
2103 <title>GNU General Public License</title>
2105 GNU GENERAL PUBLIC LICENSE
2106 Version 2, June 1991
2108 Copyright (C) 1989, 1991 Free Software Foundation, Inc.
2109 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA
2110 w Everyone is permitted to copy and distribute verbatim copies
2111 of this license document, but changing it is not allowed.
2115 The licenses for most software are designed to take away your
2116 freedom to share and change it. By contrast, the GNU General Public
2117 License is intended to guarantee your freedom to share and change free
2118 software--to make sure the software is free for all its users. This
2119 General Public License applies to most of the Free Software
2120 Foundation's software and to any other program whose authors commit to
2121 using it. (Some other Free Software Foundation software is covered by
2122 the GNU Library General Public License instead.) You can apply it to
2125 When we speak of free software, we are referring to freedom, not
2126 price. Our General Public Licenses are designed to make sure that you
2127 have the freedom to distribute copies of free software (and charge for
2128 this service if you wish), that you receive source code or can get it
2129 if you want it, that you can change the software or use pieces of it
2130 in new free programs; and that you know you can do these things.
2132 To protect your rights, we need to make restrictions that forbid
2133 anyone to deny you these rights or to ask you to surrender the rights.
2134 These restrictions translate to certain responsibilities for you if you
2135 distribute copies of the software, or if you modify it.
2137 For example, if you distribute copies of such a program, whether
2138 gratis or for a fee, you must give the recipients all the rights that
2139 you have. You must make sure that they, too, receive or can get the
2140 source code. And you must show them these terms so they know their
2143 We protect your rights with two steps: (1) copyright the software, and
2144 (2) offer you this license which gives you legal permission to copy,
2145 distribute and/or modify the software.
2147 Also, for each author's protection and ours, we want to make certain
2148 that everyone understands that there is no warranty for this free
2149 software. If the software is modified by someone else and passed on, we
2150 want its recipients to know that what they have is not the original, so
2151 that any problems introduced by others will not reflect on the original
2152 authors' reputations.
2154 Finally, any free program is threatened constantly by software
2155 patents. We wish to avoid the danger that redistributors of a free
2156 program will individually obtain patent licenses, in effect making the
2157 program proprietary. To prevent this, we have made it clear that any
2158 patent must be licensed for everyone's free use or not licensed at all.
2160 The precise terms and conditions for copying, distribution and
2161 modification follow.
2163 GNU GENERAL PUBLIC LICENSE
2164 TERMS AND CONDITIONS FOR COPYING, DISTRIBUTION AND MODIFICATION
2166 0. This License applies to any program or other work which contains
2167 a notice placed by the copyright holder saying it may be distributed
2168 under the terms of this General Public License. The "Program", below,
2169 refers to any such program or work, and a "work based on the Program"
2170 means either the Program or any derivative work under copyright law:
2171 that is to say, a work containing the Program or a portion of it,
2172 either verbatim or with modifications and/or translated into another
2173 language. (Hereinafter, translation is included without limitation in
2174 the term "modification".) Each licensee is addressed as "you".
2176 Activities other than copying, distribution and modification are not
2177 covered by this License; they are outside its scope. The act of
2178 running the Program is not restricted, and the output from the Program
2179 is covered only if its contents constitute a work based on the
2180 Program (independent of having been made by running the Program).
2181 Whether that is true depends on what the Program does.
2183 1. You may copy and distribute verbatim copies of the Program's
2184 source code as you receive it, in any medium, provided that you
2185 conspicuously and appropriately publish on each copy an appropriate
2186 copyright notice and disclaimer of warranty; keep intact all the
2187 notices that refer to this License and to the absence of any warranty;
2188 and give any other recipients of the Program a copy of this License
2189 along with the Program.
2191 You may charge a fee for the physical act of transferring a copy, and
2192 you may at your option offer warranty protection in exchange for a fee.
2194 2. You may modify your copy or copies of the Program or any portion
2195 of it, thus forming a work based on the Program, and copy and
2196 distribute such modifications or work under the terms of Section 1
2197 above, provided that you also meet all of these conditions:
2199 a) You must cause the modified files to carry prominent notices
2200 stating that you changed the files and the date of any change.
2202 b) You must cause any work that you distribute or publish, that in
2203 whole or in part contains or is derived from the Program or any
2204 part thereof, to be licensed as a whole at no charge to all third
2205 parties under the terms of this License.
2207 c) If the modified program normally reads commands interactively
2208 when run, you must cause it, when started running for such
2209 interactive use in the most ordinary way, to print or display an
2210 announcement including an appropriate copyright notice and a
2211 notice that there is no warranty (or else, saying that you provide
2212 a warranty) and that users may redistribute the program under
2213 these conditions, and telling the user how to view a copy of this
2214 License. (Exception: if the Program itself is interactive but
2215 does not normally print such an announcement, your work based on
2216 the Program is not required to print an announcement.)
2218 These requirements apply to the modified work as a whole. If
2219 identifiable sections of that work are not derived from the Program,
2220 and can be reasonably considered independent and separate works in
2221 themselves, then this License, and its terms, do not apply to those
2222 sections when you distribute them as separate works. But when you
2223 distribute the same sections as part of a whole which is a work based
2224 on the Program, the distribution of the whole must be on the terms of
2225 this License, whose permissions for other licensees extend to the
2226 entire whole, and thus to each and every part regardless of who wrote it.
2228 Thus, it is not the intent of this section to claim rights or contest
2229 your rights to work written entirely by you; rather, the intent is to
2230 exercise the right to control the distribution of derivative or
2231 collective works based on the Program.
2233 In addition, mere aggregation of another work not based on the Program
2234 with the Program (or with a work based on the Program) on a volume of
2235 a storage or distribution medium does not bring the other work under
2236 the scope of this License.
2238 3. You may copy and distribute the Program (or a work based on it,
2239 under Section 2) in object code or executable form under the terms of
2240 Sections 1 and 2 above provided that you also do one of the following:
2242 a) Accompany it with the complete corresponding machine-readable
2243 source code, which must be distributed under the terms of Sections
2244 1 and 2 above on a medium customarily used for software interchange; or,
2246 b) Accompany it with a written offer, valid for at least three
2247 years, to give any third party, for a charge no more than your
2248 cost of physically performing source distribution, a complete
2249 machine-readable copy of the corresponding source code, to be
2250 distributed under the terms of Sections 1 and 2 above on a medium
2251 customarily used for software interchange; or,
2253 c) Accompany it with the information you received as to the offer
2254 to distribute corresponding source code. (This alternative is
2255 allowed only for noncommercial distribution and only if you
2256 received the program in object code or executable form with such
2257 an offer, in accord with Subsection b above.)
2259 The source code for a work means the preferred form of the work for
2260 making modifications to it. For an executable work, complete source
2261 code means all the source code for all modules it contains, plus any
2262 associated interface definition files, plus the scripts used to
2263 control compilation and installation of the executable. However, as a
2264 special exception, the source code distributed need not include
2265 anything that is normally distributed (in either source or binary
2266 form) with the major components (compiler, kernel, and so on) of the
2267 operating system on which the executable runs, unless that component
2268 itself accompanies the executable.
2270 If distribution of executable or object code is made by offering
2271 access to copy from a designated place, then offering equivalent
2272 access to copy the source code from the same place counts as
2273 distribution of the source code, even though third parties are not
2274 compelled to copy the source along with the object code.
2276 4. You may not copy, modify, sublicense, or distribute the Program
2277 except as expressly provided under this License. Any attempt
2278 otherwise to copy, modify, sublicense or distribute the Program is
2279 void, and will automatically terminate your rights under this License.
2280 However, parties who have received copies, or rights, from you under
2281 this License will not have their licenses terminated so long as such
2282 parties remain in full compliance.
2284 5. You are not required to accept this License, since you have not
2285 signed it. However, nothing else grants you permission to modify or
2286 distribute the Program or its derivative works. These actions are
2287 prohibited by law if you do not accept this License. Therefore, by
2288 modifying or distributing the Program (or any work based on the
2289 Program), you indicate your acceptance of this License to do so, and
2290 all its terms and conditions for copying, distributing or modifying
2291 the Program or works based on it.
2293 6. Each time you redistribute the Program (or any work based on the
2294 Program), the recipient automatically receives a license from the
2295 original licensor to copy, distribute or modify the Program subject to
2296 these terms and conditions. You may not impose any further
2297 restrictions on the recipients' exercise of the rights granted herein.
2298 You are not responsible for enforcing compliance by third parties to
2301 7. If, as a consequence of a court judgment or allegation of patent
2302 infringement or for any other reason (not limited to patent issues),
2303 conditions are imposed on you (whether by court order, agreement or
2304 otherwise) that contradict the conditions of this License, they do not
2305 excuse you from the conditions of this License. If you cannot
2306 distribute so as to satisfy simultaneously your obligations under this
2307 License and any other pertinent obligations, then as a consequence you
2308 may not distribute the Program at all. For example, if a patent
2309 license would not permit royalty-free redistribution of the Program by
2310 all those who receive copies directly or indirectly through you, then
2311 the only way you could satisfy both it and this License would be to
2312 refrain entirely from distribution of the Program.
2314 If any portion of this section is held invalid or unenforceable under
2315 any particular circumstance, the balance of the section is intended to
2316 apply and the section as a whole is intended to apply in other
2319 It is not the purpose of this section to induce you to infringe any
2320 patents or other property right claims or to contest validity of any
2321 such claims; this section has the sole purpose of protecting the
2322 integrity of the free software distribution system, which is
2323 implemented by public license practices. Many people have made
2324 generous contributions to the wide range of software distributed
2325 through that system in reliance on consistent application of that
2326 system; it is up to the author/donor to decide if he or she is willing
2327 to distribute software through any other system and a licensee cannot
2330 This section is intended to make thoroughly clear what is believed to
2331 be a consequence of the rest of this License.
2333 8. If the distribution and/or use of the Program is restricted in
2334 certain countries either by patents or by copyrighted interfaces, the
2335 original copyright holder who places the Program under this License
2336 may add an explicit geographical distribution limitation excluding
2337 those countries, so that distribution is permitted only in or among
2338 countries not thus excluded. In such case, this License incorporates
2339 the limitation as if written in the body of this License.
2341 9. The Free Software Foundation may publish revised and/or new versions
2342 of the General Public License from time to time. Such new versions will
2343 be similar in spirit to the present version, but may differ in detail to
2344 address new problems or concerns.
2346 Each version is given a distinguishing version number. If the Program
2347 specifies a version number of this License which applies to it and "any
2348 later version", you have the option of following the terms and conditions
2349 either of that version or of any later version published by the Free
2350 Software Foundation. If the Program does not specify a version number of
2351 this License, you may choose any version ever published by the Free Software
2354 10. If you wish to incorporate parts of the Program into other free
2355 programs whose distribution conditions are different, write to the author
2356 to ask for permission. For software which is copyrighted by the Free
2357 Software Foundation, write to the Free Software Foundation; we sometimes
2358 make exceptions for this. Our decision will be guided by the two goals
2359 of preserving the free status of all derivatives of our free software and
2360 of promoting the sharing and reuse of software generally.
2364 11. BECAUSE THE PROGRAM IS LICENSED FREE OF CHARGE, THERE IS NO WARRANTY
2365 FOR THE PROGRAM, TO THE EXTENT PERMITTED BY APPLICABLE LAW. EXCEPT WHEN
2366 OTHERWISE STATED IN WRITING THE COPYRIGHT HOLDERS AND/OR OTHER PARTIES
2367 PROVIDE THE PROGRAM "AS IS" WITHOUT WARRANTY OF ANY KIND, EITHER EXPRESSED
2368 OR IMPLIED, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES OF
2369 MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE. THE ENTIRE RISK AS
2370 TO THE QUALITY AND PERFORMANCE OF THE PROGRAM IS WITH YOU. SHOULD THE
2371 PROGRAM PROVE DEFECTIVE, YOU ASSUME THE COST OF ALL NECESSARY SERVICING,
2372 REPAIR OR CORRECTION.
2374 12. IN NO EVENT UNLESS REQUIRED BY APPLICABLE LAW OR AGREED TO IN WRITING
2375 WILL ANY COPYRIGHT HOLDER, OR ANY OTHER PARTY WHO MAY MODIFY AND/OR
2376 REDISTRIBUTE THE PROGRAM AS PERMITTED ABOVE, BE LIABLE TO YOU FOR DAMAGES,
2377 INCLUDING ANY GENERAL, SPECIAL, INCIDENTAL OR CONSEQUENTIAL DAMAGES ARISING
2378 OUT OF THE USE OR INABILITY TO USE THE PROGRAM (INCLUDING BUT NOT LIMITED
2379 TO LOSS OF DATA OR DATA BEING RENDERED INACCURATE OR LOSSES SUSTAINED BY
2380 YOU OR THIRD PARTIES OR A FAILURE OF THE PROGRAM TO OPERATE WITH ANY OTHER
2381 PROGRAMS), EVEN IF SUCH HOLDER OR OTHER PARTY HAS BEEN ADVISED OF THE
2382 POSSIBILITY OF SUCH DAMAGES.
2384 END OF TERMS AND CONDITIONS
2391 <!-- Keep this comment at the end of the file
2396 sgml-minimize-attributes:nil
2397 sgml-always-quote-attributes:t
2400 sgml-parent-document: nil
2401 sgml-local-catalogs: nil
2402 sgml-namecase-general:t