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24 <title>Metaproxy - User's Guide and Reference</title>
27 <firstname>Adam</firstname><surname>Dickmeiss</surname>
30 <firstname>Marc</firstname><surname>Cromme</surname>
33 <firstname>Mike</firstname><surname>Taylor</surname>
36 <releaseinfo>&version;</releaseinfo>
38 <year>2005-2007</year>
39 <holder>Index Data ApS</holder>
43 This manual is part of Metaproxy version &version;.
46 Metaproxy is a universal router, proxy and encapsulated
47 metasearcher for information retrieval protocols. It accepts,
48 processes, interprets and redirects requests from IR clients using
49 standard protocols such as the binary
50 <ulink url="&url.z39.50;">ANSI/NISO Z39.50</ulink>
51 and the information search and 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 <emphasis>not</emphasis> open-source software, but
69 may be freely downloaded, unpacked, inspected, built and run for
70 evaluation purposes. Deployment requires a separate, commercial,
76 <imagedata fileref="common/id.png" format="PNG"/>
79 <imagedata fileref="common/id.eps" format="EPS"/>
86 <chapter id="introduction">
87 <title>Introduction</title>
91 <ulink url="&url.metaproxy;">Metaproxy</ulink>
92 is a stand alone program that acts as a universal router, proxy and
93 encapsulated metasearcher for information retrieval protocols such
94 as <ulink url="&url.z39.50;">Z39.50</ulink>, and in the future
95 <ulink url="&url.sru;">SRU</ulink> and <ulink url="&url.srw;">SRW</ulink>.
96 To clients, it acts as a server of these protocols: it can be searched,
97 records can be retrieved from it, etc.
98 To servers, it acts as a client: it searches in them,
99 retrieves records from them, etc. it satisfies its clients'
100 requests by transforming them, multiplexing them, forwarding them
101 on to zero or more servers, merging the results, transforming
102 them, and delivering them back to the client. In addition, it
103 acts as a simple <ulink url="&url.http;">HTTP</ulink> server; support
104 for further protocols can be added in a modular fashion, through the
105 creation of new filters.
110 Fish, bananas, cold pyjamas,
111 Mutton, beef and trout!
112 - attributed to Cole Porter.
115 Metaproxy is a more capable alternative to
116 <ulink url="&url.yazproxy;">YAZ Proxy</ulink>,
117 being more powerful, flexible, configurable and extensible. Among
118 its many advantages over the older, more pedestrian work are
119 support for multiplexing (encapsulated metasearching), routing by
120 database name, authentication and authorization and serving local
121 files via HTTP. Equally significant, its modular architecture
122 facilitites the creation of pluggable modules implementing further
126 This manual will describe how to install Metaproxy
127 before giving an overview of its architecture, then discussing the
128 key concept of a filter in some depth and giving an overview of
129 the various filter types, then discussing the configuration file
130 format. After this come several optional chapters which may be
131 freely skipped: a detailed discussion of virtual databases and
132 multi-database searching, some notes on writing extensions
133 (additional filter types) and a high-level description of the
134 source code. Finally comes the reference guide, which contains
135 instructions for invoking the <command>metaproxy</command>
136 program, and detailed information on each type of filter,
141 <chapter id="license">
142 <title>The Metaproxy License</title>
143 <orderedlist numeration="arabic">
146 You are allowed to download this software for evaluation purposes.
147 You can unpack it, build it, run it, see how it works and how it fits
148 your needs, all at zero cost.
153 You may NOT deploy the software. For the purposes of this license,
154 deployment means running it for any purpose other than evaluation,
155 whether or not you or anyone else makes a profit from doing so. If
156 you wish to deploy the software, you must first contact Index Data and
157 arrange to purchase a DEPLOYMENT LICENCE. If you are unsure
158 whether or not your proposed use of the software constitutes
159 deployment, email us at <literal>info@indexdata.com</literal>
165 You may modify your copy of the software (fix bugs, add features)
166 if you need to. We encourage you to send your changes back to us for
167 integration into the master copy, but you are not obliged to do so. You
168 may NOT pass your changes on to any other party.
173 There is NO WARRANTY for this software, to the extent permitted by
174 applicable law. We provide the software ``as is'' without warranty of
175 any kind, either expressed or implied, including, but not limited to, the
176 implied warranties of MERCHANTABILITY and FITNESS FOR A
177 PARTICULAR PURPOSE. The entire risk as to the quality and
178 performance of the software is with you. Should the software prove
179 defective, you assume the cost of all necessary servicing, repair or
180 correction. In no event unless required by applicable law will we be
181 liable to you for damages, arising out of the use of the software,
182 including but not limited to loss of data or data being rendered
188 All rights to the software are reserved by Index Data except where
189 this license explicitly says otherwise.
195 <chapter id="installation">
196 <title>Installation</title>
198 Metaproxy depends on the following tools/libraries:
200 <varlistentry><term><ulink url="&url.yazplusplus;">YAZ++</ulink></term>
203 This is a C++ library based on <ulink url="&url.yaz;">YAZ</ulink>.
207 <varlistentry><term><ulink url="&url.libxslt;">Libxslt</ulink></term>
209 <para>This is an XSLT processor - based on
210 <ulink url="&url.libxml2;">Libxml2</ulink>. Both Libxml2 and
211 Libxslt must be installed with the development components
212 (header files, etc.) as well as the run-time libraries.
216 <varlistentry><term><ulink url="&url.boost;">Boost</ulink></term>
219 The popular C++ library. Initial versions of Metaproxy
220 was built with 1.33.0. Version 1.33.1 works too.
227 In order to compile Metaproxy a modern C++ compiler is
228 required. Boost, in particular, requires the C++ compiler
229 to facilitate the newest features. Refer to Boost
230 <ulink url="&url.boost.compilers.status;">Compiler Status</ulink>
231 for more information.
234 We have successfully built Metaproxy using the compilers
235 <ulink url="&url.gcc;">GCC</ulink> version 4.0 and
236 <ulink url="&url.vstudio;">Microsoft Visual Studio</ulink> 2003/2005.
239 <section id="installation.unix">
240 <title>Installation on Unix (from Source)</title>
242 Here is a quick step-by-step guide on how to compile all the
243 tools that Metaproxy uses. Only few systems have none of the required
244 tools binary packages. If, for example, Libxml2/libxslt are already
245 installed as development packages use those (and omit compilation).
252 gunzip -c libxml2-version.tar.gz|tar xf -
260 gunzip -c libxslt-version.tar.gz|tar xf -
271 gunzip -c yaz-version.tar.gz|tar xf -
279 gunzip -c yazpp-version.tar.gz|tar xf -
290 gunzip -c boost-version.tar.gz|tar xf -
301 gunzip -c metaproxy-version.tar.gz|tar xf -
310 <section id="installation.debian">
311 <title>Installation on Debian GNU/Linux</title>
313 All dependencies for Metaproxy are available as
314 <ulink url="&url.debian;">Debian</ulink>
315 packages for the sarge (stable in 2005) and etch (testing in 2005)
319 The procedures for Debian based systems, such as
320 <ulink url="&url.ubuntu;">Ubuntu</ulink> is probably similar
323 There is currently no official Debian package for YAZ++.
324 And the Debian package for YAZ is probably too old.
325 Update the <filename>/etc/apt/sources.list</filename>
326 to include the Index Data repository.
327 See YAZ' <ulink url="&url.yaz.download.debian;">Download Debian</ulink>
328 for more information.
331 apt-get install libxslt1-dev
332 apt-get install libyazpp-dev
333 apt-get install libboost-dev
334 apt-get install libboost-thread-dev
335 apt-get install libboost-date-time-dev
336 apt-get install libboost-program-options-dev
337 apt-get install libboost-test-dev
340 With these packages installed, the usual configure + make
341 procedure can be used for Metaproxy as outlined in
342 <xref linkend="installation.unix"/>.
346 <section id="installation.windows">
347 <title>Installation on Windows</title>
349 Metaproxy can be compiled with Microsoft
350 <ulink url="&url.vstudio;">Visual Studio</ulink>.
351 Version 2003 (C 7.1) and 2005 (C 8.0) is known to work.
353 <section id="installation.windows.boost">
356 Get Boost from its <ulink url="&url.boost;">home page</ulink>.
357 You also need Boost Jam (an alternative to make).
358 That's also available from the Boost home page.
359 The files to be downloaded are called something like:
360 <filename>boost_1_33-1.exe</filename>
362 <filename>boost-jam-3.1.12-1-ntx86.zip</filename>.
363 Unpack Boost Jam first. Put <filename>bjam.exe</filename>
364 in your system path. Make a command prompt and ensure
365 it can be found automatically. If not check the PATH.
366 The Boost .exe is a self-extracting exe with
367 complete source for Boost. Compile that source with
368 Boost Jam (An alternative to Make).
369 The compilation takes a while.
370 For Visual Studio 2003, use
372 bjam "-sTOOLS=vc-7_1"
374 Here <literal>vc-7_1</literal> refers to a "Toolset" (compiler system).
375 For Visual Studio 2005, use
377 bjam "-sTOOLS=vc-8_0"
379 To install the libraries in a common place, use
381 bjam "-sTOOLS=vc-7_1" install
383 (or vc-8_0 for VS 2005).
386 By default, the Boost build process installs the resulting
387 libraries + header files in
388 <literal>\boost\lib</literal>, <literal>\boost\include</literal>.
391 For more information about installing Boost refer to the
392 <ulink url="&url.boost.getting.started;">getting started</ulink>
397 <section id="installation.windows.libxslt">
398 <title>Libxslt</title>
400 <ulink url="&url.libxslt;">Libxslt</ulink> can be downloaded
402 <ulink url="&url.libxml2.download.win32;">here</ulink>.
405 Libxslt has other dependencies, but these can all be downloaded
406 from the same site. Get the following:
407 iconv, zlib, libxml2, libxslt.
411 <section id="installation.windows.yaz">
414 <ulink url="&url.yaz;">YAZ</ulink> can be downloaded
416 <ulink url="&url.yaz.download.win32;">here</ulink>.
420 <section id="installation.windows.yazplusplus">
423 Get <ulink url="&url.yazplusplus;">YAZ++</ulink> as well.
424 Version 1.0 or later is required. For now get it from
426 <ulink url="&url.snapshot.download;">Snapshot area</ulink>.
429 YAZ++ includes NMAKE makefiles, similar to those found in the
434 <section id="installation.windows.metaproxy">
435 <title>Metaproxy</title>
437 Metaproxy is shipped with NMAKE makefiles as well - similar
438 to those found in the YAZ++/YAZ packages. Adjust this Makefile
439 to point to the proper locations of Boost, Libxslt, Libxml2,
440 zlib, iconv, yaz and yazpp.
444 <varlistentry><term><literal>DEBUG</literal></term>
446 If set to 1, the software is
447 compiled with debugging libraries (code generation is
448 multi-threaded debug DLL).
449 If set to 0, the software is compiled with release libraries
450 (code generation is multi-threaded DLL).
455 <term><literal>BOOST</literal></term>
458 Boost install location
464 <term><literal>BOOST_VERSION</literal></term>
467 Boost version (replace . with _).
473 <term><literal>BOOST_TOOLSET</literal></term>
482 <term><literal>LIBXSLT_DIR</literal>,
483 <literal>LIBXML2_DIR</literal> ..</term>
486 Specify the locations of Libxslt, libiconv, libxml2 and
495 After successful compilation you'll find
496 <literal>metaproxy.exe</literal> in the
497 <literal>bin</literal> directory.
505 <chapter id="architecture">
506 <title>The Metaproxy Architecture</title>
508 The Metaproxy architecture is based on three concepts:
509 the <emphasis>package</emphasis>,
510 the <emphasis>route</emphasis>
511 and the <emphasis>filter</emphasis>.
515 <term>Packages</term>
518 A package is request or response, encoded in some protocol,
519 issued by a client, making its way through Metaproxy, send to or
520 received from a server, or sent back to the client.
523 The core of a package is the protocol unit - for example, a
524 Z39.50 Init Request or Search Response, or an SRU searchRetrieve
525 URL or Explain Response. In addition to this core, a package
526 also carries some extra information added and used by Metaproxy
530 In general, packages are doctored as they pass through
531 Metaproxy. For example, when the proxy performs authentication
532 and authorization on a Z39.50 Init request, it removes the
533 authentication credentials from the package so that they are not
534 passed onto the back-end server; and when search-response
535 packages are obtained from multiple servers, they are merged
536 into a single unified package that makes its way back to the
545 Packages make their way through routes, which can be thought of
546 as programs that operate on the package data-type. Each
547 incoming package initially makes its way through a default
548 route, but may be switched to a different route based on various
549 considerations. Routes are made up of sequences of filters (see
558 Filters provide the individual instructions within a route, and
559 effect the necessary transformations on packages. A particular
560 configuration of Metaproxy is essentially a set of filters,
561 described by configuration details and arranged in order in one
562 or more routes. There are many kinds of filter - about a dozen
563 at the time of writing with more appearing all the time - each
564 performing a specific function and configured by different
568 The word ``filter'' is sometimes used rather loosely, in two
569 different ways: it may be used to mean a particular
570 <emphasis>type</emphasis> of filter, as when we speak of ``the
571 auth_simple filter'' or ``the multi filter''; or it may be used
572 to be a specific <emphasis>instance</emphasis> of a filter
573 within a Metaproxy configuration. For example, a single
574 configuration will often contain multiple instances of the
575 <literal>z3950_client</literal> filter. In
576 operational terms, of these is a separate filter. In practice,
577 context always make it clear which sense of the word ``filter''
581 Extensibility of Metaproxy is primarily through the creation of
582 plugins that provide new filters. The filter API is small and
583 conceptually simple, but there are many details to master. See
585 <link linkend="filters">Filters</link>.
591 Since packages are created and handled by the system itself, and
592 routes are conceptually simple, most of the remainder of this
593 document concentrates on filters. After a brief overview of the
594 filter types follows, along with some thoughts on possible future
601 <chapter id="filters">
602 <title>Filters</title>
606 <title>Introductory notes</title>
608 It's useful to think of Metaproxy as an interpreter providing a small
609 number of primitives and operations, but operating on a very
610 complex data type, namely the ``package''.
613 A package represents a Z39.50 or SRU/W request (whether for Init,
614 Search, Scan, etc.) together with information about where it came
615 from. Packages are created by front-end filters such as
616 <literal>frontend_net</literal> (see below), which reads them from
617 the network; other front-end filters are possible. They then pass
618 along a route consisting of a sequence of filters, each of which
619 transforms the package and may also have side-effects such as
620 generating logging. Eventually, the route will yield a response,
621 which is sent back to the origin.
624 There are many kinds of filter: some that are defined statically
625 as part of Metaproxy, and others may be provided by third parties
626 and dynamically loaded. They all conform to the same simple API
627 of essentially two methods: <function>configure()</function> is
628 called at startup time, and is passed an XML DOM tree representing that
629 part of the configuration file that pertains to this filter
630 instance: it is expected to walk that tree extracting relevant
631 information; and <function>process()</function> is called every
632 time the filter has to processes a package.
635 While all filters provide the same API, there are different modes
636 of functionality. Some filters are sources: they create
638 (<literal>frontend_net</literal>);
639 others are sinks: they consume packages and return a result
640 (<literal>backend_test</literal>,
641 <literal>bounce</literal>,
642 <literal>http_file</literal>,
643 <literal>z3950_client</literal>);
644 the others are true filters, that read, process and pass on the
645 packages they are fed
646 (<literal>auth_simple</literal>,
647 <literal>log</literal>,
648 <literal>multi</literal>,
649 <literal>query_rewrite</literal>,
650 <literal>record_transform</literal>,
651 <literal>session_shared</literal>,
652 <literal>sru_z3950</literal>,
653 <literal>template</literal>,
654 <literal>virt_db</literal>).
659 <section id="overview.filter.types">
660 <title>Overview of filter types</title>
662 We now briefly consider each of the types of filter supported by
663 the core Metaproxy binary. This overview is intended to give a
664 flavor of the available functionality; more detailed information
665 about each type of filter is included below in
666 <xref linkend="reference"/>.
669 The filters are here named by the string that is used as the
670 <literal>type</literal> attribute of a
671 <literal><filter></literal> element in the configuration
672 file to request them, with the name of the class that implements
673 them in parentheses. (The classname is not needed for normal
674 configuration and use of Metaproxy; it is useful only to
678 The filters are here listed in alphabetical order:
685 New virt_db-alike that does inteligent peer choice, explain merging,
686 adds FD&N to explain. Keeps init responses (like "virt_db Classic"),
687 makes routing choices based on local explain knowledge. Ref IDDI
690 Filter to convert Explain Classic to ZeeRex.
692 CQL2PQF (which needs augmented ZeeRex) - MARC for Talis.
696 Figure out what additional information we need in:
697 ZeeRex (check against D3.1)
698 Init request (e.g. loop detection)
699 Query package (e.g. number of hops)
700 Query response (e.g. record source)
705 <title><literal>auth_simple</literal>
706 (mp::filter::AuthSimple)</title>
708 Simple authentication and authorization. The configuration
709 specifies the name of a file that is the user register, which
710 lists <varname>username</varname>:<varname>password</varname>
711 pairs, one per line, colon separated. When a session begins, it
712 is rejected unless username and passsword are supplied, and match
713 a pair in the register. The configuration file may also specific
714 the name of another file that is the target register: this lists
715 lists <varname>username</varname>:<varname>dbname</varname>,<varname>dbname</varname>...
716 sets, one per line, with multiple database names separated by
717 commas. When a search is processed, it is rejected unless the
718 database to be searched is one of those listed as available to
724 <title><literal>backend_test</literal>
725 (mp::filter::Backend_test)</title>
727 A partial sink that provides dummy responses in the manner of the
728 <literal>yaz-ztest</literal> Z39.50 server. This is useful only
729 for testing. Seriously, you don't need this. Pretend you didn't
730 even read this section.
735 <title><literal>bounce</literal>
736 (mp::filter::Bounce)</title>
738 A sink that swallows <emphasis>all packages</emphasis>,
739 and returns them almost unprocessed.
740 It never sends any package of any type further down the row, but
741 sets Z39.50 packages to Z_Close, and HTTP_Request packages to
742 HTTP_Response err code 400 packages, and adds a suitable bounce
744 The bounce filter is usually added at end of each filter chain route
745 to prevent infinite hanging of for example HTTP
746 requests packages when only the Z39.50 client partial sink
747 filter is found in the
753 <title><literal>cql_rpn</literal>
754 (mp::filter::CQLtoRPN)</title>
756 A query language transforming filter which catches Z39.50
757 <literal>searchRequest</literal>
758 packages containing <literal>CQL</literal> queries, transforms
759 those to <literal>RPN</literal> queries,
760 and sends the <literal>searchRequests</literal> on to the next
761 filters. It is among other things useful in a SRU context.
766 <title><literal>frontend_net</literal>
767 (mp::filter::FrontendNet)</title>
769 A source that accepts Z39.50 connections from a port
770 specified in the configuration, reads protocol units, and
771 feeds them into the next filter in the route. When the result is
772 received, it is returned to the original origin.
777 <title><literal>http_file</literal>
778 (mp::filter::HttpFile)</title>
780 A partial sink which swallows only
781 <literal>HTTP_Request</literal> packages, and
782 returns the contents of files from the local
783 filesystem in response to HTTP requests.
784 It lets Z39.50 packages and all other forthcoming package types
787 does mean that Metaproxy is also a Web-server in its spare time. So
788 far it does not contain either an email-reader or a Lisp
789 interpreter, but that day is surely coming.)
794 <title><literal>load_balance</literal>
795 (mp::filter::LoadBalance)</title>
797 Performs load balancing for incoming Z39.50 init requests.
798 It is used together with the <literal>virt_db</literal> filter,
799 but unlike the <literal>multi</literal> filter it does send an
800 entire session to only one of the virtual backends. The
801 <literal>load_balance</literal> filter is assuming that
802 all backend targets have equal content, and chooses the backend
803 with least load cost for a new session.
806 This filter is experimental and yet not mature for heavy load
814 <title><literal>log</literal>
815 (mp::filter::Log)</title>
817 Writes logging information to standard output, and passes on
818 the package unchanged. A log file name can be specified, as well
819 as multiple different logging formats.
824 <title><literal>multi</literal>
825 (mp::filter::Multi)</title>
827 Performs multi-database searching.
829 <link linkend="multidb">the extended discussion</link>
830 of virtual databases and multi-database searching below.
835 <title><literal>query_rewrite</literal>
836 (mp::filter::QueryRewrite)</title>
838 Rewrites Z39.50 <literal>Type-1</literal>
839 and <literal>Type-101</literal> (``<literal>RPN</literal>'')
841 three-step process: the query is transliterated from Z39.50
842 packet structures into an XML representation; that XML
843 representation is transformed by an XSLT stylesheet; and the
844 resulting XML is transliterated back into the Z39.50 packet
851 <title><literal>record_transform</literal>
852 (mp::filter::RecordTransform)</title>
854 This filter acts only on Z3950 present requests, and let all
855 other types of packages and requests pass untouched. It's use is
856 twofold: blocking Z3950 present requests, which the backend
857 server does not understand and can not honor, and transforming
858 the present syntax and elementset name according to the rules
859 specified, to fetch only existing record formats, and transform
860 them on the fly to requested record syntaxes.
865 <title><literal>session_shared</literal>
866 (mp::filter::SessionShared)</title>
868 This filter implements global sharing of
869 result sets (i.e. between threads and therefore between
870 clients), yielding performance improvements by clever resource
876 <title><literal>sru_z3950</literal>
877 (mp::filter::SRUtoZ3950)</title>
879 This filter transforms valid
880 SRU GET/POST/SOAP searchRetrieve requests to Z3950 init, search,
881 and present requests, and wraps the
882 received hit counts and XML records into suitable SRU response
884 The <literal>sru_z3950</literal> filter processes also SRU
885 GET/POST/SOAP explain requests, returning
886 either the absolute minimum required by the standard, or a full
887 pre-defined ZeeReX explain record.
889 <ulink url="&url.zeerex.explain;">ZeeReX Explain</ulink>
890 standard pages and the
891 <ulink url="&url.sru.explain;">SRU Explain</ulink> pages
892 for more information on the correct explain syntax.
893 SRU scan requests are not supported yet.
898 <title><literal>template</literal>
899 (mp::filter::Template)</title>
901 Does nothing at all, merely passing the packet on. (Maybe it
902 should be called <literal>nop</literal> or
903 <literal>passthrough</literal>?) This exists not to be used, but
904 to be copied - to become the skeleton of new filters as they are
905 written. As with <literal>backend_test</literal>, this is not
906 intended for civilians.
911 <title><literal>virt_db</literal>
912 (mp::filter::VirtualDB)</title>
914 Performs virtual database selection: based on the name of the
915 database in the search request, a server is selected, and its
916 address added to the request in a <literal>VAL_PROXY</literal>
917 otherInfo packet. It will subsequently be used by a
918 <literal>z3950_client</literal> filter.
920 <link linkend="multidb">the extended discussion</link>
921 of virtual databases and multi-database searching below.
926 <title><literal>z3950_client</literal>
927 (mp::filter::Z3950Client)</title>
929 A partial sink which swallows only Z39.50 packages.
930 It performs Z39.50 searching and retrieval by proxying the
931 packages that are passed to it. Init requests are sent to the
932 address specified in the <literal>VAL_PROXY</literal> otherInfo
933 attached to the request: this may have been specified by client,
934 or generated by a <literal>virt_db</literal> filter earlier in
935 the route. Subsequent requests are sent to the same address,
936 which is remembered at Init time in a Session object.
937 HTTP_Request packages and all other forthcoming package types
938 are passed untouched.
944 <title><literal>zeerex_explain</literal>
945 (mp::filter::ZeerexExplain)</title>
947 This filter acts as a sink for
948 Z39.50 explain requests, returning a static ZeeReX
949 Explain XML record from the config section. All other packages
952 <ulink url="&url.zeerex.explain;">ZeeReX Explain</ulink>
954 for more information on the correct explain syntax.
958 This filter is not yet completed.
967 <section id="future.directions">
968 <title>Future directions</title>
970 Some other filters that do not yet exist, but which would be
971 useful, are briefly described. These may be added in future
972 releases (or may be created by third parties, as loadable
978 <term><literal>frontend_cli</literal> (source)</term>
981 Command-line interface for generating requests.
986 <term><literal>sru_client</literal> (sink)</term>
989 SRU/GET and SRU/SOAP searching and retrieval.
994 <term><literal>opensearch_client</literal> (sink)</term>
997 A9 OpenSearch searching and retrieval.
1007 <chapter id="configuration">
1008 <title>Configuration: the Metaproxy configuration file format</title>
1012 <title>Introductory notes</title>
1014 If Metaproxy is an interpreter providing operations on packages, then
1015 its configuration file can be thought of as a program for that
1016 interpreter. Configuration is by means of a single XML file, the name
1017 of which is supplied as the sole command-line argument to the
1018 <command>metaproxy</command> program. (See
1019 <xref linkend="reference"/> below for more information on invoking
1024 <section id="overview.xml.structure">
1025 <title>Overview of the config file XML structure</title>
1027 All elements and attributes are in the namespace
1028 <ulink url="http://indexdata.com/metaproxy"/>.
1029 This is most easily achieved by setting the default namespace on
1030 the top-level element, as here:
1033 <metaproxy xmlns="http://indexdata.com/metaproxy" version="1.0">
1036 The top-level element is <metaproxy>. This contains a
1037 <start> element, a <filters> element and a
1038 <routes> element, in that order. <filters> is
1039 optional; the other two are mandatory. All three are
1043 The <start> element is empty, but carries a
1044 <literal>route</literal> attribute, whose value is the name of
1045 route at which to start running - analogous to the name of the
1046 start production in a formal grammar.
1049 If present, <filters> contains zero or more <filter>
1050 elements. Each filter carries a <literal>type</literal> attribute
1051 which specifies what kind of filter is being defined
1052 (<literal>frontend_net</literal>, <literal>log</literal>, etc.)
1053 and contain various elements that provide suitable configuration
1054 for a filter of its type. The filter-specific elements are
1056 <xref linkend="reference"/>.
1057 Filters defined in this part of the file must carry an
1058 <literal>id</literal> attribute so that they can be referenced
1062 <routes> contains one or more <route> elements, each
1063 of which must carry an <literal>id</literal> element. One of the
1064 routes must have the ID value that was specified as the start
1065 route in the <start> element's <literal>route</literal>
1066 attribute. Each route contains zero or more <filter>
1067 elements. These are of two types. They may be empty, but carry a
1068 <literal>refid</literal> attribute whose value is the same as the
1069 <literal>id</literal> of a filter previously defined in the
1070 <filters> section. Alternatively, a route within a filter
1071 may omit the <literal>refid</literal> attribute, but contain
1072 configuration elements similar to those used for filters defined
1073 in the <filters> section. (In other words, each filter in a
1074 route may be included either by reference or by physical
1080 <section id="example.configuration">
1081 <title>An example configuration</title>
1083 The following is a small, but complete, Metaproxy configuration
1084 file (included in the distribution as
1085 <literal>metaproxy/etc/config1.xml</literal>).
1086 This file defines a very simple configuration that simply proxies
1087 to whatever back-end server the client requests, but logs each
1088 request and response. This can be useful for debugging complex
1089 client-server dialogues.
1091 <screen><![CDATA[<?xml version="1.0"?>
1092 <metaproxy xmlns="http://indexdata.com/metaproxy" version="1.0">
1093 <start route="start"/>
1095 <filter id="frontend" type="frontend_net">
1098 <filter id="backend" type="z3950_client">
1103 <filter refid="frontend"/>
1104 <filter type="log"/>
1105 <filter refid="backend"/>
1106 <filter type="bounce"/>
1112 It works by defining a single route, called
1113 <literal>start</literal>, which consists of a sequence of four
1114 filters. The first and last of these are included by reference:
1115 their <literal><filter></literal> elements have
1116 <literal>refid</literal> attributes that refer to filters defined
1117 within the prior <literal><filters></literal> section. The
1118 middle filter is included inline in the route.
1121 The four filters in the route are as follows: first, a
1122 <literal>frontend_net</literal> filter accepts Z39.50 requests
1123 from any host on port 9000; then these requests are passed through
1124 a <literal>log</literal> filter that emits a message for each
1125 request; they are then fed into a <literal>z3950_client</literal>
1126 filter, which forwards all Z39.50 requests to the client-specified
1127 back-end Z39.509 server. Those Z39.50 packages are returned by the
1128 <literal>z3950_client</literal> filter, with the response data
1129 filled by the external Z39.50 server targeted.
1130 All non-Z39.50 packages are passed through to the
1131 <literal>bounce</literal> filter, which definitely bounces
1132 everything, including fish, bananas, cold pyjamas,
1133 mutton, beef and trout packages.
1134 When the response arrives, it is handed
1135 back to the <literal>log</literal> filter, which emits another
1136 message; and then to the <literal>frontend_net</literal> filter,
1137 which returns the response to the client.
1141 <section id="config-file-modularity">
1142 <title>Config file modularity</title>
1144 Metaproxy XML configuration snippets can be reused by other
1145 filters using the <literal>XInclude</literal> standard, as seen in
1146 the <literal>/etc/config-sru-to-z3950.xml</literal> example SRU
1149 <filter id="sru" type="sru_z3950">
1150 <database name="Default">
1151 <xi:include xmlns:xi="http://www.w3.org/2001/XInclude"
1152 href="explain.xml"/>
1159 <section id="config-file-syntax-check">
1160 <title>Config file syntax checking</title>
1162 The distribution contains RelaxNG Compact and XML syntax checking
1163 files, as well as XML Schema files. These are found in the
1166 xml/schema/metaproxy.rnc
1167 xml/schema/metaproxy.rng
1168 xml/schema/metaproxy.xsd
1170 and can be used to verify or debug the XML structure of
1171 configuration files. For example, using the utility
1172 <filename>xmllint</filename>, syntax checking is done like this:
1174 xmllint --noout --schema xml/schema/metaproxy.xsd etc/config-local.xml
1175 xmllint --noout --relaxng xml/schema/metaproxy.rng etc/config-local.xml
1177 (A recent version of <literal>libxml2</literal> is required, as
1178 support for XML Schemas is a relatively recent addition.)
1181 You can of course use any other RelaxNG or XML Schema compliant tool
1189 <chapter id="multidb">
1190 <title>Virtual databases and multi-database searching</title>
1194 <title>Introductory notes</title>
1196 Two of Metaproxy's filters are concerned with multiple-database
1197 operations. Of these, <literal>virt_db</literal> can work alone
1198 to control the routing of searches to one of a number of servers,
1199 while <literal>multi</literal> can work together with
1200 <literal>virt_db</literal> to perform multi-database searching, merging
1201 the results into a unified result-set - ``metasearch in a box''.
1204 The interaction between
1205 these two filters is necessarily complex: it reflects the real,
1206 irreducible complexity of multi-database searching in a protocol such
1207 as Z39.50 that separates initialization from searching, and in
1208 which the database to be searched is not known at initialization
1212 It's possible to use these filters without understanding the
1213 details of their functioning and the interaction between them; the
1214 next two sections of this chapter are ``HOW-TO'' guides for doing
1215 just that. However, debugging complex configurations will require
1216 a deeper understanding, which the last two sections of this
1217 chapters attempt to provide.
1222 <section id="multidb.virt_db">
1223 <title>Virtual databases with the <literal>virt_db</literal> filter</title>
1225 Working alone, the purpose of the
1226 <literal>virt_db</literal>
1227 filter is to route search requests to one of a selection of
1228 back-end databases. In this way, a single Z39.50 endpoint
1229 (running Metaproxy) can provide access to several different
1230 underlying services, including those that would otherwise be
1231 inaccessible due to firewalls. In many useful configurations, the
1232 back-end databases are local to the Metaproxy installation, but
1233 the software does not enforce this, and any valid Z39.50 servers
1234 may be used as back-ends.
1237 For example, a <literal>virt_db</literal>
1238 filter could be set up so that searches in the virtual database
1239 ``lc'' are forwarded to the Library of Congress bibliographic
1240 catalogue server, and searches in the virtual database ``marc''
1241 are forwarded to the toy database of MARC records that Index Data
1242 hosts for testing purposes. A <literal>virt_db</literal>
1243 configuration to make this switch would look like this:
1245 <screen><![CDATA[<filter type="virt_db">
1247 <database>lc</database>
1248 <target>z3950.loc.gov:7090/voyager</target>
1251 <database>marc</database>
1252 <target>indexdata.com/marc</target>
1254 </filter>]]></screen>
1256 As well as being useful in it own right, this filter also provides
1257 the foundation for multi-database searching.
1262 <section id="multidb.multi">
1263 <title>Multi-database search with the <literal>multi</literal> filter</title>
1265 To arrange for Metaproxy to broadcast searches to multiple back-end
1266 servers, the configuration needs to include two components: a
1267 <literal>virt_db</literal>
1268 filter that specifies multiple
1269 <literal><target></literal>
1270 elements, and a subsequent
1271 <literal>multi</literal>
1272 filter. Here, for example, is a complete configuration that
1273 broadcasts searches to both the Library of Congress catalogue and
1274 Index Data's tiny testing database of MARC records:
1276 <screen><![CDATA[<?xml version="1.0"?>
1277 <metaproxy xmlns="http://indexdata.com/metaproxy" version="1.0">
1278 <start route="start"/>
1281 <filter type="frontend_net">
1282 <threads>10</threads>
1285 <filter type="virt_db">
1287 <database>lc</database>
1288 <target>z3950.loc.gov:7090/voyager</target>
1291 <database>marc</database>
1292 <target>indexdata.com/marc</target>
1295 <database>all</database>
1296 <target>z3950.loc.gov:7090/voyager</target>
1297 <target>indexdata.com/marc</target>
1300 <filter type="multi"/>
1301 <filter type="z3950_client">
1302 <timeout>30</timeout>
1304 <filter type="bounce"/>
1307 </metaproxy>]]></screen>
1310 <literal>virt_db</literal>
1311 filter that specifies multiple
1312 <literal><target></literal>
1313 elements but without a subsequent
1314 <literal>multi</literal>
1315 filter yields surprising and undesirable results, as will be
1316 described below. Don't do that.)
1319 Metaproxy can be invoked with this configuration as follows:
1321 <screen>../src/metaproxy --config config-simple-multi.xml</screen>
1323 And thereafter, Z39.50 clients can connect to the running server
1324 (on port 9000, as specified in the configuration) and search in
1325 any of the databases
1326 <literal>lc</literal> (the Library of Congress catalogue),
1327 <literal>marc</literal> (Index Data's test database of MARC records)
1329 <literal>all</literal> (both of these). As an example, a session
1330 using the YAZ command-line client <literal>yaz-client</literal> is
1331 here included (edited for brevity and clarity):
1333 <screen><![CDATA[$ yaz-client @:9000
1337 Search was a success.
1338 Number of hits: 10000, setno 1
1342 Search was a success.
1343 Number of hits: 10, setno 3
1347 Search was a success.
1348 Number of hits: 10010, setno 4
1351 [marc]Record type: USmarc
1354 005 00000000000000.0
1355 008 910710c19910701nju 00010 eng
1359 100 10 $a Jack Collins
1360 245 10 $a How to program a computer
1366 [VOYAGER]Record type: USmarc
1368 005 20041229102447.0
1369 008 030910s2004 caua 000 0 eng
1370 035 $a (DLC) 2003112666
1371 906 $a 7 $b cbc $c orignew $d 4 $e epcn $f 20 $g y-gencatlg
1372 925 0 $a acquire $b 1 shelf copy $x policy default
1373 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.
1376 040 $a DLC $c DLC $d DLC
1377 050 00 $a MLCM 2004/03312 (G)
1378 245 10 $a 007, everything or nothing : $b Prima's official strategy guide / $c created by Kaizen Media Group.
1379 246 3 $a Double-O-seven, everything or nothing
1380 246 30 $a Prima's official strategy guide
1381 260 $a Roseville, CA : $b Prima Games, $c c2004.
1382 300 $a 161 p. : $b col. ill. ; $c 28 cm.
1383 500 $a "Platforms: Nintendo GameCube, Macintosh, PC, PlayStation 2 computer entertainment system, Xbox"--P. [4] of cover.
1384 650 0 $a Video games.
1385 710 2 $a Kaizen Media Group.
1386 856 42 $3 Publisher description $u http://www.loc.gov/catdir/description/random052/2003112666.html
1391 As can be seen, the first record in the result set is from the
1392 Index Data test database, and the second from the Library of
1393 Congress database. The result-set continues alternating records
1394 round-robin style until the point where one of the databases'
1395 records are exhausted.
1398 This example uses only two back-end databases; more may be used.
1399 There is no limitation imposed on the number of databases that may
1400 be metasearched in this way: issues of resource usage and
1401 administrative complexity dictate the practical limits.
1404 What happens when one of the databases doesn't respond? By default,
1405 the entire multi-database search fails, and the appropriate
1406 diagnostic is returned to the client. This is usually appropriate
1407 during development, when technicians need maximum information, but
1408 can be inconvenient in deployment, when users typically don't want
1409 to be bothered with problems of this kind and prefer just to get
1410 the records from the databases that are available. To obtain this
1411 latter behavior add an empty
1412 <literal><hideunavailable></literal>
1414 <literal>multi</literal> filter:
1416 <screen><![CDATA[ <filter type="multi">
1418 </filter>]]></screen>
1420 Under this regime, an error is reported to the client only if
1421 <emphasis>all</emphasis> the databases in a multi-database search
1427 <section id="multidb.what">
1428 <title>What's going on?</title>
1430 <title>Lark's vomit</title>
1432 This section goes into a level of technical detail that is
1433 probably not necessary in order to configure and use Metaproxy.
1434 It is provided only for those who like to know how things work.
1435 You should feel free to skip on to the next section if this one
1436 doesn't seem like fun.
1440 Hold on tight - this may get a little hairy.
1443 In the general course of things, a Z39.50 Init request may carry
1444 with it an otherInfo packet of type <literal>VAL_PROXY</literal>,
1445 whose value indicates the address of a Z39.50 server to which the
1446 ultimate connection is to be made. (This otherInfo packet is
1447 supported by YAZ-based Z39.50 clients and servers, but has not yet
1448 been ratified by the Maintenance Agency and so is not widely used
1449 in non-Index Data software. We're working on it.)
1450 The <literal>VAL_PROXY</literal> packet functions
1451 analogously to the absoluteURI-style Request-URI used with the GET
1452 method when a web browser asks a proxy to forward its request: see
1454 <ulink url="http://www.w3.org/Protocols/rfc2616/rfc2616-sec5.html#sec5.1.2"
1455 >Request-URI</ulink>
1457 <ulink url="http://www.w3.org/Protocols/rfc2616/rfc2616.html"
1458 >the HTTP 1.1 specification</ulink>.
1461 Within Metaproxy, Search requests that are part of the same
1462 session as an Init request that carries a
1463 <literal>VAL_PROXY</literal> otherInfo are also annotated with the
1464 same information. The role of the <literal>virt_db</literal>
1465 filter is to rewrite this otherInfo packet dependent on the
1466 virtual database that the client wants to search.
1469 When Metaproxy receives a Z39.50 Init request from a client, it
1470 doesn't immediately forward that request to the back-end server.
1471 Why not? Because it doesn't know <emphasis>which</emphasis>
1472 back-end server to forward it to until the client sends a Search
1473 request that specifies the database that it wants to search in.
1474 Instead, it just treasures the Init request up in its heart; and,
1475 later, the first time the client does a search on one of the
1476 specified virtual databases, a connection is forged to the
1477 appropriate server and the Init request is forwarded to it. If,
1478 later in the session, the same client searches in a different
1479 virtual database, then a connection is forged to the server that
1480 hosts it, and the same cached Init request is forwarded there,
1484 All of this clever Init-delaying is done by the
1485 <literal>frontend_net</literal> filter. The
1486 <literal>virt_db</literal> filter knows nothing about it; in
1487 fact, because the Init request that is received from the client
1488 doesn't get forwarded until a Search request is received, the
1489 <literal>virt_db</literal> filter (and the
1490 <literal>z3950_client</literal> filter behind it) doesn't even get
1491 invoked at Init time. The <emphasis>only</emphasis> thing that a
1492 <literal>virt_db</literal> filter ever does is rewrite the
1493 <literal>VAL_PROXY</literal> otherInfo in the requests that pass
1497 It is possible for a <literal>virt_db</literal> filter to contain
1499 <literal><target></literal>
1500 elements. What does this mean? Only that the filter will add
1501 multiple <literal>VAL_PROXY</literal> otherInfo packets to the
1502 Search requests that pass through it. That's because the virtual
1503 DB filter is dumb, and does exactly what it's told - no more, no
1505 If a Search request with multiple <literal>VAL_PROXY</literal>
1506 otherInfo packets reaches a <literal>z3950_client</literal>
1507 filter, this is an error. That filter doesn't know how to deal
1508 with multiple targets, so it will either just pick one and search
1509 in it, or (better) fail with an error message.
1512 The <literal>multi</literal> filter comes to the rescue! This is
1513 the only filter that knows how to deal with multiple
1514 <literal>VAL_PROXY</literal> otherInfo packets, and it does so by
1515 making multiple copies of the entire Search request: one for each
1516 <literal>VAL_PROXY</literal>. Each of these new copies is then
1517 passed down through the remaining filters in the route. (The
1518 copies are handled in parallel though the
1519 spawning of new threads.) Since the copies each have only one
1520 <literal>VAL_PROXY</literal> otherInfo, they can be handled by the
1521 <literal>z3950_client</literal> filter, which happily deals with
1522 each one individually. When the results of the individual
1523 searches come back up to the <literal>multi</literal> filter, it
1524 merges them into a single Search response, which is what
1525 eventually makes it back to the client.
1530 <section id="multidb.picture">
1531 <title>A picture is worth a thousand words (but only five hundred on 64-bit architectures)</title>
1535 <imagedata fileref="multi.pdf" format="PDF" scale="50"/>
1538 <imagedata fileref="multi.png" format="PNG"/>
1541 <!-- Fall back if none of the images can be used -->
1543 [Here there should be a diagram showing the progress of
1544 packages through the filters during a simple virtual-database
1545 search and a multi-database search, but is seems that your
1546 tool chain has not been able to include the diagram in this
1550 <!-- ### This used to work with an older version of DocBook
1552 <para>Caption: progress of packages through filters.</para>
1555 </inlinemediaobject>
1561 <chapter id="sru-server">
1562 <title>Combined SRU webservice and Z39.50 server configuration</title>
1564 Metaproxy can act as
1565 <ulink url="&url.sru;">SRU</ulink> and
1566 <ulink url="&url.srw;">SRW</ulink>
1567 web service server, which translates web service requests to
1568 <ulink url="&url.z39.50;">ANSI/NISO Z39.50</ulink> packages and
1569 sends them off to common available targets.
1572 A typical setup for this operation needs a filter route including the
1576 <table id="sru-server-table-config" frame="top">
1577 <title>SRU/Z39.50 Server Filter Route Configuration</title>
1581 <entry>Filter</entry>
1582 <entry>Importance</entry>
1583 <entry>Purpose</entry>
1589 <entry><literal>frontend_net</literal></entry>
1590 <entry>required</entry>
1591 <entry>Accepting HTTP connections and passing them to following
1592 filters. Since this filter also accepts Z39.50 connections, the
1593 server works as SRU and Z39.50 server on the same port.</entry>
1596 <entry><literal>sru_z3950</literal></entry>
1597 <entry>required</entry>
1598 <entry>Accepting SRU GET/POST/SOAP explain and
1599 searchRetrieve requests for the the configured databases.
1600 Explain requests are directly served from the static XML configuration.
1601 SearchRetrieve requests are
1602 transformed to Z39.50 search and present packages.
1603 All other HTTP and Z39.50 packages are passed unaltered.</entry>
1606 <entry><literal>http_file</literal></entry>
1607 <entry>optional</entry>
1608 <entry>Serving HTTP requests from the filesystem. This is only
1609 needed if the server should serve XSLT stylesheets, static HTML
1610 files or Java Script for thin browser based clients.
1611 Z39.50 packages are passed unaltered.</entry>
1614 <entry><literal>cql_rpn</literal></entry>
1615 <entry>required</entry>
1616 <entry>Usually, Z39.50 servers do not talk CQL, hence the
1617 translation of the CQL query language to RPN is mandatory in
1618 most cases. Affects only Z39.50 search packages.</entry>
1621 <entry><literal>record_transform</literal></entry>
1622 <entry>optional</entry>
1623 <entry>Some Z39.50 backend targets can not present XML record
1624 syntaxes in common wanted element sets. using this filter, one
1625 can transform binary MARC records to MARCXML records, and
1626 further transform those to any needed XML schema/format by XSLT
1627 transformations. Changes only Z39.50 present packages.</entry>
1630 <entry><literal>session_shared</literal></entry>
1631 <entry>optional</entry>
1632 <entry>The stateless nature of web services requires frequent
1633 re-searching of the same targets for display of paged result set
1634 records. This might be an unacceptable burden for the accessed
1635 backend Z39.50 targets, and this mosule can be added for
1636 efficient backend target resource pooling.</entry>
1639 <entry><literal>z3950_client</literal></entry>
1640 <entry>required</entry>
1641 <entry>Finally, a Z39.50 package sink is needed in the filter
1642 chain to provide the response packages. The Z39.50 client module
1643 is used to access external targets over the network, but any
1644 coming local Z39.50 package sink could be used instead of.</entry>
1647 <entry><literal>bounce</literal></entry>
1648 <entry>required</entry>
1649 <entry>Any Metaproxy package arriving here did not do so by
1650 purpose, and is bounced back with connection closure. this
1651 prevents inifinite package hanging inside the SRU server.</entry>
1657 A typical minimal example <ulink url="&url.sru;">SRU</ulink> and
1658 <ulink url="&url.srw;">SRW</ulink> server configuration file is found
1659 in the tarball distribution at
1660 <literal>etc/config-sru-to-z3950.xml</literal>.
1663 Off course, any other metaproxy modules can be integrated into a
1664 SRU server solution, including, but not limited to, load balancing,
1665 multiple target querying
1666 (see <xref linkend="multidb"/>), and complex RPN query rewrites.
1673 <chapter id="extensions">
1674 <title>Writing extensions for Metaproxy</title>
1675 <para>### To be written</para>
1681 <chapter id="classes">
1682 <title>Classes in the Metaproxy source code</title>
1686 <title>Introductory notes</title>
1688 <emphasis>Stop! Do not read this!</emphasis>
1689 You won't enjoy it at all. You should just skip ahead to
1690 <xref linkend="reference"/>,
1692 <!-- The remainder of this paragraph is lifted verbatim from
1693 Douglas Adams' _Hitch Hiker's Guide to the Galaxy_, chapter 8 -->
1694 you things you really need to know, like the fact that the
1695 fabulously beautiful planet Bethselamin is now so worried about
1696 the cumulative erosion by ten billion visiting tourists a year
1697 that any net imbalance between the amount you eat and the amount
1698 you excrete whilst on the planet is surgically removed from your
1699 bodyweight when you leave: so every time you go to the lavatory it
1700 is vitally important to get a receipt.
1703 This chapter contains documentation of the Metaproxy source code, and is
1704 of interest only to maintainers and developers. If you need to
1705 change Metaproxy's behavior or write a new filter, then you will most
1706 likely find this chapter helpful. Otherwise it's a waste of your
1707 good time. Seriously: go and watch a film or something.
1708 <citetitle>This is Spinal Tap</citetitle> is particularly good.
1711 Still here? OK, let's continue.
1714 In general, classes seem to be named big-endianly, so that
1715 <literal>FactoryFilter</literal> is not a filter that filters
1716 factories, but a factory that produces filters; and
1717 <literal>FactoryStatic</literal> is a factory for the statically
1718 registered filters (as opposed to those that are dynamically
1723 <section id="individual.classes">
1724 <title>Individual classes</title>
1726 The classes making up the Metaproxy application are here listed by
1727 class-name, with the names of the source files that define them in
1732 <title><literal>mp::FactoryFilter</literal>
1733 (<filename>factory_filter.cpp</filename>)</title>
1735 A factory class that exists primarily to provide the
1736 <literal>create()</literal> method, which takes the name of a
1737 filter class as its argument and returns a new filter of that
1738 type. To enable this, the factory must first be populated by
1739 calling <literal>add_creator()</literal> for static filters (this
1740 is done by the <literal>FactoryStatic</literal> class, see below)
1741 and <literal>add_creator_dyn()</literal> for filters loaded
1747 <title><literal>mp::FactoryStatic</literal>
1748 (<filename>factory_static.cpp</filename>)</title>
1750 A subclass of <literal>FactoryFilter</literal> which is
1751 responsible for registering all the statically defined filter
1752 types. It does this by knowing about all those filters'
1753 structures, which are listed in its constructor. Merely
1754 instantiating this class registers all the static classes. It is
1755 for the benefit of this class that <literal>struct
1756 metaproxy_1_filter_struct</literal> exists, and that all the filter
1757 classes provide a static object of that type.
1762 <title><literal>mp::filter::Base</literal>
1763 (<filename>filter.cpp</filename>)</title>
1765 The virtual base class of all filters. The filter API is, on the
1766 surface at least, extremely simple: two methods.
1767 <literal>configure()</literal> is passed an XML DOM tree representing
1768 that part of the configuration file that pertains to this filter
1769 instance, and is expected to walk that tree extracting relevant
1770 information. And <literal>process()</literal> processes a
1771 package (see below). That surface simplicity is a bit
1772 misleading, as <literal>process()</literal> needs to know a lot
1773 about the <literal>Package</literal> class in order to do
1779 <title><literal>mp::filter::AuthSimple</literal>,
1780 <literal>Backend_test</literal>, etc.
1781 (<filename>filter_auth_simple.cpp</filename>,
1782 <filename>filter_backend_test.cpp</filename>, etc.)</title>
1784 Individual filters. Each of these is implemented by a header and
1785 a source file, named <filename>filter_*.hpp</filename> and
1786 <filename>filter_*.cpp</filename> respectively. All the header
1787 files should be pretty much identical, in that they declare the
1788 class, including a private <literal>Rep</literal> class and a
1789 member pointer to it, and the two public methods.
1792 The source file for each filter needs to supply:
1797 A definition of the private <literal>Rep</literal> class.
1802 Some boilerplate constructors and destructors.
1807 A <literal>configure()</literal> method that uses the
1808 appropriate XML fragment.
1813 Most important, the <literal>process()</literal> method that
1814 does all the actual work.
1821 <title><literal>mp::Package</literal>
1822 (<filename>package.cpp</filename>)</title>
1824 Represents a package on its way through the series of filters
1825 that make up a route. This is essentially a Z39.50 or SRU APDU
1826 together with information about where it came from, which is
1827 modified as it passes through the various filters.
1832 <title><literal>mp::Pipe</literal>
1833 (<filename>pipe.cpp</filename>)</title>
1835 This class provides a compatibility layer so that we have an IPC
1836 mechanism that works the same under Unix and Windows. It's not
1837 particularly exciting.
1842 <title><literal>mp::RouterChain</literal>
1843 (<filename>router_chain.cpp</filename>)</title>
1850 <title><literal>mp::RouterFleXML</literal>
1851 (<filename>router_flexml.cpp</filename>)</title>
1858 <title><literal>mp::Session</literal>
1859 (<filename>session.cpp</filename>)</title>
1866 <title><literal>mp::ThreadPoolSocketObserver</literal>
1867 (<filename>thread_pool_observer.cpp</filename>)</title>
1874 <title><literal>mp::util</literal>
1875 (<filename>util.cpp</filename>)</title>
1877 A namespace of various small utility functions and classes,
1878 collected together for convenience. Most importantly, includes
1879 the <literal>mp::util::odr</literal> class, a wrapper for YAZ's
1885 <title><literal>mp::xml</literal>
1886 (<filename>xmlutil.cpp</filename>)</title>
1888 A namespace of various XML utility functions and classes,
1889 collected together for convenience.
1895 <section id="other.source.files">
1896 <title>Other Source Files</title>
1898 In addition to the Metaproxy source files that define the classes
1899 described above, there are a few additional files which are
1900 briefly described here:
1904 <term><literal>metaproxy_prog.cpp</literal></term>
1907 The main function of the <command>metaproxy</command> program.
1912 <term><literal>ex_router_flexml.cpp</literal></term>
1915 Identical to <literal>metaproxy_prog.cpp</literal>: it's not clear why.
1920 <term><literal>test_*.cpp</literal></term>
1923 Unit-tests for various modules.
1929 ### Still to be described:
1930 <literal>ex_filter_frontend_net.cpp</literal>,
1931 <literal>filter_dl.cpp</literal>,
1932 <literal>plainfile.cpp</literal>,
1933 <literal>tstdl.cpp</literal>.
1939 <reference id="reference">
1940 <title>Reference</title>
1943 The material in this chapter is drawn directly from the individual
1944 manual entries. In particular, the Metaproxy invocation section is
1945 available using <command>man metaproxy</command>, and the section
1946 on each individual filter is available using the name of the filter
1947 as the argument to the <command>man</command> command.
1954 <!-- Keep this comment at the end of the file
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