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23 <title>Metaproxy - User's Guide and Reference</title>
25 <firstname>Mike</firstname><surname>Taylor</surname>
28 <firstname>Adam</firstname><surname>Dickmeiss</surname>
32 <holder>Index Data ApS</holder>
36 Metaproxy is a universal router, proxy and encapsulated
37 metasearcher for information retrieval protocols. It accepts,
38 processes, interprets and redirects requests from IR clients using
39 standard protocols such as
40 <ulink url="&url.z39.50;">ANSI/NISO Z39.50</ulink>
41 (and in the future <ulink url="&url.sru;">SRU</ulink>
42 and <ulink url="&url.srw;">SRW</ulink>), as
43 well as functioning as a limited
44 <ulink url="&url.http;">HTTP</ulink> server.
45 Metaproxy is configured by an XML file which
46 specifies how the software should function in terms of routes that
47 the request packets can take through the proxy, each step on a
48 route being an instantiation of a filter. Filters come in many
49 types, one for each operation: accepting Z39.50 packets, logging,
50 query transformation, multiplexing, etc. Further filter-types can
51 be added as loadable modules to extend Metaproxy functionality,
55 Metaproxy is <emphasis>not</emphasis> open-source software, but
56 may be freely downloaded, unpacked, inspected, built and run for
57 evaluation purposes. Deployment requires a separate, commercial,
63 <imagedata fileref="common/id.png" format="PNG"/>
66 <imagedata fileref="common/id.eps" format="EPS"/>
73 <chapter id="introduction">
74 <title>Introduction</title>
78 <ulink url="&url.metaproxy;">Metaproxy</ulink>
79 is a standalone program that acts as a universal router, proxy and
80 encapsulated metasearcher for information retrieval protocols such
81 as <ulink url="&url.z39.50;">Z39.50</ulink>, and in the future
82 <ulink url="&url.sru;">SRU</ulink> and <ulink url="&url.srw;">SRW</ulink>.
83 To clients, it acts as a server of these protocols: it can be searched,
84 records can be retrieved from it, etc.
85 To servers, it acts as a client: it searches in them,
86 retrieves records from them, etc. it satisfies its clients'
87 requests by transforming them, multiplexing them, forwarding them
88 on to zero or more servers, merging the results, transforming
89 them, and delivering them back to the client. In addition, it
90 acts as a simple <ulink url="&url.http;">HTTP</ulink> server; support
91 for further protocols can be added in a modular fashion, through the
92 creation of new filters.
97 Fish, bananas, cold pyjamas,
98 Mutton, beef and trout!
99 - attributed to Cole Porter.
102 Metaproxy is a more capable alternative to
103 <ulink url="&url.yazproxy;">YAZ Proxy</ulink>,
104 being more powerful, flexible, configurable and extensible. Among
105 its many advantages over the older, more pedestrian work are
106 support for multiplexing (encapsulated metasearching), routing by
107 database name, authentication and authorisation and serving local
108 files via HTTP. Equally significant, its modular architecture
109 facilitites the creation of pluggable modules implementing further
113 This manual will describe how to install Metaproxy
114 before giving an overview of its architecture, then discussing the
115 key concept of a filter in some depth and giving an overview of
116 the various filter types, then discussing the configuration file
117 format. After this come several optional chapters which may be
118 freely skipped: a detailed discussion of virtual databases and
119 multi-database searching, some notes on writing extensions
120 (additional filter types) and a high-level description of the
121 source code. Finally comes the reference guide, which contains
122 instructions for invoking the <command>metaproxy</command>
123 program, and detailed information on each type of filter,
128 <chapter id="license">
129 <title>The Metaproxy License</title>
130 <orderedlist numeration="arabic">
133 You are allowed to download this software for evaluation purposes.
134 You can unpack it, build it, run it, see how it works and how it fits
135 your needs, all at zero cost.
140 You may NOT deploy the software. For the purposes of this license,
141 deployment means running it for any purpose other than evaluation,
142 whether or not you or anyone else makes a profit from doing so. If
143 you wish to deploy the software, you must first contact Index Data and
144 arrange to purchase a DEPLOYMENT LICENCE. If you are unsure
145 whether or not your proposed use of the software constitutes
146 deployment, email us at <literal>info@indexdata.com</literal>
152 You may modify your copy of the software (fix bugs, add features)
153 if you need to. We encourage you to send your changes back to us for
154 integration into the master copy, but you are not obliged to do so. You
155 may NOT pass your changes on to any other party.
160 There is NO WARRANTY for this software, to the extent permitted by
161 applicable law. We provide the software ``as is'' without warranty of
162 any kind, either expressed or implied, including, but not limited to, the
163 implied warranties of MERCHANTABILITY and FITNESS FOR A
164 PARTICULAR PURPOSE. The entire risk as to the quality and
165 performance of the software is with you. Should the software prove
166 defective, you assume the cost of all necessary servicing, repair or
167 correction. In no event unless required by applicable law will we be
168 liable to you for damages, arising out of the use of the software,
169 including but not limited to loss of data or data being rendered
175 All rights to the software are reserved by Index Data except where
176 this license explicitly says otherwise.
182 <chapter id="installation">
183 <title>Installation</title>
185 Metaproxy depends on the following tools/libraries:
187 <varlistentry><term><ulink url="&url.yazplusplus;">YAZ++</ulink></term>
190 This is a C++ library based on <ulink url="&url.yaz;">YAZ</ulink>.
194 <varlistentry><term><ulink url="&url.libxslt;">Libxslt</ulink></term>
196 <para>This is an XSLT processor - based on
197 <ulink url="&url.libxml2;">Libxml2</ulink>. Both Libxml2 and
198 Libxslt must be installed with the development components
199 (header files, etc.) as well as the run-time libraries.
203 <varlistentry><term><ulink url="&url.boost;">Boost</ulink></term>
206 The popular C++ library. Initial versions of Metaproxy
207 was built with 1.33.0. Version 1.33.1 works too.
214 In order to compile Metaproxy a modern C++ compiler is
215 required. Boost, in particular, requires the C++ compiler
216 to facilitate the newest features. Refer to Boost
217 <ulink url="&url.boost.compilers.status;">Compiler Status</ulink>
218 for more information.
221 We have succesfully built Metaproxy using the compilers
222 <ulink url="&url.gcc;">GCC</ulink> version 4.0 and
223 <ulink url="&url.vstudio;">Microsoft Visual Studio</ulink> 2003/2005.
226 <section id="installation.unix">
227 <title>Installation on Unix (from Source)</title>
229 Here is a quick step-by-step guide on how to compile all the
230 tools that Metaproxy uses. Only few systems have none of the required
231 tools binary packages. If, for example, Libxml2/libxslt are already
232 installed as development packages use those (and omit compilation).
239 gunzip -c libxml2-version.tar.gz|tar xf -
247 gunzip -c libxslt-version.tar.gz|tar xf -
258 gunzip -c yaz-version.tar.gz|tar xf -
266 gunzip -c yazpp-version.tar.gz|tar xf -
277 gunzip -c boost-version.tar.gz|tar xf -
288 gunzip -c metaproxy-version.tar.gz|tar xf -
297 <section id="installation.debian">
298 <title>Installation on Debian GNU/Linux</title>
300 All dependencies for Metaproxy are available as
301 <ulink url="&url.debian;">Debian</ulink>
302 packages for the sarge (stable in 2005) and etch (testing in 2005)
306 The procedures for Debian based systems, such as
307 <ulink url="&url.ubuntu;">Ubuntu</ulink> is probably similar
310 There is currently no official Debian package for YAZ++.
311 And the Debian package for YAZ is probably too old.
312 Update the <filename>/etc/apt/sources.list</filename>
313 to include the Index Data repository.
314 See YAZ' <ulink url="&url.yaz.download.debian;">Download Debian</ulink>
315 for more information.
318 apt-get install libxslt1-dev
319 apt-get install libyazpp-dev
320 apt-get install libboost-dev
321 apt-get install libboost-thread-dev
322 apt-get install libboost-date-time-dev
323 apt-get install libboost-program-options-dev
324 apt-get install libboost-test-dev
327 With these packages installed, the usual configure + make
328 procedure can be used for Metaproxy as outlined in
329 <xref linkend="installation.unix"/>.
333 <section id="installation.windows">
334 <title>Installation on Windows</title>
336 Metaproxy can be compiled with Microsoft
337 <ulink url="&url.vstudio;">Visual Studio</ulink>.
338 Version 2003 (C 7.1) and 2005 (C 8.0) is known to work.
340 <section id="installation.windows.boost">
343 Get Boost from its <ulink url="&url.boost;">home page</ulink>.
344 You also need Boost Jam (an alternative to make).
345 That's also available from the Boost home page.
346 The files to be downloaded are called something like:
347 <filename>boost_1_33-1.exe</filename>
349 <filename>boost-jam-3.1.12-1-ntx86.zip</filename>.
350 Unpack Boost Jam first. Put <filename>bjam.exe</filename>
351 in your system path. Make a command prompt and ensure
352 it can be found automatically. If not check the PATH.
353 The Boost .exe is a self-extracting exe with
354 complete source for Boost. Compile that source with
355 Boost Jam (An alternative to Make).
356 The compilation takes a while.
357 For Visual Studio 2003, use
359 bjam "-sTOOLS=vc-7_1"
361 Here <literal>vc-7_1</literal> refers to a "Toolset" (compiler system).
362 For Visual Studio 2005, use
364 bjam "-sTOOLS=vc-8_0"
366 To install the libraries in a common place, use
368 bjam "-sTOOLS=vc-7_1" install
370 (or vc-8_0 for VS 2005).
373 By default, the Boost build process installs the resulting
374 libraries + header files in
375 <literal>\boost\lib</literal>, <literal>\boost\include</literal>.
378 For more informatation about installing Boost refer to the
379 <ulink url="&url.boost.getting.started;">getting started</ulink>
384 <section id="installation.windows.libxslt">
385 <title>Libxslt</title>
387 <ulink url="&url.libxslt;">Libxslt</ulink> can be downloaded
389 <ulink url="&url.libxml2.download.win32;">here</ulink>.
392 Libxslt has other dependencies, but thes can all be downloaded
393 from the same site. Get the following:
394 iconv, zlib, libxml2, libxslt.
398 <section id="installation.windows.yaz">
401 <ulink url="&url.yaz;">YAZ</ulink> can be downloaded
403 <ulink url="&url.yaz.download.win32;">here</ulink>.
407 <section id="installation.windows.yazplusplus">
410 Get <ulink url="&url.yazplusplus;">YAZ++</ulink> as well.
411 Version 1.0 or later is required. For now get it from
413 <ulink url="&url.snapshot.download;">Snapshot area</ulink>.
416 YAZ++ includes NMAKE makefiles, similar to those found in the
421 <section id="installation.windows.metaproxy">
422 <title>Metaproxy</title>
424 Metaproxy is shipped with NMAKE makfiles as well - similar
425 to those found in the YAZ++/YAZ packages. Adjust this Makefile
426 to point to the proper locations of Boost, Libxslt, Libxml2,
427 zlib, iconv, yaz and yazpp.
431 <varlistentry><term><literal>DEBUG</literal></term>
433 If set to 1, the software is
434 compiled with debugging libraries (code generation is
435 multi-threaded debug DLL).
436 If set to 0, the software is compiled with release libraries
437 (code generation is multi-threaded DLL).
442 <term><literal>BOOST</literal></term>
445 Boost install location
451 <term><literal>BOOST_VERSION</literal></term>
454 Boost version (replace . with _).
460 <term><literal>BOOST_TOOLSET</literal></term>
469 <term><literal>LIBXSLT_DIR</literal>,
470 <literal>LIBXML2_DIR</literal> ..</term>
473 Specify the locations of Libxslt, libiconv, libxml2 and
482 After succesful compilation you'll find
483 <literal>metaproxy.exe</literal> in the
484 <literal>bin</literal> directory.
492 <chapter id="architecture">
493 <title>The Metaproxy Architecture</title>
495 The Metaproxy architecture is based on three concepts:
496 the <emphasis>package</emphasis>,
497 the <emphasis>route</emphasis>
498 and the <emphasis>filter</emphasis>.
502 <term>Packages</term>
505 A package is request or response, encoded in some protocol,
506 issued by a client, making its way through Metaproxy, send to or
507 received from a server, or sent back to the client.
510 The core of a package is the protocol unit - for example, a
511 Z39.50 Init Request or Search Response, or an SRU searchRetrieve
512 URL or Explain Response. In addition to this core, a package
513 also carries some extra information added and used by Metaproxy
517 In general, packages are doctored as they pass through
518 Metaproxy. For example, when the proxy performs authentication
519 and authorisation on a Z39.50 Init request, it removes the
520 authentication credentials from the package so that they are not
521 passed onto the back-end server; and when search-response
522 packages are obtained from multiple servers, they are merged
523 into a single unified package that makes its way back to the
532 Packages make their way through routes, which can be thought of
533 as programs that operate on the package data-type. Each
534 incoming package initially makes its way through a default
535 route, but may be switched to a different route based on various
536 considerations. Routes are made up of sequences of filters (see
545 Filters provide the individual instructions within a route, and
546 effect the necessary transformations on packages. A particular
547 configuration of Metaproxy is essentially a set of filters,
548 described by configuration details and arranged in order in one
549 or more routes. There are many kinds of filter - about a dozen
550 at the time of writing with more appearing all the time - each
551 performing a specific function and configured by different
555 The word ``filter'' is sometimes used rather loosely, in two
556 different ways: it may be used to mean a particular
557 <emphasis>type</emphasis> of filter, as when we speak of ``the
558 auth_simplefilter'' or ``the multi filter''; or it may be used
559 to be a specific <emphasis>instance</emphasis> of a filter
560 within a Metaproxy configuration. For example, a single
561 configuration will often contain multiple instances of the
562 <literal>z3950_client</literal> filter. In
563 operational terms, of these is a separate filter. In practice,
564 context always make it clear which sense of the word ``filter''
568 Extensibility of Metaproxy is primarily through the creation of
569 plugins that provide new filters. The filter API is small and
570 conceptually simple, but there are many details to master. See
572 <link linkend="extensions">extensions</link>.
578 Since packages are created and handled by the system itself, and
579 routes are conceptually simple, most of the remainder of this
580 document concentrates on filters. After a brief overview of the
581 filter types follows, along with some thoughts on possible future
588 <chapter id="filters">
589 <title>Filters</title>
593 <title>Introductory notes</title>
595 It's useful to think of Metaproxy as an interpreter providing a small
596 number of primitives and operations, but operating on a very
597 complex data type, namely the ``package''.
600 A package represents a Z39.50 or SRU/W request (whether for Init,
601 Search, Scan, etc.) together with information about where it came
602 from. Packages are created by front-end filters such as
603 <literal>frontend_net</literal> (see below), which reads them from
604 the network; other front-end filters are possible. They then pass
605 along a route consisting of a sequence of filters, each of which
606 transforms the package and may also have side-effects such as
607 generating logging. Eventually, the route will yield a response,
608 which is sent back to the origin.
611 There are many kinds of filter: some that are defined statically
612 as part of Metaproxy, and others may be provided by third parties
613 and dynamically loaded. They all conform to the same simple API
614 of essentially two methods: <function>configure()</function> is
615 called at startup time, and is passed a DOM tree representing that
616 part of the configuration file that pertains to this filter
617 instance: it is expected to walk that tree extracting relevant
618 information; and <function>process()</function> is called every
619 time the filter has to processes a package.
622 While all filters provide the same API, there are different modes
623 of functionality. Some filters are sources: they create
625 (<literal>frontend_net</literal>);
626 others are sinks: they consume packages and return a result
627 (<literal>z3950_client</literal>,
628 <literal>backend_test</literal>,
629 <literal>http_file</literal>);
630 the others are true filters, that read, process and pass on the
631 packages they are fed
632 (<literal>auth_simple</literal>,
633 <literal>log</literal>,
634 <literal>multi</literal>,
635 <literal>query_rewrite</literal>,
636 <literal>session_shared</literal>,
637 <literal>template</literal>,
638 <literal>virt_db</literal>).
643 <section id="overview.filter.types">
644 <title>Overview of filter types</title>
646 We now briefly consider each of the types of filter supported by
647 the core Metaproxy binary. This overview is intended to give a
648 flavour of the available functionality; more detailed information
649 about each type of filter is included below in
650 <link linkend="filterref"
651 >the reference guide to Metaproxy filters</link>.
654 The filters are here named by the string that is used as the
655 <literal>type</literal> attribute of a
656 <literal><filter></literal> element in the configuration
657 file to request them, with the name of the class that implements
658 them in parentheses. (The classname is not needed for normal
659 configuration and use of Metaproxy; it is useful only to
663 The filters are here listed in alphabetical order:
667 <title><literal>auth_simple</literal>
668 (mp::filter::AuthSimple)</title>
670 Simple authentication and authorisation. The configuration
671 specifies the name of a file that is the user register, which
672 lists <varname>username</varname>:<varname>password</varname>
673 pairs, one per line, colon separated. When a session begins, it
674 is rejected unless username and passsword are supplied, and match
675 a pair in the register. The configuration file may also specific
676 the name of another file that is the target register: this lists
677 lists <varname>username</varname>:<varname>dbname</varname>,<varname>dbname</varname>...
678 sets, one per line, with multiple database names separated by
679 commas. When a search is processed, it is rejected unless the
680 database to be searched is one of those listed as available to
686 <title><literal>backend_test</literal>
687 (mp::filter::Backend_test)</title>
689 A sink that provides dummy responses in the manner of the
690 <literal>yaz-ztest</literal> Z39.50 server. This is useful only
691 for testing. Seriously, you don't need this. Pretend you didn't
692 even read this section.
697 <title><literal>frontend_net</literal>
698 (mp::filter::FrontendNet)</title>
700 A source that accepts Z39.50 connections from a port
701 specified in the configuration, reads protocol units, and
702 feeds them into the next filter in the route. When the result is
703 revceived, it is returned to the original origin.
708 <title><literal>http_file</literal>
709 (mp::filter::HttpFile)</title>
711 A sink that returns the contents of files from the local
712 filesystem in response to HTTP requests. (Yes, Virginia, this
713 does mean that Metaproxy is also a Web-server in its spare time. So
714 far it does not contain either an email-reader or a Lisp
715 interpreter, but that day is surely coming.)
720 <title><literal>log</literal>
721 (mp::filter::Log)</title>
723 Writes logging information to standard output, and passes on
724 the package unchanged.
729 <title><literal>multi</literal>
730 (mp::filter::Multi)</title>
732 Performs multi-database searching.
734 <link linkend="multidb">the extended discussion</link>
735 of virtual databases and multi-database searching below.
740 <title><literal>query_rewrite</literal>
741 (mp::filter::QueryRewrite)</title>
743 Rewrites Z39.50 Type-1 and Type-101 (``RPN'') queries by a
744 three-step process: the query is transliterated from Z39.50
745 packet structures into an XML representation; that XML
746 representation is transformed by an XSLT stylesheet; and the
747 resulting XML is transliterated back into the Z39.50 packet
753 <title><literal>session_shared</literal>
754 (mp::filter::SessionShared)</title>
756 When this is finished, it will implement global sharing of
757 result sets (i.e. between threads and therefore between
758 clients), yielding performance improvements especially when
759 incoming requests are from a stateless environment such as a
760 web-server, in which the client process representing a session
761 might be any one of many. However:
765 This filter is not yet completed.
771 <title><literal>template</literal>
772 (mp::filter::Template)</title>
774 Does nothing at all, merely passing the packet on. (Maybe it
775 should be called <literal>nop</literal> or
776 <literal>passthrough</literal>?) This exists not to be used, but
777 to be copied - to become the skeleton of new filters as they are
778 written. As with <literal>backend_test</literal>, this is not
779 intended for civilians.
784 <title><literal>virt_db</literal>
785 (mp::filter::Virt_db)</title>
787 Performs virtual database selection: based on the name of the
788 database in the search request, a server is selected, and its
789 address added to the request in a <literal>VAL_PROXY</literal>
790 otherInfo packet. It will subsequently be used by a
791 <literal>z3950_client</literal> filter.
793 <link linkend="multidb">the extended discussion</link>
794 of virtual databases and multi-database searching below.
799 <title><literal>z3950_client</literal>
800 (mp::filter::Z3950Client)</title>
802 Performs Z39.50 searching and retrieval by proxying the
803 packages that are passed to it. Init requests are sent to the
804 address specified in the <literal>VAL_PROXY</literal> otherInfo
805 attached to the request: this may have been specified by client,
806 or generated by a <literal>virt_db</literal> filter earlier in
807 the route. Subsequent requests are sent to the same address,
808 which is remembered at Init time in a Session object.
814 <section id="future.directions">
815 <title>Future directions</title>
817 Some other filters that do not yet exist, but which would be
818 useful, are briefly described. These may be added in future
819 releases (or may be created by third parties, as loadable
825 <term><literal>frontend_cli</literal> (source)</term>
828 Command-line interface for generating requests.
833 <term><literal>frontend_sru</literal> (source)</term>
836 Receive SRU (and perhaps SRW) requests.
841 <term><literal>sru2z3950</literal> (filter)</term>
844 Translate SRU requests into Z39.50 requests.
849 <term><literal>sru_client</literal> (sink)</term>
852 SRU searching and retrieval.
857 <term><literal>srw_client</literal> (sink)</term>
860 SRW searching and retrieval.
865 <term><literal>opensearch_client</literal> (sink)</term>
868 A9 OpenSearch searching and retrieval.
878 <chapter id="configuration">
879 <title>Configuration: the Metaproxy configuration file format</title>
883 <title>Introductory notes</title>
885 If Metaproxy is an interpreter providing operations on packages, then
886 its configuration file can be thought of as a program for that
887 interpreter. Configuration is by means of a single file, the name
888 of which is supplied as the sole command-line argument to the
889 <command>metaproxy</command> program. (See
890 <link linkend="progref">the reference guide</link>
891 below for more information on invoking Metaproxy.)
894 The configuration files are written in XML. (But that's just an
895 implementation detail - they could just as well have been written
896 in YAML or Lisp-like S-expressions, or in a custom syntax.)
899 Since XML has been chosen, an XML schema,
900 <filename>config.xsd</filename>, is provided for validating
901 configuration files. This file is supplied in the
902 <filename>etc</filename> directory of the Metaproxy distribution. It
903 can be used by (among other tools) the <command>xmllint</command>
904 program supplied as part of the <literal>libxml2</literal>
908 xmllint --noout --schema etc/config.xsd my-config-file.xml
911 (A recent version of <literal>libxml2</literal> is required, as
912 support for XML Schemas is a relatively recent addition.)
916 <section id="overview.xml.structure">
917 <title>Overview of XML structure</title>
919 All elements and attributes are in the namespace
920 <ulink url="http://indexdata.dk/yp2/config/1"/>.
921 This is most easily achieved by setting the default namespace on
922 the top-level element, as here:
925 <yp2 xmlns="http://indexdata.dk/yp2/config/1">
928 The top-level element is <yp2>. This contains a
929 <start> element, a <filters> element and a
930 <routes> element, in that order. <filters> is
931 optional; the other two are mandatory. All three are
935 The <start> element is empty, but carries a
936 <literal>route</literal> attribute, whose value is the name of
937 route at which to start running - analogous to the name of the
938 start production in a formal grammar.
941 If present, <filters> contains zero or more <filter>
942 elements. Each filter carries a <literal>type</literal> attribute
943 which specifies what kind of filter is being defined
944 (<literal>frontend_net</literal>, <literal>log</literal>, etc.)
945 and contain various elements that provide suitable configuration
946 for a filter of its type. The filter-specific elements are
948 <link linkend="filterref">the reference guide below</link>.
949 Filters defined in this part of the file must carry an
950 <literal>id</literal> attribute so that they can be referenced
954 <routes> contains one or more <route> elements, each
955 of which must carry an <literal>id</literal> element. One of the
956 routes must have the ID value that was specified as the start
957 route in the <start> element's <literal>route</literal>
958 attribute. Each route contains zero or more <filter>
959 elements. These are of two types. They may be empty, but carry a
960 <literal>refid</literal> attribute whose value is the same as the
961 <literal>id</literal> of a filter previously defined in the
962 <filters> section. Alternatively, a route within a filter
963 may omit the <literal>refid</literal> attribute, but contain
964 configuration elements similar to those used for filters defined
965 in the <filters> section. (In other words, each filter in a
966 route may be included either by reference or by physical
972 <section id="example.configuration">
973 <title>An example configuration</title>
975 The following is a small, but complete, Metaproxy configuration
976 file (included in the distribution as
977 <literal>metaproxy/etc/config0.xml</literal>).
978 This file defines a very simple configuration that simply proxies
979 to whatever back-end server the client requests, but logs each
980 request and response. This can be useful for debugging complex
981 client-server dialogues.
983 <screen><![CDATA[<?xml version="1.0"?>
984 <yp2 xmlns="http://indexdata.dk/yp2/config/1">
985 <start route="start"/>
987 <filter id="frontend" type="frontend_net">
990 <filter id="backend" type="z3950_client">
995 <filter refid="frontend"/>
997 <filter refid="backend"/>
1003 It works by defining a single route, called
1004 <literal>start</literal>, which consists of a sequence of three
1005 filters. The first and last of these are included by reference:
1006 their <literal><filter></literal> elements have
1007 <literal>refid</literal> attributes that refer to filters defined
1008 within the prior <literal><filters></literal> section. The
1009 middle filter is included inline in the route.
1012 The three filters in the route are as follows: first, a
1013 <literal>frontend_net</literal> filter accepts Z39.50 requests
1014 from any host on port 9000; then these requests are passed through
1015 a <literal>log</literal> filter that emits a message for each
1016 request; they are then fed into a <literal>z3950_client</literal>
1017 filter, which forwards the requests to the client-specified
1018 back-end Z39.509 server. When the response arrives, it is handed
1019 back to the <literal>log</literal> filter, which emits another
1020 message; and then to the front-end filter, which returns the
1021 response to the client.
1028 <chapter id="multidb">
1029 <title>Virtual databases and multi-database searching</title>
1033 <title>Introductory notes</title>
1035 Two of Metaproxy's filters are concerned with multiple-database
1036 operations. Of these, <literal>virt_db</literal> can work alone
1037 to control the routing of searches to one of a number of servers,
1038 while <literal>multi</literal> can work together with
1039 <literal>virt_db</literal> to perform multi-database searching, merging
1040 the results into a unified result-set - ``metasearch in a box''.
1043 The interaction between
1044 these two filters is necessarily complex: it reflects the real,
1045 irreducible complexity of multi-database searching in a protocol such
1046 as Z39.50 that separates initialisation from searching, and in
1047 which the database to be searched is not known at initialisation
1051 It's possible to use these filters without understanding the
1052 details of their functioning and the interaction between them; the
1053 next two sections of this chapter are ``HOWTO'' guides for doing
1054 just that. However, debugging complex configurations will require
1055 a deeper understanding, which the last two sections of this
1056 chapters attempt to provide.
1061 <section id="multidb.virt_db">
1062 <title>Virtual databases with the <literal>virt_db</literal> filter</title>
1064 Working alone, the purpose of the
1065 <literal>virt_db</literal>
1066 filter is to route search requests to one of a selection of
1067 back-end databases. In this way, a single Z39.50 endpoint
1068 (running Metaproxy) can provide access to several different
1069 underlying services, including those that would otherwise be
1070 inaccessible due to firewalls. In many useful configurations, the
1071 back-end databases are local to the Metaproxy installation, but
1072 the software does not enforce this, and any valid Z39.50 servers
1073 may be used as back-ends.
1076 For example, a <literal>virt_db</literal>
1077 filter could be set up so that searches in the virtual database
1078 ``lc'' are forwarded to the Library of Congress bibliographic
1079 catalogue server, and searches in the virtual database ``marc''
1080 are forwarded to the toy database of MARC records that Index Data
1081 hosts for testing purposes. A <literal>virt_db</literal>
1082 configuration to make this switch would look like this:
1084 <screen><![CDATA[<filter type="virt_db">
1086 <database>lc</database>
1087 <target>z3950.loc.gov:7090/voyager</target>
1090 <database>marc</database>
1091 <target>indexdata.dk/marc</target>
1093 </filter>]]></screen>
1095 As well as being useful in it own right, this filter also provides
1096 the foundation for multi-database searching.
1101 <section id="multidb.multi">
1102 <title>Multi-database search with the <literal>multi</literal> filter</title>
1104 To arrange for Metaproxy to broadcast searches to multiple back-end
1105 servers, the configuration needs to include two components: a
1106 <literal>virt_db</literal>
1107 filter that specifies multiple
1108 <literal><target></literal>
1109 elements, and a subsequent
1110 <literal>multi</literal>
1111 filter. Here, for example, is a complete configuration that
1112 broadcasts searches to both the Library of Congress catalogue and
1113 Index Data's tiny testing database of MARC records:
1115 <screen><![CDATA[<?xml version="1.0"?>
1116 <yp2 xmlns="http://indexdata.dk/yp2/config/1">
1117 <start route="start"/>
1120 <filter type="frontend_net">
1121 <threads>10</threads>
1124 <filter type="virt_db">
1126 <database>lc</database>
1127 <target>z3950.loc.gov:7090/voyager</target>
1130 <database>marc</database>
1131 <target>indexdata.dk/marc</target>
1134 <database>all</database>
1135 <target>z3950.loc.gov:7090/voyager</target>
1136 <target>indexdata.dk/marc</target>
1139 <filter type="multi"/>
1140 <filter type="z3950_client">
1141 <timeout>30</timeout>
1148 <literal>virt_db</literal>
1149 filter that specifies multiple
1150 <literal><target></literal>
1151 elements but without a subsequent
1152 <literal>multi</literal>
1153 filter yields surprising and undesirable results, as will be
1154 described below. Don't do that.)
1157 Metaproxy can be invoked with this configuration as follows:
1159 <screen>../src/metaproxy --config config-simple-multi.xml</screen>
1161 And thereafter, Z39.50 clients can connect to the running server
1162 (on port 9000, as specified in the configuration) and search in
1163 any of the databases
1164 <literal>lc</literal> (the Library of Congress catalogue),
1165 <literal>marc</literal> (Index Data's test database of MARC records)
1167 <literal>all</literal> (both of these). As an example, a session
1168 using the YAZ command-line client <literal>yaz-client</literal> is
1169 here included (edited for brevity and clarity):
1171 <screen><![CDATA[$ yaz-client @:9000
1175 Search was a success.
1176 Number of hits: 10000, setno 1
1180 Search was a success.
1181 Number of hits: 10, setno 3
1185 Search was a success.
1186 Number of hits: 10010, setno 4
1189 [marc]Record type: USmarc
1192 005 00000000000000.0
1193 008 910710c19910701nju 00010 eng
1197 100 10 $a Jack Collins
1198 245 10 $a How to program a computer
1204 [VOYAGER]Record type: USmarc
1206 005 20041229102447.0
1207 008 030910s2004 caua 000 0 eng
1208 035 $a (DLC) 2003112666
1209 906 $a 7 $b cbc $c orignew $d 4 $e epcn $f 20 $g y-gencatlg
1210 925 0 $a acquire $b 1 shelf copy $x policy default
1211 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.
1214 040 $a DLC $c DLC $d DLC
1215 050 00 $a MLCM 2004/03312 (G)
1216 245 10 $a 007, everything or nothing : $b Prima's official strategy guide / $c created by Kaizen Media Group.
1217 246 3 $a Double-O-seven, everything or nothing
1218 246 30 $a Prima's official strategy guide
1219 260 $a Roseville, CA : $b Prima Games, $c c2004.
1220 300 $a 161 p. : $b col. ill. ; $c 28 cm.
1221 500 $a "Platforms: Nintendo GameCube, Macintosh, PC, PlayStation 2 computer entertainment system, Xbox"--P. [4] of cover.
1222 650 0 $a Video games.
1223 710 2 $a Kaizen Media Group.
1224 856 42 $3 Publisher description $u http://www.loc.gov/catdir/description/random052/2003112666.html
1229 As can be seen, the first record in the result set is from the
1230 Index Data test database, and the second from the Library of
1231 Congress database. The result-set continues alternating records
1232 round-robin style until the point where one of the databases'
1233 records are exhausted.
1236 This example uses only two back-end databases; more may be used.
1237 There is no limitation imposed on the number of databases that may
1238 be metasearched in this way: issues of resource usage and
1239 administrative complexity dictate the practical limits.
1242 What happens when one of the databases doesn't respond? By default,
1243 the entire multi-database search fails, and the appropriate
1244 diagnostic is returned to the client. This is usually appropriate
1245 during development, when technicians need maximum information, but
1246 can be inconvenient in deployment, when users typically don't want
1247 to be bothered with problems of this kind and prefer just to get
1248 the records from the databases that are available. To obtain this
1249 latter behaviour add an empty
1250 <literal><hideunavailable></literal>
1252 <literal>multi</literal> filter:
1254 <screen><![CDATA[ <filter type="multi">
1256 </filter>]]></screen>
1258 Under this regime, an error is reported to the client only if
1259 <emphasis>all</emphasis> the databases in a multi-database search
1265 <section id="multidb.what">
1266 <title>What's going on?</title>
1268 <title>Lark's vomit</title>
1270 This section goes into a level of technical detail that is
1271 probably not necessary in order to configure and use Metaproxy.
1272 It is provided only for those who like to know how things work.
1273 You should feel free to skip on to the next section if this one
1274 doesn't seem like fun.
1278 Hold on tight - this may get a little hairy.
1281 In the general course of things, a Z39.50 Init request may carry
1282 with it an otherInfo packet of type <literal>VAL_PROXY</literal>,
1283 whose value indicates the address of a Z39.50 server to which the
1284 ultimate connection is to be made. (This otherInfo packet is
1285 supported by YAZ-based Z39.50 clients and servers, but has not yet
1286 been ratified by the Maintenance Agency and so is not widely used
1287 in non-Index Data software. We're working on it.)
1288 The <literal>VAL_PROXY</literal> packet functions
1289 analogously to the absoluteURI-style Request-URI used with the GET
1290 method when a web browser asks a proxy to forward its request: see
1292 <ulink url="http://www.w3.org/Protocols/rfc2616/rfc2616-sec5.html#sec5.1.2"
1293 >Request-URI</ulink>
1295 <ulink url="http://www.w3.org/Protocols/rfc2616/rfc2616.html"
1296 >the HTTP 1.1 specification</ulink>.
1299 Within Metaproxy, Search requests that are part of the same
1300 session as an Init request that carries a
1301 <literal>VAL_PROXY</literal> otherInfo are also annotated with the
1302 same information. The role of the <literal>virt_db</literal>
1303 filter is to rewrite this otherInfo packet dependent on the
1304 virtual database that the client wants to search.
1307 When Metaproxy receives a Z39.50 Init request from a client, it
1308 doesn't immediately forward that request to the back-end server.
1309 Why not? Because it doesn't know <emphasis>which</emphasis>
1310 back-end server to forward it to until the client sends a Search
1311 request that specifies the database that it wants to search in.
1312 Instead, it just treasures the Init request up in its heart; and,
1313 later, the first time the client does a search on one of the
1314 specified virtual databases, a connection is forged to the
1315 appropriate server and the Init request is forwarded to it. If,
1316 later in the session, the same client searches in a different
1317 virtual database, then a connection is forged to the server that
1318 hosts it, and the same cached Init request is forwarded there,
1322 All of this clever Init-delaying is done by the
1323 <literal>frontend_net</literal> filter. The
1324 <literal>virt_db</literal> filter knows nothing about it; in
1325 fact, because the Init request that is received from the client
1326 doesn't get forwarded until a Search request is received, the
1327 <literal>virt_db</literal> filter (and the
1328 <literal>z3950_client</literal> filter behind it) doesn't even get
1329 invoked at Init time. The <emphasis>only</emphasis> thing that a
1330 <literal>virt_db</literal> filter ever does is rewrite the
1331 <literal>VAL_PROXY</literal> otherInfo in the requests that pass
1335 It is possible for a <literal>virt_db</literal> filter to contain
1337 <literal><target></literal>
1338 elements. What does this mean? Only that the filter will add
1339 multiple <literal>VAL_PROXY</literal> otherInfo packets to the
1340 Search requests that pass through it. That's because the virtual
1341 DB filter is dumb, and does exactly what it's told - no more, no
1343 If a Search request with multiple <literal>VAL_PROXY</literal>
1344 otherInfo packets reaches a <literal>z3950_client</literal>
1345 filter, this is an error. That filter doesn't know how to deal
1346 with multiple targets, so it will either just pick one and search
1347 in it, or (better) fail with an error message.
1350 The <literal>multi</literal> filter comes to the rescue! This is
1351 the only filter that knows how to deal with multiple
1352 <literal>VAL_PROXY</literal> otherInfo packets, and it does so by
1353 making multiple copies of the entire Search request: one for each
1354 <literal>VAL_PROXY</literal>. Each of these new copies is then
1355 passed down through the remaining filters in the route. (The
1356 copies are handled in parallel though the
1357 spawning of new threads.) Since the copies each have only one
1358 <literal>VAL_PROXY</literal> otherInfo, they can be handled by the
1359 <literal>z3950_client</literal> filter, which happily deals with
1360 each one individually. When the results of the individual
1361 searches come back up to the <literal>multi</literal> filter, it
1362 merges them into a single Search response, which is what
1363 eventually makes it back to the client.
1368 <section id="multidb.picture">
1369 <title>A picture is worth a thousand words (but only five hundred on 64-bit architectures)</title>
1373 <imagedata fileref="multi.pdf" format="PDF" scale="50"/>
1376 <imagedata fileref="multi.png" format="PNG"/>
1379 <!-- Fall back if none of the images can be used -->
1381 [Here there should be a diagram showing the progress of
1382 packages through the filters during a simple virtual-database
1383 search and a multi-database search, but is seems that your
1384 toolchain has not been able to include the diagram in this
1385 document. This is because of LaTeX suckage. Time to move to
1386 OpenOffice. Yes, really.]
1389 <!-- ### This used to work with an older version of DocBook
1391 <para>Caption: progress of packages through filters.</para>
1394 </inlinemediaobject>
1401 <chapter id="extensions">
1402 <title>Writing extensions for Metaproxy</title>
1403 <para>### To be written</para>
1409 <chapter id="classes">
1410 <title>Classes in the Metaproxy source code</title>
1414 <title>Introductory notes</title>
1416 <emphasis>Stop! Do not read this!</emphasis>
1417 You won't enjoy it at all. You should just skip ahead to
1418 <link linkend="refguide">the reference guide</link>,
1420 <!-- The remainder of this paragraph is lifted verbatim from
1421 Douglas Adams' _Hitch Hiker's Guide to the Galaxy_, chapter 8 -->
1422 you things you really need to know, like the fact that the
1423 fabulously beautiful planet Bethselamin is now so worried about
1424 the cumulative erosion by ten billion visiting tourists a year
1425 that any net imbalance between the amount you eat and the amount
1426 you excrete whilst on the planet is surgically removed from your
1427 bodyweight when you leave: so every time you go to the lavatory it
1428 is vitally important to get a receipt.
1431 This chapter contains documentation of the Metaproxy source code, and is
1432 of interest only to maintainers and developers. If you need to
1433 change Metaproxy's behaviour or write a new filter, then you will most
1434 likely find this chapter helpful. Otherwise it's a waste of your
1435 good time. Seriously: go and watch a film or something.
1436 <citetitle>This is Spinal Tap</citetitle> is particularly good.
1439 Still here? OK, let's continue.
1442 In general, classes seem to be named big-endianly, so that
1443 <literal>FactoryFilter</literal> is not a filter that filters
1444 factories, but a factory that produces filters; and
1445 <literal>FactoryStatic</literal> is a factory for the statically
1446 registered filters (as opposed to those that are dynamically
1451 <section id="individual.classes">
1452 <title>Individual classes</title>
1454 The classes making up the Metaproxy application are here listed by
1455 class-name, with the names of the source files that define them in
1460 <title><literal>mp::FactoryFilter</literal>
1461 (<filename>factory_filter.cpp</filename>)</title>
1463 A factory class that exists primarily to provide the
1464 <literal>create()</literal> method, which takes the name of a
1465 filter class as its argument and returns a new filter of that
1466 type. To enable this, the factory must first be populated by
1467 calling <literal>add_creator()</literal> for static filters (this
1468 is done by the <literal>FactoryStatic</literal> class, see below)
1469 and <literal>add_creator_dyn()</literal> for filters loaded
1475 <title><literal>mp::FactoryStatic</literal>
1476 (<filename>factory_static.cpp</filename>)</title>
1478 A subclass of <literal>FactoryFilter</literal> which is
1479 responsible for registering all the statically defined filter
1480 types. It does this by knowing about all those filters'
1481 structures, which are listed in its constructor. Merely
1482 instantiating this class registers all the static classes. It is
1483 for the benefit of this class that <literal>struct
1484 metaproxy_1_filter_struct</literal> exists, and that all the filter
1485 classes provide a static object of that type.
1490 <title><literal>mp::filter::Base</literal>
1491 (<filename>filter.cpp</filename>)</title>
1493 The virtual base class of all filters. The filter API is, on the
1494 surface at least, extremely simple: two methods.
1495 <literal>configure()</literal> is passed a DOM tree representing
1496 that part of the configuration file that pertains to this filter
1497 instance, and is expected to walk that tree extracting relevant
1498 information. And <literal>process()</literal> processes a
1499 package (see below). That surface simplicitly is a bit
1500 misleading, as <literal>process()</literal> needs to know a lot
1501 about the <literal>Package</literal> class in order to do
1507 <title><literal>mp::filter::AuthSimple</literal>,
1508 <literal>Backend_test</literal>, etc.
1509 (<filename>filter_auth_simple.cpp</filename>,
1510 <filename>filter_backend_test.cpp</filename>, etc.)</title>
1512 Individual filters. Each of these is implemented by a header and
1513 a source file, named <filename>filter_*.hpp</filename> and
1514 <filename>filter_*.cpp</filename> respectively. All the header
1515 files should be pretty much identical, in that they declare the
1516 class, including a private <literal>Rep</literal> class and a
1517 member pointer to it, and the two public methods. The only extra
1518 information in any filter header is additional private types and
1519 members (which should really all be in the <literal>Rep</literal>
1520 anyway) and private methods (which should also remain known only
1521 to the source file, but C++'s brain-damaged design requires this
1522 dirty laundry to be exhibited in public. Thanks, Bjarne!)
1525 The source file for each filter needs to supply:
1530 A definition of the private <literal>Rep</literal> class.
1535 Some boilerplate constructors and destructors.
1540 A <literal>configure()</literal> method that uses the
1541 appropriate XML fragment.
1546 Most important, the <literal>process()</literal> method that
1547 does all the actual work.
1554 <title><literal>mp::Package</literal>
1555 (<filename>package.cpp</filename>)</title>
1557 Represents a package on its way through the series of filters
1558 that make up a route. This is essentially a Z39.50 or SRU APDU
1559 together with information about where it came from, which is
1560 modified as it passes through the various filters.
1565 <title><literal>mp::Pipe</literal>
1566 (<filename>pipe.cpp</filename>)</title>
1568 This class provides a compatibility layer so that we have an IPC
1569 mechanism that works the same under Unix and Windows. It's not
1570 particularly exciting.
1575 <title><literal>mp::RouterChain</literal>
1576 (<filename>router_chain.cpp</filename>)</title>
1583 <title><literal>mp::RouterFleXML</literal>
1584 (<filename>router_flexml.cpp</filename>)</title>
1591 <title><literal>mp::Session</literal>
1592 (<filename>session.cpp</filename>)</title>
1599 <title><literal>mp::ThreadPoolSocketObserver</literal>
1600 (<filename>thread_pool_observer.cpp</filename>)</title>
1607 <title><literal>mp::util</literal>
1608 (<filename>util.cpp</filename>)</title>
1610 A namespace of various small utility functions and classes,
1611 collected together for convenience. Most importantly, includes
1612 the <literal>mp::util::odr</literal> class, a wrapper for YAZ's
1618 <title><literal>mp::xml</literal>
1619 (<filename>xmlutil.cpp</filename>)</title>
1621 A namespace of various XML utility functions and classes,
1622 collected together for convenience.
1628 <section id="other.source.files">
1629 <title>Other Source Files</title>
1631 In addition to the Metaproxy source files that define the classes
1632 described above, there are a few additional files which are
1633 briefly described here:
1637 <term><literal>metaproxy_prog.cpp</literal></term>
1640 The main function of the <command>metaproxy</command> program.
1645 <term><literal>ex_router_flexml.cpp</literal></term>
1648 Identical to <literal>metaproxy_prog.cpp</literal>: it's not clear why.
1653 <term><literal>test_*.cpp</literal></term>
1656 Unit-tests for various modules.
1662 ### Still to be described:
1663 <literal>ex_filter_frontend_net.cpp</literal>,
1664 <literal>filter_dl.cpp</literal>,
1665 <literal>plainfile.cpp</literal>,
1666 <literal>tstdl.cpp</literal>.
1673 <chapter id="refguide">
1674 <title>Reference guide</title>
1676 The material in this chapter is drawn directly from the individual
1677 manual entries. In particular, the Metaproxy invocation section is
1678 available using <command>man metaproxy</command>, and the section
1679 on each individual filter is available using the name of the filter
1680 as the argument to the <command>man</command> command.
1684 <section id="progref">
1685 <title>Metaproxy invocation</title>
1690 <section id="filterref">
1691 <title>Reference guide to Metaproxy filters</title>
1697 <!-- Keep this comment at the end of the file
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1703 sgml-always-quote-attributes:t
1706 sgml-parent-document: nil
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