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23 <title>Metaproxy - User's Guide and Reference</title>
25 <firstname>Adam</firstname><surname>Dickmeiss</surname>
28 <firstname>Marc</firstname><surname>Cromme</surname>
31 <firstname>Mike</firstname><surname>Taylor</surname>
35 <holder>Index Data ApS</holder>
39 Metaproxy is a universal router, proxy and encapsulated
40 metasearcher for information retrieval protocols. It accepts,
41 processes, interprets and redirects requests from IR clients using
42 standard protocols such as
43 <ulink url="&url.z39.50;">ANSI/NISO Z39.50</ulink>
44 (and in the future <ulink url="&url.sru;">SRU</ulink>
45 and <ulink url="&url.srw;">SRW</ulink>), as
46 well as functioning as a limited
47 <ulink url="&url.http;">HTTP</ulink> server.
48 Metaproxy is configured by an XML file which
49 specifies how the software should function in terms of routes that
50 the request packets can take through the proxy, each step on a
51 route being an instantiation of a filter. Filters come in many
52 types, one for each operation: accepting Z39.50 packets, logging,
53 query transformation, multiplexing, etc. Further filter-types can
54 be added as loadable modules to extend Metaproxy functionality,
58 Metaproxy is <emphasis>not</emphasis> open-source software, but
59 may be freely downloaded, unpacked, inspected, built and run for
60 evaluation purposes. Deployment requires a separate, commercial,
66 <imagedata fileref="common/id.png" format="PNG"/>
69 <imagedata fileref="common/id.eps" format="EPS"/>
76 <chapter id="introduction">
77 <title>Introduction</title>
81 <ulink url="&url.metaproxy;">Metaproxy</ulink>
82 is a standalone program that acts as a universal router, proxy and
83 encapsulated metasearcher for information retrieval protocols such
84 as <ulink url="&url.z39.50;">Z39.50</ulink>, and in the future
85 <ulink url="&url.sru;">SRU</ulink> and <ulink url="&url.srw;">SRW</ulink>.
86 To clients, it acts as a server of these protocols: it can be searched,
87 records can be retrieved from it, etc.
88 To servers, it acts as a client: it searches in them,
89 retrieves records from them, etc. it satisfies its clients'
90 requests by transforming them, multiplexing them, forwarding them
91 on to zero or more servers, merging the results, transforming
92 them, and delivering them back to the client. In addition, it
93 acts as a simple <ulink url="&url.http;">HTTP</ulink> server; support
94 for further protocols can be added in a modular fashion, through the
95 creation of new filters.
100 Fish, bananas, cold pyjamas,
101 Mutton, beef and trout!
102 - attributed to Cole Porter.
105 Metaproxy is a more capable alternative to
106 <ulink url="&url.yazproxy;">YAZ Proxy</ulink>,
107 being more powerful, flexible, configurable and extensible. Among
108 its many advantages over the older, more pedestrian work are
109 support for multiplexing (encapsulated metasearching), routing by
110 database name, authentication and authorisation and serving local
111 files via HTTP. Equally significant, its modular architecture
112 facilitites the creation of pluggable modules implementing further
116 This manual will describe how to install Metaproxy
117 before giving an overview of its architecture, then discussing the
118 key concept of a filter in some depth and giving an overview of
119 the various filter types, then discussing the configuration file
120 format. After this come several optional chapters which may be
121 freely skipped: a detailed discussion of virtual databases and
122 multi-database searching, some notes on writing extensions
123 (additional filter types) and a high-level description of the
124 source code. Finally comes the reference guide, which contains
125 instructions for invoking the <command>metaproxy</command>
126 program, and detailed information on each type of filter,
131 <chapter id="license">
132 <title>The Metaproxy License</title>
133 <orderedlist numeration="arabic">
136 You are allowed to download this software for evaluation purposes.
137 You can unpack it, build it, run it, see how it works and how it fits
138 your needs, all at zero cost.
143 You may NOT deploy the software. For the purposes of this license,
144 deployment means running it for any purpose other than evaluation,
145 whether or not you or anyone else makes a profit from doing so. If
146 you wish to deploy the software, you must first contact Index Data and
147 arrange to purchase a DEPLOYMENT LICENCE. If you are unsure
148 whether or not your proposed use of the software constitutes
149 deployment, email us at <literal>info@indexdata.com</literal>
155 You may modify your copy of the software (fix bugs, add features)
156 if you need to. We encourage you to send your changes back to us for
157 integration into the master copy, but you are not obliged to do so. You
158 may NOT pass your changes on to any other party.
163 There is NO WARRANTY for this software, to the extent permitted by
164 applicable law. We provide the software ``as is'' without warranty of
165 any kind, either expressed or implied, including, but not limited to, the
166 implied warranties of MERCHANTABILITY and FITNESS FOR A
167 PARTICULAR PURPOSE. The entire risk as to the quality and
168 performance of the software is with you. Should the software prove
169 defective, you assume the cost of all necessary servicing, repair or
170 correction. In no event unless required by applicable law will we be
171 liable to you for damages, arising out of the use of the software,
172 including but not limited to loss of data or data being rendered
178 All rights to the software are reserved by Index Data except where
179 this license explicitly says otherwise.
185 <chapter id="installation">
186 <title>Installation</title>
188 Metaproxy depends on the following tools/libraries:
190 <varlistentry><term><ulink url="&url.yazplusplus;">YAZ++</ulink></term>
193 This is a C++ library based on <ulink url="&url.yaz;">YAZ</ulink>.
197 <varlistentry><term><ulink url="&url.libxslt;">Libxslt</ulink></term>
199 <para>This is an XSLT processor - based on
200 <ulink url="&url.libxml2;">Libxml2</ulink>. Both Libxml2 and
201 Libxslt must be installed with the development components
202 (header files, etc.) as well as the run-time libraries.
206 <varlistentry><term><ulink url="&url.boost;">Boost</ulink></term>
209 The popular C++ library. Initial versions of Metaproxy
210 was built with 1.33.0. Version 1.33.1 works too.
217 In order to compile Metaproxy a modern C++ compiler is
218 required. Boost, in particular, requires the C++ compiler
219 to facilitate the newest features. Refer to Boost
220 <ulink url="&url.boost.compilers.status;">Compiler Status</ulink>
221 for more information.
224 We have succesfully built Metaproxy using the compilers
225 <ulink url="&url.gcc;">GCC</ulink> version 4.0 and
226 <ulink url="&url.vstudio;">Microsoft Visual Studio</ulink> 2003/2005.
229 <section id="installation.unix">
230 <title>Installation on Unix (from Source)</title>
232 Here is a quick step-by-step guide on how to compile all the
233 tools that Metaproxy uses. Only few systems have none of the required
234 tools binary packages. If, for example, Libxml2/libxslt are already
235 installed as development packages use those (and omit compilation).
242 gunzip -c libxml2-version.tar.gz|tar xf -
250 gunzip -c libxslt-version.tar.gz|tar xf -
261 gunzip -c yaz-version.tar.gz|tar xf -
269 gunzip -c yazpp-version.tar.gz|tar xf -
280 gunzip -c boost-version.tar.gz|tar xf -
291 gunzip -c metaproxy-version.tar.gz|tar xf -
300 <section id="installation.debian">
301 <title>Installation on Debian GNU/Linux</title>
303 All dependencies for Metaproxy are available as
304 <ulink url="&url.debian;">Debian</ulink>
305 packages for the sarge (stable in 2005) and etch (testing in 2005)
309 The procedures for Debian based systems, such as
310 <ulink url="&url.ubuntu;">Ubuntu</ulink> is probably similar
313 There is currently no official Debian package for YAZ++.
314 And the Debian package for YAZ is probably too old.
315 Update the <filename>/etc/apt/sources.list</filename>
316 to include the Index Data repository.
317 See YAZ' <ulink url="&url.yaz.download.debian;">Download Debian</ulink>
318 for more information.
321 apt-get install libxslt1-dev
322 apt-get install libyazpp-dev
323 apt-get install libboost-dev
324 apt-get install libboost-thread-dev
325 apt-get install libboost-date-time-dev
326 apt-get install libboost-program-options-dev
327 apt-get install libboost-test-dev
330 With these packages installed, the usual configure + make
331 procedure can be used for Metaproxy as outlined in
332 <xref linkend="installation.unix"/>.
336 <section id="installation.windows">
337 <title>Installation on Windows</title>
339 Metaproxy can be compiled with Microsoft
340 <ulink url="&url.vstudio;">Visual Studio</ulink>.
341 Version 2003 (C 7.1) and 2005 (C 8.0) is known to work.
343 <section id="installation.windows.boost">
346 Get Boost from its <ulink url="&url.boost;">home page</ulink>.
347 You also need Boost Jam (an alternative to make).
348 That's also available from the Boost home page.
349 The files to be downloaded are called something like:
350 <filename>boost_1_33-1.exe</filename>
352 <filename>boost-jam-3.1.12-1-ntx86.zip</filename>.
353 Unpack Boost Jam first. Put <filename>bjam.exe</filename>
354 in your system path. Make a command prompt and ensure
355 it can be found automatically. If not check the PATH.
356 The Boost .exe is a self-extracting exe with
357 complete source for Boost. Compile that source with
358 Boost Jam (An alternative to Make).
359 The compilation takes a while.
360 For Visual Studio 2003, use
362 bjam "-sTOOLS=vc-7_1"
364 Here <literal>vc-7_1</literal> refers to a "Toolset" (compiler system).
365 For Visual Studio 2005, use
367 bjam "-sTOOLS=vc-8_0"
369 To install the libraries in a common place, use
371 bjam "-sTOOLS=vc-7_1" install
373 (or vc-8_0 for VS 2005).
376 By default, the Boost build process installs the resulting
377 libraries + header files in
378 <literal>\boost\lib</literal>, <literal>\boost\include</literal>.
381 For more informatation about installing Boost refer to the
382 <ulink url="&url.boost.getting.started;">getting started</ulink>
387 <section id="installation.windows.libxslt">
388 <title>Libxslt</title>
390 <ulink url="&url.libxslt;">Libxslt</ulink> can be downloaded
392 <ulink url="&url.libxml2.download.win32;">here</ulink>.
395 Libxslt has other dependencies, but thes can all be downloaded
396 from the same site. Get the following:
397 iconv, zlib, libxml2, libxslt.
401 <section id="installation.windows.yaz">
404 <ulink url="&url.yaz;">YAZ</ulink> can be downloaded
406 <ulink url="&url.yaz.download.win32;">here</ulink>.
410 <section id="installation.windows.yazplusplus">
413 Get <ulink url="&url.yazplusplus;">YAZ++</ulink> as well.
414 Version 1.0 or later is required. For now get it from
416 <ulink url="&url.snapshot.download;">Snapshot area</ulink>.
419 YAZ++ includes NMAKE makefiles, similar to those found in the
424 <section id="installation.windows.metaproxy">
425 <title>Metaproxy</title>
427 Metaproxy is shipped with NMAKE makfiles as well - similar
428 to those found in the YAZ++/YAZ packages. Adjust this Makefile
429 to point to the proper locations of Boost, Libxslt, Libxml2,
430 zlib, iconv, yaz and yazpp.
434 <varlistentry><term><literal>DEBUG</literal></term>
436 If set to 1, the software is
437 compiled with debugging libraries (code generation is
438 multi-threaded debug DLL).
439 If set to 0, the software is compiled with release libraries
440 (code generation is multi-threaded DLL).
445 <term><literal>BOOST</literal></term>
448 Boost install location
454 <term><literal>BOOST_VERSION</literal></term>
457 Boost version (replace . with _).
463 <term><literal>BOOST_TOOLSET</literal></term>
472 <term><literal>LIBXSLT_DIR</literal>,
473 <literal>LIBXML2_DIR</literal> ..</term>
476 Specify the locations of Libxslt, libiconv, libxml2 and
485 After succesful compilation you'll find
486 <literal>metaproxy.exe</literal> in the
487 <literal>bin</literal> directory.
495 <chapter id="architecture">
496 <title>The Metaproxy Architecture</title>
498 The Metaproxy architecture is based on three concepts:
499 the <emphasis>package</emphasis>,
500 the <emphasis>route</emphasis>
501 and the <emphasis>filter</emphasis>.
505 <term>Packages</term>
508 A package is request or response, encoded in some protocol,
509 issued by a client, making its way through Metaproxy, send to or
510 received from a server, or sent back to the client.
513 The core of a package is the protocol unit - for example, a
514 Z39.50 Init Request or Search Response, or an SRU searchRetrieve
515 URL or Explain Response. In addition to this core, a package
516 also carries some extra information added and used by Metaproxy
520 In general, packages are doctored as they pass through
521 Metaproxy. For example, when the proxy performs authentication
522 and authorisation on a Z39.50 Init request, it removes the
523 authentication credentials from the package so that they are not
524 passed onto the back-end server; and when search-response
525 packages are obtained from multiple servers, they are merged
526 into a single unified package that makes its way back to the
535 Packages make their way through routes, which can be thought of
536 as programs that operate on the package data-type. Each
537 incoming package initially makes its way through a default
538 route, but may be switched to a different route based on various
539 considerations. Routes are made up of sequences of filters (see
548 Filters provide the individual instructions within a route, and
549 effect the necessary transformations on packages. A particular
550 configuration of Metaproxy is essentially a set of filters,
551 described by configuration details and arranged in order in one
552 or more routes. There are many kinds of filter - about a dozen
553 at the time of writing with more appearing all the time - each
554 performing a specific function and configured by different
558 The word ``filter'' is sometimes used rather loosely, in two
559 different ways: it may be used to mean a particular
560 <emphasis>type</emphasis> of filter, as when we speak of ``the
561 auth_simple filter'' or ``the multi filter''; or it may be used
562 to be a specific <emphasis>instance</emphasis> of a filter
563 within a Metaproxy configuration. For example, a single
564 configuration will often contain multiple instances of the
565 <literal>z3950_client</literal> filter. In
566 operational terms, of these is a separate filter. In practice,
567 context always make it clear which sense of the word ``filter''
571 Extensibility of Metaproxy is primarily through the creation of
572 plugins that provide new filters. The filter API is small and
573 conceptually simple, but there are many details to master. See
575 <link linkend="extensions">extensions</link>.
581 Since packages are created and handled by the system itself, and
582 routes are conceptually simple, most of the remainder of this
583 document concentrates on filters. After a brief overview of the
584 filter types follows, along with some thoughts on possible future
591 <chapter id="filters">
592 <title>Filters</title>
596 <title>Introductory notes</title>
598 It's useful to think of Metaproxy as an interpreter providing a small
599 number of primitives and operations, but operating on a very
600 complex data type, namely the ``package''.
603 A package represents a Z39.50 or SRU/W request (whether for Init,
604 Search, Scan, etc.) together with information about where it came
605 from. Packages are created by front-end filters such as
606 <literal>frontend_net</literal> (see below), which reads them from
607 the network; other front-end filters are possible. They then pass
608 along a route consisting of a sequence of filters, each of which
609 transforms the package and may also have side-effects such as
610 generating logging. Eventually, the route will yield a response,
611 which is sent back to the origin.
614 There are many kinds of filter: some that are defined statically
615 as part of Metaproxy, and others may be provided by third parties
616 and dynamically loaded. They all conform to the same simple API
617 of essentially two methods: <function>configure()</function> is
618 called at startup time, and is passed an XML DOM tree representing that
619 part of the configuration file that pertains to this filter
620 instance: it is expected to walk that tree extracting relevant
621 information; and <function>process()</function> is called every
622 time the filter has to processes a package.
625 While all filters provide the same API, there are different modes
626 of functionality. Some filters are sources: they create
628 (<literal>frontend_net</literal>);
629 others are sinks: they consume packages and return a result
630 (<literal>z3950_client</literal>,
631 <literal>backend_test</literal>,
632 <literal>bounce</literal>,
633 <literal>http_file</literal>);
634 the others are true filters, that read, process and pass on the
635 packages they are fed
636 (<literal>auth_simple</literal>,
637 <literal>log</literal>,
638 <literal>multi</literal>,
639 <literal>query_rewrite</literal>,
640 <literal>session_shared</literal>,
641 <literal>template</literal>,
642 <literal>virt_db</literal>).
647 <section id="overview.filter.types">
648 <title>Overview of filter types</title>
650 We now briefly consider each of the types of filter supported by
651 the core Metaproxy binary. This overview is intended to give a
652 flavour of the available functionality; more detailed information
653 about each type of filter is included below in
654 <link linkend="filterref"
655 >the reference guide to Metaproxy filters</link>.
658 The filters are here named by the string that is used as the
659 <literal>type</literal> attribute of a
660 <literal><filter></literal> element in the configuration
661 file to request them, with the name of the class that implements
662 them in parentheses. (The classname is not needed for normal
663 configuration and use of Metaproxy; it is useful only to
667 The filters are here listed in alphabetical order:
674 New virt_db-alike that does inteligent peer choice, explain merging,
675 adds FD&N to explain. Keeps init responses (like "virt_db Classic"),
676 makes routing choices based on local explain knowledge. Ref IDDI
679 Filter to convert Explain Classic to ZeeRex.
681 CQL2PQF (which needs augmented ZeeRex) - MARC for Talis.
685 Figure out what additional information we need in:
686 ZeeRex (check against D3.1)
687 Init request (e.g. loop detection)
688 Query package (e.g. number of hops)
689 Query response (e.g. record source)
694 <title><literal>auth_simple</literal>
695 (mp::filter::AuthSimple)</title>
697 Simple authentication and authorisation. The configuration
698 specifies the name of a file that is the user register, which
699 lists <varname>username</varname>:<varname>password</varname>
700 pairs, one per line, colon separated. When a session begins, it
701 is rejected unless username and passsword are supplied, and match
702 a pair in the register. The configuration file may also specific
703 the name of another file that is the target register: this lists
704 lists <varname>username</varname>:<varname>dbname</varname>,<varname>dbname</varname>...
705 sets, one per line, with multiple database names separated by
706 commas. When a search is processed, it is rejected unless the
707 database to be searched is one of those listed as available to
713 <title><literal>backend_test</literal>
714 (mp::filter::Backend_test)</title>
716 A partial sink that provides dummy responses in the manner of the
717 <literal>yaz-ztest</literal> Z39.50 server. This is useful only
718 for testing. Seriously, you don't need this. Pretend you didn't
719 even read this section.
724 <title><literal>bounce</literal>
725 (mp::filter::Bounce)</title>
727 A sink that swallows <emphasis>all packages</emphasis>,
728 and returns them almost unprocessed.
729 It never sends any package of any type further down the row, but
730 sets Z39.50 packages to Z_Close, and HTTP_Request packages to
731 HTTP_Response err code 400 packages, and adds a suitable bounce
733 The bounce filter is usually added at end of each filter chain
734 config.xml to prevent infinite hanging of for example HTTP
735 requests packages when only the Z39.50 client partial sink
736 filter is found in the
742 <title><literal>frontend_net</literal>
743 (mp::filter::FrontendNet)</title>
745 A source that accepts Z39.50 connections from a port
746 specified in the configuration, reads protocol units, and
747 feeds them into the next filter in the route. When the result is
748 revceived, it is returned to the original origin.
753 <title><literal>http_file</literal>
754 (mp::filter::HttpFile)</title>
756 A partial sink which swallows only HTTP_Request packages, and
757 returns the contents of files from the local
758 filesystem in response to HTTP requests.
759 It lets Z39.50 packages and all other forthcoming package types
762 does mean that Metaproxy is also a Web-server in its spare time. So
763 far it does not contain either an email-reader or a Lisp
764 interpreter, but that day is surely coming.)
769 <title><literal>log</literal>
770 (mp::filter::Log)</title>
772 Writes logging information to standard output, and passes on
773 the package unchanged. A log file name can be specified, as well
774 as multiple different logging formats.
779 <title><literal>multi</literal>
780 (mp::filter::Multi)</title>
782 Performs multi-database searching.
784 <link linkend="multidb">the extended discussion</link>
785 of virtual databases and multi-database searching below.
790 <title><literal>query_rewrite</literal>
791 (mp::filter::QueryRewrite)</title>
793 Rewrites Z39.50 Type-1 and Type-101 (``RPN'') queries by a
794 three-step process: the query is transliterated from Z39.50
795 packet structures into an XML representation; that XML
796 representation is transformed by an XSLT stylesheet; and the
797 resulting XML is transliterated back into the Z39.50 packet
803 <title><literal>session_shared</literal>
804 (mp::filter::SessionShared)</title>
806 When this is finished, it will implement global sharing of
807 result sets (i.e. between threads and therefore between
808 clients), yielding performance improvements especially when
809 incoming requests are from a stateless environment such as a
810 web-server, in which the client process representing a session
811 might be any one of many. However:
815 This filter is not yet completed.
821 <title><literal>template</literal>
822 (mp::filter::Template)</title>
824 Does nothing at all, merely passing the packet on. (Maybe it
825 should be called <literal>nop</literal> or
826 <literal>passthrough</literal>?) This exists not to be used, but
827 to be copied - to become the skeleton of new filters as they are
828 written. As with <literal>backend_test</literal>, this is not
829 intended for civilians.
834 <title><literal>virt_db</literal>
835 (mp::filter::Virt_db)</title>
837 Performs virtual database selection: based on the name of the
838 database in the search request, a server is selected, and its
839 address added to the request in a <literal>VAL_PROXY</literal>
840 otherInfo packet. It will subsequently be used by a
841 <literal>z3950_client</literal> filter.
843 <link linkend="multidb">the extended discussion</link>
844 of virtual databases and multi-database searching below.
849 <title><literal>z3950_client</literal>
850 (mp::filter::Z3950Client)</title>
852 A partial sink which swallows only Z39.50 packages.
853 It performs Z39.50 searching and retrieval by proxying the
854 packages that are passed to it. Init requests are sent to the
855 address specified in the <literal>VAL_PROXY</literal> otherInfo
856 attached to the request: this may have been specified by client,
857 or generated by a <literal>virt_db</literal> filter earlier in
858 the route. Subsequent requests are sent to the same address,
859 which is remembered at Init time in a Session object.
860 HTTP_Request packages and all other forthcoming package types
861 are passed untouched.
867 <section id="future.directions">
868 <title>Future directions</title>
870 Some other filters that do not yet exist, but which would be
871 useful, are briefly described. These may be added in future
872 releases (or may be created by third parties, as loadable
878 <term><literal>frontend_cli</literal> (source)</term>
881 Command-line interface for generating requests.
886 <term><literal>frontend_sru</literal> (source)</term>
889 Receive SRU (and perhaps SRW) requests.
894 <term><literal>sru2z3950</literal> (filter)</term>
897 Translate SRU requests into Z39.50 requests.
902 <term><literal>sru_client</literal> (sink)</term>
905 SRU searching and retrieval.
910 <term><literal>srw_client</literal> (sink)</term>
913 SRW searching and retrieval.
918 <term><literal>opensearch_client</literal> (sink)</term>
921 A9 OpenSearch searching and retrieval.
931 <chapter id="configuration">
932 <title>Configuration: the Metaproxy configuration file format</title>
936 <title>Introductory notes</title>
938 If Metaproxy is an interpreter providing operations on packages, then
939 its configuration file can be thought of as a program for that
940 interpreter. Configuration is by means of a single file, the name
941 of which is supplied as the sole command-line argument to the
942 <command>metaproxy</command> program. (See
943 <link linkend="progref">the reference guide</link>
944 below for more information on invoking Metaproxy.)
947 The configuration files are written in XML. (But that's just an
948 implementation detail - they could just as well have been written
949 in YAML or Lisp-like S-expressions, or in a custom syntax.)
952 Since XML has been chosen, an XML schema,
953 <filename>config.xsd</filename>, is provided for validating
954 configuration files. This file is supplied in the
955 <filename>etc</filename> directory of the Metaproxy distribution. It
956 can be used by (among other tools) the <command>xmllint</command>
957 program supplied as part of the <literal>libxml2</literal>
961 xmllint --noout --schema etc/config.xsd my-config-file.xml
964 (A recent version of <literal>libxml2</literal> is required, as
965 support for XML Schemas is a relatively recent addition.)
969 <section id="overview.xml.structure">
970 <title>Overview of XML structure</title>
972 All elements and attributes are in the namespace
973 <ulink url="http://indexdata.dk/yp2/config/1"/>.
974 This is most easily achieved by setting the default namespace on
975 the top-level element, as here:
978 <yp2 xmlns="http://indexdata.dk/yp2/config/1">
981 The top-level element is <yp2>. This contains a
982 <start> element, a <filters> element and a
983 <routes> element, in that order. <filters> is
984 optional; the other two are mandatory. All three are
988 The <start> element is empty, but carries a
989 <literal>route</literal> attribute, whose value is the name of
990 route at which to start running - analogous to the name of the
991 start production in a formal grammar.
994 If present, <filters> contains zero or more <filter>
995 elements. Each filter carries a <literal>type</literal> attribute
996 which specifies what kind of filter is being defined
997 (<literal>frontend_net</literal>, <literal>log</literal>, etc.)
998 and contain various elements that provide suitable configuration
999 for a filter of its type. The filter-specific elements are
1001 <link linkend="filterref">the reference guide below</link>.
1002 Filters defined in this part of the file must carry an
1003 <literal>id</literal> attribute so that they can be referenced
1007 <routes> contains one or more <route> elements, each
1008 of which must carry an <literal>id</literal> element. One of the
1009 routes must have the ID value that was specified as the start
1010 route in the <start> element's <literal>route</literal>
1011 attribute. Each route contains zero or more <filter>
1012 elements. These are of two types. They may be empty, but carry a
1013 <literal>refid</literal> attribute whose value is the same as the
1014 <literal>id</literal> of a filter previously defined in the
1015 <filters> section. Alternatively, a route within a filter
1016 may omit the <literal>refid</literal> attribute, but contain
1017 configuration elements similar to those used for filters defined
1018 in the <filters> section. (In other words, each filter in a
1019 route may be included either by reference or by physical
1025 <section id="example.configuration">
1026 <title>An example configuration</title>
1028 The following is a small, but complete, Metaproxy configuration
1029 file (included in the distribution as
1030 <literal>metaproxy/etc/config1.xml</literal>).
1031 This file defines a very simple configuration that simply proxies
1032 to whatever back-end server the client requests, but logs each
1033 request and response. This can be useful for debugging complex
1034 client-server dialogues.
1036 <screen><![CDATA[<?xml version="1.0"?>
1037 <yp2 xmlns="http://indexdata.dk/yp2/config/1">
1038 <start route="start"/>
1040 <filter id="frontend" type="frontend_net">
1043 <filter id="backend" type="z3950_client">
1048 <filter refid="frontend"/>
1049 <filter type="log"/>
1050 <filter refid="backend"/>
1051 <filter type="bounce"/>
1057 It works by defining a single route, called
1058 <literal>start</literal>, which consists of a sequence of four
1059 filters. The first and last of these are included by reference:
1060 their <literal><filter></literal> elements have
1061 <literal>refid</literal> attributes that refer to filters defined
1062 within the prior <literal><filters></literal> section. The
1063 middle filter is included inline in the route.
1066 The four filters in the route are as follows: first, a
1067 <literal>frontend_net</literal> filter accepts Z39.50 requests
1068 from any host on port 9000; then these requests are passed through
1069 a <literal>log</literal> filter that emits a message for each
1070 request; they are then fed into a <literal>z3950_client</literal>
1071 filter, which forwards all Z39.50 requests to the client-specified
1072 back-end Z39.509 server. Those Z39.50 packages are returned by the
1073 <literal>z3950_client</literal> filter, with the response data
1074 filled by the external Z39.50 server targeted.
1075 All non-Z39.50 packages are passed through to the
1076 <literal>bounce</literal> filter, which defitely bounces
1077 everything, including fish, bananas, cold pyjamas,
1078 mutton, beef and trout packages.
1079 When the response arrives, it is handed
1080 back to the <literal>log</literal> filter, which emits another
1081 message; and then to the <literal>frontend_net</literal> filter,
1082 which returns the response to the client.
1089 <chapter id="multidb">
1090 <title>Virtual databases and multi-database searching</title>
1094 <title>Introductory notes</title>
1096 Two of Metaproxy's filters are concerned with multiple-database
1097 operations. Of these, <literal>virt_db</literal> can work alone
1098 to control the routing of searches to one of a number of servers,
1099 while <literal>multi</literal> can work together with
1100 <literal>virt_db</literal> to perform multi-database searching, merging
1101 the results into a unified result-set - ``metasearch in a box''.
1104 The interaction between
1105 these two filters is necessarily complex: it reflects the real,
1106 irreducible complexity of multi-database searching in a protocol such
1107 as Z39.50 that separates initialisation from searching, and in
1108 which the database to be searched is not known at initialisation
1112 It's possible to use these filters without understanding the
1113 details of their functioning and the interaction between them; the
1114 next two sections of this chapter are ``HOWTO'' guides for doing
1115 just that. However, debugging complex configurations will require
1116 a deeper understanding, which the last two sections of this
1117 chapters attempt to provide.
1122 <section id="multidb.virt_db">
1123 <title>Virtual databases with the <literal>virt_db</literal> filter</title>
1125 Working alone, the purpose of the
1126 <literal>virt_db</literal>
1127 filter is to route search requests to one of a selection of
1128 back-end databases. In this way, a single Z39.50 endpoint
1129 (running Metaproxy) can provide access to several different
1130 underlying services, including those that would otherwise be
1131 inaccessible due to firewalls. In many useful configurations, the
1132 back-end databases are local to the Metaproxy installation, but
1133 the software does not enforce this, and any valid Z39.50 servers
1134 may be used as back-ends.
1137 For example, a <literal>virt_db</literal>
1138 filter could be set up so that searches in the virtual database
1139 ``lc'' are forwarded to the Library of Congress bibliographic
1140 catalogue server, and searches in the virtual database ``marc''
1141 are forwarded to the toy database of MARC records that Index Data
1142 hosts for testing purposes. A <literal>virt_db</literal>
1143 configuration to make this switch would look like this:
1145 <screen><![CDATA[<filter type="virt_db">
1147 <database>lc</database>
1148 <target>z3950.loc.gov:7090/voyager</target>
1151 <database>marc</database>
1152 <target>indexdata.dk/marc</target>
1154 </filter>]]></screen>
1156 As well as being useful in it own right, this filter also provides
1157 the foundation for multi-database searching.
1162 <section id="multidb.multi">
1163 <title>Multi-database search with the <literal>multi</literal> filter</title>
1165 To arrange for Metaproxy to broadcast searches to multiple back-end
1166 servers, the configuration needs to include two components: a
1167 <literal>virt_db</literal>
1168 filter that specifies multiple
1169 <literal><target></literal>
1170 elements, and a subsequent
1171 <literal>multi</literal>
1172 filter. Here, for example, is a complete configuration that
1173 broadcasts searches to both the Library of Congress catalogue and
1174 Index Data's tiny testing database of MARC records:
1176 <screen><![CDATA[<?xml version="1.0"?>
1177 <yp2 xmlns="http://indexdata.dk/yp2/config/1">
1178 <start route="start"/>
1181 <filter type="frontend_net">
1182 <threads>10</threads>
1185 <filter type="virt_db">
1187 <database>lc</database>
1188 <target>z3950.loc.gov:7090/voyager</target>
1191 <database>marc</database>
1192 <target>indexdata.dk/marc</target>
1195 <database>all</database>
1196 <target>z3950.loc.gov:7090/voyager</target>
1197 <target>indexdata.dk/marc</target>
1200 <filter type="multi"/>
1201 <filter type="z3950_client">
1202 <timeout>30</timeout>
1204 <filter type="bounce"/>
1210 <literal>virt_db</literal>
1211 filter that specifies multiple
1212 <literal><target></literal>
1213 elements but without a subsequent
1214 <literal>multi</literal>
1215 filter yields surprising and undesirable results, as will be
1216 described below. Don't do that.)
1219 Metaproxy can be invoked with this configuration as follows:
1221 <screen>../src/metaproxy --config config-simple-multi.xml</screen>
1223 And thereafter, Z39.50 clients can connect to the running server
1224 (on port 9000, as specified in the configuration) and search in
1225 any of the databases
1226 <literal>lc</literal> (the Library of Congress catalogue),
1227 <literal>marc</literal> (Index Data's test database of MARC records)
1229 <literal>all</literal> (both of these). As an example, a session
1230 using the YAZ command-line client <literal>yaz-client</literal> is
1231 here included (edited for brevity and clarity):
1233 <screen><![CDATA[$ yaz-client @:9000
1237 Search was a success.
1238 Number of hits: 10000, setno 1
1242 Search was a success.
1243 Number of hits: 10, setno 3
1247 Search was a success.
1248 Number of hits: 10010, setno 4
1251 [marc]Record type: USmarc
1254 005 00000000000000.0
1255 008 910710c19910701nju 00010 eng
1259 100 10 $a Jack Collins
1260 245 10 $a How to program a computer
1266 [VOYAGER]Record type: USmarc
1268 005 20041229102447.0
1269 008 030910s2004 caua 000 0 eng
1270 035 $a (DLC) 2003112666
1271 906 $a 7 $b cbc $c orignew $d 4 $e epcn $f 20 $g y-gencatlg
1272 925 0 $a acquire $b 1 shelf copy $x policy default
1273 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.
1276 040 $a DLC $c DLC $d DLC
1277 050 00 $a MLCM 2004/03312 (G)
1278 245 10 $a 007, everything or nothing : $b Prima's official strategy guide / $c created by Kaizen Media Group.
1279 246 3 $a Double-O-seven, everything or nothing
1280 246 30 $a Prima's official strategy guide
1281 260 $a Roseville, CA : $b Prima Games, $c c2004.
1282 300 $a 161 p. : $b col. ill. ; $c 28 cm.
1283 500 $a "Platforms: Nintendo GameCube, Macintosh, PC, PlayStation 2 computer entertainment system, Xbox"--P. [4] of cover.
1284 650 0 $a Video games.
1285 710 2 $a Kaizen Media Group.
1286 856 42 $3 Publisher description $u http://www.loc.gov/catdir/description/random052/2003112666.html
1291 As can be seen, the first record in the result set is from the
1292 Index Data test database, and the second from the Library of
1293 Congress database. The result-set continues alternating records
1294 round-robin style until the point where one of the databases'
1295 records are exhausted.
1298 This example uses only two back-end databases; more may be used.
1299 There is no limitation imposed on the number of databases that may
1300 be metasearched in this way: issues of resource usage and
1301 administrative complexity dictate the practical limits.
1304 What happens when one of the databases doesn't respond? By default,
1305 the entire multi-database search fails, and the appropriate
1306 diagnostic is returned to the client. This is usually appropriate
1307 during development, when technicians need maximum information, but
1308 can be inconvenient in deployment, when users typically don't want
1309 to be bothered with problems of this kind and prefer just to get
1310 the records from the databases that are available. To obtain this
1311 latter behaviour add an empty
1312 <literal><hideunavailable></literal>
1314 <literal>multi</literal> filter:
1316 <screen><![CDATA[ <filter type="multi">
1318 </filter>]]></screen>
1320 Under this regime, an error is reported to the client only if
1321 <emphasis>all</emphasis> the databases in a multi-database search
1327 <section id="multidb.what">
1328 <title>What's going on?</title>
1330 <title>Lark's vomit</title>
1332 This section goes into a level of technical detail that is
1333 probably not necessary in order to configure and use Metaproxy.
1334 It is provided only for those who like to know how things work.
1335 You should feel free to skip on to the next section if this one
1336 doesn't seem like fun.
1340 Hold on tight - this may get a little hairy.
1343 In the general course of things, a Z39.50 Init request may carry
1344 with it an otherInfo packet of type <literal>VAL_PROXY</literal>,
1345 whose value indicates the address of a Z39.50 server to which the
1346 ultimate connection is to be made. (This otherInfo packet is
1347 supported by YAZ-based Z39.50 clients and servers, but has not yet
1348 been ratified by the Maintenance Agency and so is not widely used
1349 in non-Index Data software. We're working on it.)
1350 The <literal>VAL_PROXY</literal> packet functions
1351 analogously to the absoluteURI-style Request-URI used with the GET
1352 method when a web browser asks a proxy to forward its request: see
1354 <ulink url="http://www.w3.org/Protocols/rfc2616/rfc2616-sec5.html#sec5.1.2"
1355 >Request-URI</ulink>
1357 <ulink url="http://www.w3.org/Protocols/rfc2616/rfc2616.html"
1358 >the HTTP 1.1 specification</ulink>.
1361 Within Metaproxy, Search requests that are part of the same
1362 session as an Init request that carries a
1363 <literal>VAL_PROXY</literal> otherInfo are also annotated with the
1364 same information. The role of the <literal>virt_db</literal>
1365 filter is to rewrite this otherInfo packet dependent on the
1366 virtual database that the client wants to search.
1369 When Metaproxy receives a Z39.50 Init request from a client, it
1370 doesn't immediately forward that request to the back-end server.
1371 Why not? Because it doesn't know <emphasis>which</emphasis>
1372 back-end server to forward it to until the client sends a Search
1373 request that specifies the database that it wants to search in.
1374 Instead, it just treasures the Init request up in its heart; and,
1375 later, the first time the client does a search on one of the
1376 specified virtual databases, a connection is forged to the
1377 appropriate server and the Init request is forwarded to it. If,
1378 later in the session, the same client searches in a different
1379 virtual database, then a connection is forged to the server that
1380 hosts it, and the same cached Init request is forwarded there,
1384 All of this clever Init-delaying is done by the
1385 <literal>frontend_net</literal> filter. The
1386 <literal>virt_db</literal> filter knows nothing about it; in
1387 fact, because the Init request that is received from the client
1388 doesn't get forwarded until a Search request is received, the
1389 <literal>virt_db</literal> filter (and the
1390 <literal>z3950_client</literal> filter behind it) doesn't even get
1391 invoked at Init time. The <emphasis>only</emphasis> thing that a
1392 <literal>virt_db</literal> filter ever does is rewrite the
1393 <literal>VAL_PROXY</literal> otherInfo in the requests that pass
1397 It is possible for a <literal>virt_db</literal> filter to contain
1399 <literal><target></literal>
1400 elements. What does this mean? Only that the filter will add
1401 multiple <literal>VAL_PROXY</literal> otherInfo packets to the
1402 Search requests that pass through it. That's because the virtual
1403 DB filter is dumb, and does exactly what it's told - no more, no
1405 If a Search request with multiple <literal>VAL_PROXY</literal>
1406 otherInfo packets reaches a <literal>z3950_client</literal>
1407 filter, this is an error. That filter doesn't know how to deal
1408 with multiple targets, so it will either just pick one and search
1409 in it, or (better) fail with an error message.
1412 The <literal>multi</literal> filter comes to the rescue! This is
1413 the only filter that knows how to deal with multiple
1414 <literal>VAL_PROXY</literal> otherInfo packets, and it does so by
1415 making multiple copies of the entire Search request: one for each
1416 <literal>VAL_PROXY</literal>. Each of these new copies is then
1417 passed down through the remaining filters in the route. (The
1418 copies are handled in parallel though the
1419 spawning of new threads.) Since the copies each have only one
1420 <literal>VAL_PROXY</literal> otherInfo, they can be handled by the
1421 <literal>z3950_client</literal> filter, which happily deals with
1422 each one individually. When the results of the individual
1423 searches come back up to the <literal>multi</literal> filter, it
1424 merges them into a single Search response, which is what
1425 eventually makes it back to the client.
1430 <section id="multidb.picture">
1431 <title>A picture is worth a thousand words (but only five hundred on 64-bit architectures)</title>
1435 <imagedata fileref="multi.pdf" format="PDF" scale="50"/>
1438 <imagedata fileref="multi.png" format="PNG"/>
1441 <!-- Fall back if none of the images can be used -->
1443 [Here there should be a diagram showing the progress of
1444 packages through the filters during a simple virtual-database
1445 search and a multi-database search, but is seems that your
1446 toolchain has not been able to include the diagram in this
1447 document. This is because of LaTeX suckage. Time to move to
1448 OpenOffice. Yes, really.]
1451 <!-- ### This used to work with an older version of DocBook
1453 <para>Caption: progress of packages through filters.</para>
1456 </inlinemediaobject>
1463 <chapter id="extensions">
1464 <title>Writing extensions for Metaproxy</title>
1465 <para>### To be written</para>
1471 <chapter id="classes">
1472 <title>Classes in the Metaproxy source code</title>
1476 <title>Introductory notes</title>
1478 <emphasis>Stop! Do not read this!</emphasis>
1479 You won't enjoy it at all. You should just skip ahead to
1480 <link linkend="refguide">the reference guide</link>,
1482 <!-- The remainder of this paragraph is lifted verbatim from
1483 Douglas Adams' _Hitch Hiker's Guide to the Galaxy_, chapter 8 -->
1484 you things you really need to know, like the fact that the
1485 fabulously beautiful planet Bethselamin is now so worried about
1486 the cumulative erosion by ten billion visiting tourists a year
1487 that any net imbalance between the amount you eat and the amount
1488 you excrete whilst on the planet is surgically removed from your
1489 bodyweight when you leave: so every time you go to the lavatory it
1490 is vitally important to get a receipt.
1493 This chapter contains documentation of the Metaproxy source code, and is
1494 of interest only to maintainers and developers. If you need to
1495 change Metaproxy's behaviour or write a new filter, then you will most
1496 likely find this chapter helpful. Otherwise it's a waste of your
1497 good time. Seriously: go and watch a film or something.
1498 <citetitle>This is Spinal Tap</citetitle> is particularly good.
1501 Still here? OK, let's continue.
1504 In general, classes seem to be named big-endianly, so that
1505 <literal>FactoryFilter</literal> is not a filter that filters
1506 factories, but a factory that produces filters; and
1507 <literal>FactoryStatic</literal> is a factory for the statically
1508 registered filters (as opposed to those that are dynamically
1513 <section id="individual.classes">
1514 <title>Individual classes</title>
1516 The classes making up the Metaproxy application are here listed by
1517 class-name, with the names of the source files that define them in
1522 <title><literal>mp::FactoryFilter</literal>
1523 (<filename>factory_filter.cpp</filename>)</title>
1525 A factory class that exists primarily to provide the
1526 <literal>create()</literal> method, which takes the name of a
1527 filter class as its argument and returns a new filter of that
1528 type. To enable this, the factory must first be populated by
1529 calling <literal>add_creator()</literal> for static filters (this
1530 is done by the <literal>FactoryStatic</literal> class, see below)
1531 and <literal>add_creator_dyn()</literal> for filters loaded
1537 <title><literal>mp::FactoryStatic</literal>
1538 (<filename>factory_static.cpp</filename>)</title>
1540 A subclass of <literal>FactoryFilter</literal> which is
1541 responsible for registering all the statically defined filter
1542 types. It does this by knowing about all those filters'
1543 structures, which are listed in its constructor. Merely
1544 instantiating this class registers all the static classes. It is
1545 for the benefit of this class that <literal>struct
1546 metaproxy_1_filter_struct</literal> exists, and that all the filter
1547 classes provide a static object of that type.
1552 <title><literal>mp::filter::Base</literal>
1553 (<filename>filter.cpp</filename>)</title>
1555 The virtual base class of all filters. The filter API is, on the
1556 surface at least, extremely simple: two methods.
1557 <literal>configure()</literal> is passed an XML DOM tree representing
1558 that part of the configuration file that pertains to this filter
1559 instance, and is expected to walk that tree extracting relevant
1560 information. And <literal>process()</literal> processes a
1561 package (see below). That surface simplicitly is a bit
1562 misleading, as <literal>process()</literal> needs to know a lot
1563 about the <literal>Package</literal> class in order to do
1569 <title><literal>mp::filter::AuthSimple</literal>,
1570 <literal>Backend_test</literal>, etc.
1571 (<filename>filter_auth_simple.cpp</filename>,
1572 <filename>filter_backend_test.cpp</filename>, etc.)</title>
1574 Individual filters. Each of these is implemented by a header and
1575 a source file, named <filename>filter_*.hpp</filename> and
1576 <filename>filter_*.cpp</filename> respectively. All the header
1577 files should be pretty much identical, in that they declare the
1578 class, including a private <literal>Rep</literal> class and a
1579 member pointer to it, and the two public methods. The only extra
1580 information in any filter header is additional private types and
1581 members (which should really all be in the <literal>Rep</literal>
1582 anyway) and private methods (which should also remain known only
1583 to the source file, but C++'s brain-damaged design requires this
1584 dirty laundry to be exhibited in public. Thanks, Bjarne!)
1587 The source file for each filter needs to supply:
1592 A definition of the private <literal>Rep</literal> class.
1597 Some boilerplate constructors and destructors.
1602 A <literal>configure()</literal> method that uses the
1603 appropriate XML fragment.
1608 Most important, the <literal>process()</literal> method that
1609 does all the actual work.
1616 <title><literal>mp::Package</literal>
1617 (<filename>package.cpp</filename>)</title>
1619 Represents a package on its way through the series of filters
1620 that make up a route. This is essentially a Z39.50 or SRU APDU
1621 together with information about where it came from, which is
1622 modified as it passes through the various filters.
1627 <title><literal>mp::Pipe</literal>
1628 (<filename>pipe.cpp</filename>)</title>
1630 This class provides a compatibility layer so that we have an IPC
1631 mechanism that works the same under Unix and Windows. It's not
1632 particularly exciting.
1637 <title><literal>mp::RouterChain</literal>
1638 (<filename>router_chain.cpp</filename>)</title>
1645 <title><literal>mp::RouterFleXML</literal>
1646 (<filename>router_flexml.cpp</filename>)</title>
1653 <title><literal>mp::Session</literal>
1654 (<filename>session.cpp</filename>)</title>
1661 <title><literal>mp::ThreadPoolSocketObserver</literal>
1662 (<filename>thread_pool_observer.cpp</filename>)</title>
1669 <title><literal>mp::util</literal>
1670 (<filename>util.cpp</filename>)</title>
1672 A namespace of various small utility functions and classes,
1673 collected together for convenience. Most importantly, includes
1674 the <literal>mp::util::odr</literal> class, a wrapper for YAZ's
1680 <title><literal>mp::xml</literal>
1681 (<filename>xmlutil.cpp</filename>)</title>
1683 A namespace of various XML utility functions and classes,
1684 collected together for convenience.
1690 <section id="other.source.files">
1691 <title>Other Source Files</title>
1693 In addition to the Metaproxy source files that define the classes
1694 described above, there are a few additional files which are
1695 briefly described here:
1699 <term><literal>metaproxy_prog.cpp</literal></term>
1702 The main function of the <command>metaproxy</command> program.
1707 <term><literal>ex_router_flexml.cpp</literal></term>
1710 Identical to <literal>metaproxy_prog.cpp</literal>: it's not clear why.
1715 <term><literal>test_*.cpp</literal></term>
1718 Unit-tests for various modules.
1724 ### Still to be described:
1725 <literal>ex_filter_frontend_net.cpp</literal>,
1726 <literal>filter_dl.cpp</literal>,
1727 <literal>plainfile.cpp</literal>,
1728 <literal>tstdl.cpp</literal>.
1735 <reference id="refguide">
1736 <title>Reference guide</title>
1738 The material in this chapter is drawn directly from the individual
1739 manual entries. In particular, the Metaproxy invocation section is
1740 available using <command>man metaproxy</command>, and the section
1741 on each individual filter is available using the name of the filter
1742 as the argument to the <command>man</command> command.
1748 <!-- Keep this comment at the end of the file
1753 sgml-minimize-attributes:nil
1754 sgml-always-quote-attributes:t
1757 sgml-parent-document: nil
1758 sgml-local-catalogs: nil
1759 sgml-namecase-general:t