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   Description of the virtual host mechanism in YAZ GFS
   Included in both manual and man page for yaz-ztest
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<para>
 The Virtual hosts mechanism allows a YAZ front-end server to
 support multiple back-ends. A back-end is selected on the basis of
 the TCP/IP binding (port+listening address) and/or the virtual host.
</para>
<para>
 A back-end can be configured to execute in a particular working
 directory. Or the YAZ front-end may perform CQL to RPN conversion, thus
 allowing traditional Z39.50 back-ends to be offered as a SRW/SRU
 service. SRW/SRU Explain information for a particular back-end may also
 be specified.
</para>
<para>
 For the HTTP protocol, the virtual host is specified in the Host header.
 For the Z39.50 protocol, the virtual host is specified as in the
 Initialize Request in the OtherInfo, OID 1.2.840.10003.10.1000.81.1.
</para>
<note>
 <para>
  Not all Z39.50 clients allow the VHOST information to be set.
  For those, the selection of the back-end must rely on the
  TCP/IP information alone (port and address).
 </para>
</note>
<para>
 The YAZ front-end server uses XML to describe the back-end
 configurations. Command-line option <literal>-f</literal>
 specifies filename of the XML configuration.
</para>
<para>
 The configuration uses the root element <literal>yazgfs</literal>.
 This element includes a list of <literal>listen</literal> elements,
 followed by one or more <literal>server</literal> elements.
</para>
<para>
 The <literal>listen</literal> describes listener (transport end point),
 such as TCP/IP, Unix file socket or SSL server. Content for
 a listener:
 <variablelist>
  <varlistentry><term>CDATA (required)</term>
   <listitem>
    <para>
     The CDATA for the <literal>listen</literal> element holds the
     listener string, such as <literal>tcp:@:210</literal>,
     <literal>tcp:server1:2100</literal>,
     etc.
    </para>
   </listitem>
  </varlistentry>
  <varlistentry><term>attribute <literal>id</literal> (optional)</term>
    <listitem>
     <para>
      Identifier for this listener. This may be referred to from
      server sections.
     </para>
    </listitem>
   </varlistentry>
 </variablelist>
 <note>
  <para>
   We expect more information to be added for the listen section in
   a future version, such as CERT file for SSL servers.
  </para>
 </note>
</para>
<para>
 The <literal>server</literal> describes a server and the parameters
 for this server type. Content for a server:
 <variablelist>
  <varlistentry><term>attribute <literal>id</literal> (optional)</term>
   <listitem>
    <para>
     Identifier for this server. Currently not used for anything,
     but it might be for logging purposes.
   </para>
   </listitem>
  </varlistentry>

  <varlistentry><term>attribute <literal>listenref</literal> (optional)</term>
   <listitem>
    <para>
     Specifies one or more listeners for this server. Each server ID is
     separated by a comma.
     If this attribute is not given, the server is accessible from all
     listeners. In order for the server to be used for real, however, the
     virtual host must match if specified in the configuration.
    </para>
   </listitem>
  </varlistentry>

  <varlistentry><term>element <literal>config</literal> (optional)</term>
   <listitem>
    <para>
     Specifies the server configuration. This is equivalent
     to the config specified using command line option
     <literal>-c</literal>.
    </para>
   </listitem>
  </varlistentry>

  <varlistentry><term>element <literal>directory</literal> (optional)</term>
   <listitem>
    <para>
     Specifies a working directory for this backend server. If
     specified, the YAZ frontend changes current working directory
     to this directory whenever a backend of this type is
     started (backend handler bend_start), stopped (backend handler hand_stop)
     and initialized (bend_init).
    </para>
   </listitem>
  </varlistentry>

  <varlistentry><term>element <literal>host</literal> (optional)</term>
   <listitem>
    <para>
     Specifies the virtual host for this server. If this is specified
     a client <emphasis>must</emphasis> specify this host string in
     order to use this backend.
    </para>
   </listitem>
  </varlistentry>

  <varlistentry><term>element <literal>cql2rpn</literal> (optional)</term>
   <listitem>
    <para>
     Specifies a filename that includes CQL to RPN conversion for this
     backend server. See &reference-tools-cql-map;.
     If given, the backend server will only "see" a Type-1/RPN query.
    </para>
   </listitem>
  </varlistentry>

  <varlistentry><term>element <literal>ccl2rpn</literal> (optional)</term>
   <listitem>
    <para>
     Specifies a filename that includes CCL to RPN conversion for this
     backend server. See &reference-tools-ccl-qualifiers;.
     If given, the backend server will only "see" a Type-1/RPN query.
    </para>
   </listitem>
  </varlistentry>

  <varlistentry><term>element <literal>stylesheet</literal> (optional)</term>
   <listitem>
    <para>
     Specifies the stylesheet reference to be part of SRU HTTP responses
     when the client does not specify one. If none is given, then if
     the client does not specify one, then no stylesheet reference is part of the
     SRU HTTP response.
    </para>
   </listitem>
  </varlistentry>

  <varlistentry><term>element <literal>client_query_charset</literal> (optional)</term>
   <listitem>
    <para>
     If specified, a conversion from the character set given to UTF-8 is
     performed by the generic frontend server. It is only executed for
     Z39.50 search requests (SRU/Solr are assumed to be UTF-8 encoded already).
    </para>
   </listitem>
  </varlistentry>

  <varlistentry><term>element <literal>docpath</literal> (optional)</term>
   <listitem>
    <para>
     Specifies a path for local file access using HTTP. All URLs with
     a leading prefix (/ excluded) that matches the value of <literal>docpath</literal>
     are used for file access. For example, if the server is to offer
     access in directory <literal>xsl</literal>, the docpath would be
     <literal>xsl</literal> and all URLs of the form
     <literal>http://host/xsl</literal> will result in a local file access.
    </para>
   </listitem>
  </varlistentry>

  <varlistentry><term>element <literal>explain</literal> (optional)</term>
   <listitem>
    <para>
     Specifies SRW/SRU ZeeRex content for this server. Copied verbatim
     to the client. As things are now, some of the Explain content
     seem redundant because host information, etc. is also stored
     elsewhere.
    </para>
   </listitem>
  </varlistentry>

  <varlistentry><term>element <literal>maximumrecordsize</literal> (optional)</term>
   <listitem>
    <para>
     Specifies maximum record size/message size, in bytes. This
     value also serves as the maximum size of <emphasis>incoming</emphasis>
     packages (for Record Updates etc). It's the same value as that
     given by the <literal>-k</literal> option.
    </para>
   </listitem>
  </varlistentry>

  <varlistentry><term>element <literal>retrievalinfo</literal> (optional)</term>
   <listitem>
    <para>
     Enables the retrieval facility to support conversions and
     specifications of record formats/types.
     See <xref linkend="tools.retrieval"/> for
     more information.
    </para>
   </listitem>
  </varlistentry>

 </variablelist>
</para>

<para>
 The XML below configures a server that accepts connections from
 two ports, TCP/IP port 9900 and a local UNIX file socket.
 We name the TCP/IP server <literal>public</literal> and the
 other server <literal>internal</literal>.
 </para>
 <screen>
  <![CDATA[
 <yazgfs>
  <listen id="public">tcp:@:9900</listen>
  <listen id="internal">unix:/var/tmp/socket</listen>
  <server id="server1">
    <host>server1.mydomain</host>
    <directory>/var/www/s1</directory>
    <config>config.cfg</config>
  </server>
  <server id="server2" listenref="public,internal">
    <host>server2.mydomain</host>
    <directory>/var/www/s2</directory>
    <config>config.cfg</config>
    <cql2rpn>../etc/pqf.properties</cql2rpn>
    <explain xmlns="http://explain.z3950.org/dtd/2.0/">
      <serverInfo>
        <host>server2.mydomain</host>
        <port>9900</port>
        <database>a</database>
      </serverInfo>
    </explain>
  </server>
  <server id="server3" listenref="internal">
    <directory>/var/www/s3</directory>
    <config>config.cfg</config>
  </server>
 </yazgfs>
]]>
 </screen>
<para>
 There are three configured backend servers. The first two
 servers, <literal>"server1"</literal> and <literal>"server2"</literal>,
 can be reached by both listener addresses.
 <literal>"server1"</literal> is reached by all (two) since no
  <literal>listenref</literal> attribute is specified.
 <literal>"server2"</literal> is reached by the two listeners specified.
 In order to distinguish between the two, a virtual host has
 been specified for each server in the <literal>host</literal>
 elements.
</para>
<para>
 For <literal>"server2"</literal> elements for CQL to RPN conversion
 is supported and explain information has been added (a short one here
 to keep the example small).
</para>
<para>
 The third server, <literal>"server3"</literal> can only be reached
 via listener <literal>"internal"</literal>.
</para>

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