source: yaz/trunk/fuentes/doc/gfs-virtual.xml @ 265

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1<!--
2   Description of the virtual host mechanism in YAZ GFS
3   Included in both manual and man page for yaz-ztest
4-->
5
6<para>
7 The Virtual hosts mechanism allows a YAZ frontend server to
8 support multiple backends. A backend is selected on the basis of
9 the TCP/IP binding (port+listening adddress) and/or the virtual host.
10</para>
11<para>
12 A backend can be configured to execute in a particular working
13 directory. Or the YAZ frontend may perform CQL to RPN conversion, thus
14 allowing traditional Z39.50 backends to be offered as a SRW/SRU
15 service. SRW/SRU Explain information for a particular backend may also
16 be specified.
17</para>
18<para>
19 For the HTTP protocol, the virtual host is specified in the Host header.
20 For the Z39.50 protocol, the virtual host is specified as in the
21 Initialize Request in the OtherInfo, OID 1.2.840.10003.10.1000.81.1.
22</para>
23<note>
24 <para>
25  Not all Z39.50 clients allow the VHOST information to be set.
26  For those, the selection of the backend must rely on the
27  TCP/IP information alone (port and address).
28 </para>
29</note>
30<para>
31 The YAZ frontend server uses XML to describe the backend
32 configurations. Command-line option <literal>-f</literal>
33 specifies filename of the XML configuration.
34</para>
35<para>
36 The configuration uses the root element <literal>yazgfs</literal>.
37 This element includes a list of <literal>listen</literal> elements,
38 followed by one or more <literal>server</literal> elements.
39</para>
40<para>
41 The <literal>listen</literal> describes listener (transport end point),
42 such as TCP/IP, Unix file socket or SSL server. Content for
43 a listener:
44 <variablelist>
45  <varlistentry><term>CDATA (required)</term>
46   <listitem>
47    <para>
48     The CDATA for the <literal>listen</literal> element holds the
49     listener string, such as <literal>tcp:@:210</literal>,
50     <literal>tcp:server1:2100</literal>,
51     etc.
52    </para>
53   </listitem>
54  </varlistentry>
55  <varlistentry><term>attribute <literal>id</literal> (optional)</term>
56    <listitem>
57     <para>
58      Identifier for this listener. This may be referred to from
59      server sections.
60     </para>
61    </listitem>
62   </varlistentry>
63 </variablelist>
64 <note>
65  <para>
66   We expect more information to be added for the listen section in
67   a future version, such as CERT file for SSL servers.
68  </para>
69 </note>
70</para>
71<para>
72 The <literal>server</literal> describes a server and the parameters
73 for this server type. Content for a server:
74 <variablelist>
75  <varlistentry><term>attribute <literal>id</literal> (optional)</term>
76   <listitem>
77    <para>
78     Identifier for this server. Currently not used for anything,
79     but it might be for logging purposes.
80   </para>
81   </listitem>
82  </varlistentry>
83
84  <varlistentry><term>attribute <literal>listenref</literal> (optional)</term>
85   <listitem>
86    <para>
87     Specifies one or more listeners for this server. Each server ID is
88     separated by a comma.
89     If this attribute is not given, the server is accessible from all
90     listeners. In order for the server to be used for real, however, the
91     virtual host must match if specified in the configuration.
92    </para>
93   </listitem>
94  </varlistentry>
95
96  <varlistentry><term>element <literal>config</literal> (optional)</term>
97   <listitem>
98    <para>
99     Specifies the server configuration. This is equivalent
100     to the config specified using command line option
101     <literal>-c</literal>.
102    </para>
103   </listitem>
104  </varlistentry>
105
106  <varlistentry><term>element <literal>directory</literal> (optional)</term>
107   <listitem>
108    <para>
109     Specifies a working directory for this backend server. If
110     specified, the YAZ frontend changes current working directory
111     to this directory whenever a backend of this type is
112     started (backend handler bend_start), stopped (backend handler hand_stop)
113     and initialized (bend_init).
114    </para>
115   </listitem>
116  </varlistentry>
117
118  <varlistentry><term>element <literal>host</literal> (optional)</term>
119   <listitem>
120    <para>
121     Specifies the virtual host for this server. If this is specified
122     a client <emphasis>must</emphasis> specify this host string in
123     order to use this backend.
124    </para>
125   </listitem>
126  </varlistentry>
127
128  <varlistentry><term>element <literal>cql2rpn</literal> (optional)</term>
129   <listitem>
130    <para>
131     Specifies a filename that includes CQL to RPN conversion for this
132     backend server. See &reference-tools-cql-map;.
133     If given, the backend server will only "see" a Type-1/RPN query.
134    </para>
135   </listitem>
136  </varlistentry>
137
138  <varlistentry><term>element <literal>ccl2rpn</literal> (optional)</term>
139   <listitem>
140    <para>
141     Specifies a filename that includes CCL to RPN conversion for this
142     backend server. See &reference-tools-ccl-qualifiers;.
143     If given, the backend server will only "see" a Type-1/RPN query.
144    </para>
145   </listitem>
146  </varlistentry>
147
148  <varlistentry><term>element <literal>stylesheet</literal> (optional)</term>
149   <listitem>
150    <para>
151     Specifies the stylesheet reference to be part of SRU HTTP responses
152     when the client does not specify one. If none is given, then if
153     the client does not specify one, then no stylesheet reference is part of the
154     SRU HTTP response.
155    </para>
156   </listitem>
157  </varlistentry>
158
159  <varlistentry><term>element <literal>client_query_charset</literal> (optional)</term>
160   <listitem>
161    <para>
162     If specified, a conversion from the character set given to UTF-8 is
163     performed by the generic frontend server. It is only executed for
164     Z39.50 search requests (SRU/Solr are assumed to be UTF-8 encoded already).
165    </para>
166   </listitem>
167  </varlistentry>
168
169  <varlistentry><term>element <literal>docpath</literal> (optional)</term>
170   <listitem>
171    <para>
172     Specifies a path for local file access using HTTP. All URLs with
173     a leading prefix (/ excluded) that matches the value of <literal>docpath</literal>
174     are used for file access. For example, if the server is to offer
175     access in directory <literal>xsl</literal>, the docpath would be
176     <literal>xsl</literal> and all URLs of the form
177     <literal>http://host/xsl</literal> will result in a local file access.
178    </para>
179   </listitem>
180  </varlistentry>
181
182  <varlistentry><term>element <literal>explain</literal> (optional)</term>
183   <listitem>
184    <para>
185     Specifies SRW/SRU ZeeRex content for this server. Copied verbatim
186     to the client. As things are now, some of the Explain content
187     seem redundant because host information, etc. is also stored
188     elsewhere.
189    </para>
190   </listitem>
191  </varlistentry>
192
193  <varlistentry><term>element <literal>maximumrecordsize</literal> (optional)</term>
194   <listitem>
195    <para>
196     Specifies maximum record size/message size, in bytes. This
197     value also serves as the maximum size of <emphasis>incoming</emphasis>
198     packages (for Record Updates etc). It's the same value as that
199     given by the <literal>-k</literal> option.
200    </para>
201   </listitem>
202  </varlistentry>
203
204  <varlistentry><term>element <literal>retrievalinfo</literal> (optional)</term>
205   <listitem>
206    <para>
207     Enables the retrieval facility to support conversions and
208     specifications of record formats/types.
209     See <xref linkend="tools.retrieval"/> for
210     more information.
211    </para>
212   </listitem>
213  </varlistentry>
214
215 </variablelist>
216</para>
217
218<para>
219 The XML below configures a server that accepts connections from
220 two ports, TCP/IP port 9900 and a local UNIX file socket.
221 We name the TCP/IP server <literal>public</literal> and the
222 other server <literal>internal</literal>.
223 </para>
224 <screen>
225  <![CDATA[
226 <yazgfs>
227  <listen id="public">tcp:@:9900</listen>
228  <listen id="internal">unix:/var/tmp/socket</listen>
229  <server id="server1">
230    <host>server1.mydomain</host>
231    <directory>/var/www/s1</directory>
232    <config>config.cfg</config>
233  </server>
234  <server id="server2" listenref="public,internal">
235    <host>server2.mydomain</host>
236    <directory>/var/www/s2</directory>
237    <config>config.cfg</config>
238    <cql2rpn>../etc/pqf.properties</cql2rpn>
239    <explain xmlns="http://explain.z3950.org/dtd/2.0/">
240      <serverInfo>
241        <host>server2.mydomain</host>
242        <port>9900</port>
243        <database>a</database>
244      </serverInfo>
245    </explain>
246  </server>
247  <server id="server3" listenref="internal">
248    <directory>/var/www/s3</directory>
249    <config>config.cfg</config>
250  </server>
251 </yazgfs>
252]]>
253 </screen>
254<para>
255 There are three configured backend servers. The first two
256 servers, <literal>"server1"</literal> and <literal>"server2"</literal>,
257 can be reached by both listener addresses.
258 <literal>"server1"</literal> is reached by all (two) since no
259  <literal>listenref</literal> attribute is specified.
260 <literal>"server2"</literal> is reached by the two listeners specified.
261 In order to distinguish between the two, a virtual host has
262 been specified for each server in the <literal>host</literal>
263 elements.
264</para>
265<para>
266 For <literal>"server2"</literal> elements for CQL to RPN conversion
267 is supported and explain information has been added (a short one here
268 to keep the example small).
269</para>
270<para>
271 The third server, <literal>"server3"</literal> can only be reached
272 via listener <literal>"internal"</literal>.
273</para>
274
275<!-- Keep this comment at the end of the file
276Local variables:
277mode: nxml
278nxml-child-indent: 1
279End:
280-->
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