YAZ User's Guide and Reference
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# Virtual Hosts

The Virtual hosts mechanism allows a YAZ frontend server to support multiple backends. A backend is selected on the basis of the TCP/IP binding (port+listening adddress) and/or the virtual host.

A backend can be configured to execute in a particular working directory. Or the YAZ frontend may perform CQL to RPN conversion, thus allowing traditional Z39.50 backends to be offered as a SRW/SRU service. SRW/SRU Explain information for a particular backend may also be specified.

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.

> Note: Not all Z39.50 clients allows the VHOST information to be set. For those the selection of the backend must rely on the TCP/IP information alone (port and address).

The YAZ frontend server uses XML to describe the backend configurations. Command-line option `-f` specifies filename of the XML configuration.

The configuration uses the root element `yazgfs`. This element includes a list of `listen` elements, followed by one or more `server` elements.

The `listen` describes listener (transport end point), such as TCP/IP, Unix file socket or SSL server. Content for a listener:

CDATA (required)

  The CDATA for the `listen` element holds the listener string, such as `tcp:@:210`, `tcp:server1:2100`, etc.

attribute `id` (optional)

  identifier for this listener. This may be referred to from server sections.

> Note: We expect more information to be added for the listen section in a future version, such as CERT file for SSL servers.

The `server` describes a server and the parameters for this server type. Content for a server:

attribute `id` (optional)

  Identifier for this server. Currently not used for anything, but it might be for logging purposes.

attribute `listenref` (optional)

  Specifies listener for this server. If this attribute is not given, the server is accessible from all listener. In order for the server to be used for real, howeever, the virtual host must match (if specified in the configuration).

element `config` (optional)

  Specifies the server configuration. This is equivalent to the config specified using command line option `-c`.

element `directory` (optional)

  Specifies a working directory for this backend server. If specifid, the YAZ fronend 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).

element `host` (optional)

  Specifies the virtual host for this server. If this is specified a client must specify this host string in order to use this backend.

element `cql2rpn` (optional)

  Specifies a filename that includes CQL to RPN conversion for this backend server. See the Section called Specification of CQL to RPN mappings in Chapter 8 If given, the backend server will only "see" a Type-1/RPN query.

element `explain` (optional)

  Specifies SRW/SRU ZeeRex content for this server. Copied verbatim to the client. As things are now, some of the Explain content seeem redundant because host information, etc. is also stored elsewhere.

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 `public` and the other server `internal`.

      <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">
        <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>

There are three configured backend servers. The first two servers, `"server1"` and `"server2"`, can be reached by both listener addresses - since no `listenref` attribute is specified. In order to distinguish between the two a virtual host has been specified for each of server in the `host` elements.

For `"server2"` elements for CQL to RPN conversion is supported and explain information has been added (a short one here to keep the example small).

The third server, `"server3"` can only be reached via listener `"internal"`.

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