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. 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. 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 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. 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 &reference-tools-cql-map; If given, the backend server will only "see" a Type-1/RPN query. element ccl2rpn (optional) 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. element stylesheet (optional) Specifies the stylesheet reference to be part of SRU HTTP responses when the client does not specify one. If neither this is given, nor the client specifies one, no stylesheet reference is part of the SRU HTTP response. element docpath (optional) Specifies a path for local file access using HTTP. All URLs with a leading prefix (/ exluded) that matches the value of docpath are used for file access. For example, if the server is to offer access in directory xsl, the docpath would be xsl and all URLs of the form http://host/exl will result in a local file access. 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. element maximumrecordsize (optional) Specifies maximum record size/message size, in bytes. This value also servers as maximum size of incoming packages (for Record Updates etc). It's the same value as that given by the -k option. element retrievalinfo (optional) Enables the retrieval facility supporting conversions and specifications of record formats/types. See for more information. 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. tcp:@:9900 unix:/var/tmp/socket server1.mydomain /var/www/s1 config.cfg server2.mydomain /var/www/s2 config.cfg ../etc/pqf.properties server2.mydomain 9900 a /var/www/s3 config.cfg ]]> There are three configured backend servers. The first two servers, "server1" and "server2", can be reached by both listener addresses. "server1" is reached by all (two) since no listenref attribute is specified. "server2" is reached by the two listeners 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".