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<!-- $Id: frontend.xml,v 1.37 2008-01-17 21:44:18 adam Exp $ -->
 <chapter id="server"><title>Generic server</title>
  <sect1 id="server.introduction"><title>Introduction</title>
   
   <para>
    If you aren't into documentation, a good way to learn how the
    back end interface works is to look at the <filename>backend.h</filename>
    file. Then, look at the small dummy-server in
    <filename>ztest/ztest.c</filename>. The <filename>backend.h</filename>
    file also makes a good reference, once you've chewed your way through
    the prose of this file.
   </para>

   <para>
    If you have a database system that you would like to make available by
    means of Z39.50 or SRU, &yaz; basically offers your two options. You
    can use the APIs provided by the &asn;, &odr;, and &comstack;
    modules to
    create and decode PDUs, and exchange them with a client.
    Using this low-level interface gives you access to all fields and
    options of the protocol, and you can construct your server as close
    to your existing database as you like.
    It is also a fairly involved process, requiring
    you to set up an event-handling mechanism, protocol state machine,
    etc. To simplify server implementation, we have implemented a compact
    and simple, but reasonably full-functioned server-frontend that will
    handle most of the protocol mechanics, while leaving you to
    concentrate on your database interface.
   </para>

   <note>
    <para>
     The backend interface was designed in anticipation of a specific
     integration task, while still attempting to achieve some degree of
     generality. We realize fully that there are points where the
     interface can be improved significantly. If you have specific
     functions or parameters that you think could be useful, send us a
     mail (or better, sign on to the mailing list referred to in the
     top-level README file). We will try to fit good suggestions into future
     releases, to the extent that it can be done without requiring
     too many structural changes in existing applications.
    </para>
   </note>

   <note>
    <para>
     The &yaz; server does not support XCQL.
     </para>
   </note>
  </sect1>
  
  <sect1 id="server.frontend"><title>The Database Frontend</title>

   <para>
    We refer to this software as a generic database frontend. Your
    database system is the <emphasis>backend database</emphasis>, and the
    interface between the two is called the <emphasis>backend API</emphasis>.
    The backend API consists of a small number of function handlers and
    structure definitions. You are required to provide the
    <function>main()</function> routine for the server (which can be
    quite simple), as well as a set of handlers to match each of the
    prototypes.
    The interface functions that you write can use any mechanism you like
    to communicate with your database system: You might link the whole
    thing together with your database application and access it by
    function calls; you might use IPC to talk to a database server
    somewhere; or you might link with third-party software that handles
    the communication for you (like a commercial database client library).
    At any rate, the handlers will perform the tasks of:
   </para>

   <itemizedlist>

    <listitem><para>
      Initialization.
     </para></listitem>

    <listitem><para>
      Searching.
     </para></listitem>

    <listitem><para>
      Fetching records.
     </para></listitem>

    <listitem><para>
      Scanning the database index (optional - if you wish to implement SCAN).
     </para></listitem>

    <listitem><para>
      Extended Services (optional).
     </para></listitem>
    
    <listitem><para>
      Result-Set Delete (optional).
     </para></listitem>

    <listitem><para>
      Result-Set Sort (optional).
     </para></listitem>
    
    <listitem><para>
      Return Explain for SRU (optional).
     </para></listitem>
    
   </itemizedlist>

   <para>
    (more functions will be added in time to support as much of
    Z39.50-1995 as possible).
   </para>

  </sect1>
  <sect1 id="server.backend"><title>The Backend API</title>

   <para>
    The header file that you need to use the interface are in the
    <filename>include/yaz</filename> directory. It's called
    <filename>backend.h</filename>. It will include other files from
    the <filename>include/yaz</filename> directory, so you'll
    probably want to use the -I option of your compiler to tell it
    where to find the files. When you run
    <literal>make</literal> in the top-level &yaz; directory,
    everything you need to create your server is to link with the
    <filename>lib/libyaz.la</filename> library.
   </para>
  </sect1>

  <sect1 id="server.main"><title>Your main() Routine</title>

   <para>
    As mentioned, your <function>main()</function> routine can be quite brief.
    If you want to initialize global parameters, or read global configuration
    tables, this is the place to do it. At the end of the routine, you should
    call the function
   </para>

   <synopsis>
int statserv_main(int argc, char **argv,
                  bend_initresult *(*bend_init)(bend_initrequest *r),
                  void (*bend_close)(void *handle));
   </synopsis>

   <para>
    The third and fourth arguments are pointers to handlers. Handler
    <function>bend_init</function> is called whenever the server receives
    an Initialize Request, so it serves as a Z39.50 session initializer. The
    <function>bend_close</function> handler is called when the session is
    closed.
   </para>

   <para>
    <function>statserv_main</function> will establish listening sockets
    according to the parameters given. When connection requests are received,
    the event handler will typically <function>fork()</function> and
    create a sub-process to handle a new connection.
    Alternatively the server may be setup to create threads for each
    connection.
    If you do use global variables and forking, you should be aware, then,
    that these cannot be shared between associations, unless you explicitly
    disable forking by command line parameters. 
   </para>
   
   <para>
    The server provides a mechanism for controlling some of its behavior
    without using command-line options. The function
   </para>

   <synopsis>
    statserv_options_block *statserv_getcontrol(void);
   </synopsis>

   <para>
    will return a pointer to a <literal>struct statserv_options_block</literal>
    describing the current default settings of the server. The structure
    contains these elements:
    
    <variablelist>
     <varlistentry><term>
       <literal>int dynamic</literal></term><listitem><para>
        A boolean value, which determines whether the server
        will fork on each incoming request (TRUE), or not (FALSE). Default is
        TRUE. This flag is only read by UNIX-based servers (WIN32 based servers
        doesn't fork).
       </para></listitem></varlistentry>
     
     <varlistentry><term>
       <literal>int threads</literal></term><listitem><para>
        A boolean value, which determines whether the server
        will create a thread on each incoming request (TRUE), or not (FALSE).
        Default is FALSE. This flag is only read by UNIX-based servers
        that offer POSIX Threads support.
        WIN32-based servers always operate in threaded mode.
       </para></listitem></varlistentry>
     
     <varlistentry><term>
       <literal>int inetd</literal></term><listitem><para>
        A boolean value, which determines whether the server
        will operates under a UNIX INET daemon (inetd). Default is FALSE.
       </para></listitem></varlistentry>
     
     <varlistentry><term>
       <literal>char logfile[ODR_MAXNAME+1]</literal></term>
      <listitem><para>File for diagnostic output (&quot;&quot;: stderr).
       </para></listitem></varlistentry>
     
     <varlistentry><term>
       <literal>char apdufile[ODR_MAXNAME+1]</literal></term>
      <listitem><para>
        Name of file for logging incoming and outgoing APDUs
        (&quot;&quot;: don't log APDUs, &quot;-&quot;:
        <literal>stderr</literal>).
       </para></listitem></varlistentry>

     <varlistentry><term>
      <literal>char default_listen[1024]</literal></term>
      <listitem><para>Same form as the command-line specification of
        listener address. &quot;&quot;: no default listener address.
        Default is to listen at &quot;tcp:@:9999&quot;. You can only
        specify one default listener address in this fashion.
       </para></listitem></varlistentry>

     <varlistentry><term>
      <literal>enum oid_proto default_proto;</literal></term>
      <listitem><para>Either <literal>PROTO_Z3950</literal> or
        <literal>PROTO_SR</literal>.
        Default is <literal>PROTO_Z39_50</literal>.
       </para></listitem></varlistentry>
     
     <varlistentry><term>
       <literal>int idle_timeout;</literal></term>
      <listitem><para>Maximum session idle-time, in minutes. Zero indicates
        no (infinite) timeout. Default is 15 minutes.
       </para></listitem></varlistentry>
     
     <varlistentry><term>
       <literal>int maxrecordsize;</literal></term>
      <listitem><para>Maximum permissible record (message) size. Default
        is 1Mb. This amount of memory will only be allocated if a
        client requests a very large amount of records in one operation
        (or a big record).
        Set it to a lower number if you are worried about resource
        consumption on your host system.
       </para></listitem></varlistentry>

     <varlistentry><term>
       <literal>char configname[ODR_MAXNAME+1]</literal></term>
      <listitem><para>Passed to the backend when a new connection is received.
       </para></listitem></varlistentry>

     <varlistentry><term>
       <literal>char setuid[ODR_MAXNAME+1]</literal></term>
      <listitem><para>Set user id to the user specified, after binding
        the listener addresses.
       </para></listitem></varlistentry>
     
     <varlistentry><term>
       <literal>void (*bend_start)(struct statserv_options_block *p)</literal>
      </term>
      <listitem><para>Pointer to function which is called after the
        command line options have been parsed - but before the server
        starts listening.
        For forked UNIX servers this handler is called in the mother
        process; for threaded servers this handler is called in the
        main thread.
        The default value of this pointer is NULL in which case it
        isn't invoked by the frontend server.
        When the server operates as an NT service this handler is called
        whenever the service is started. 
       </para></listitem></varlistentry>
     
     <varlistentry><term>
       <literal>void (*bend_stop)(struct statserv_options_block *p)</literal>
      </term>
      <listitem><para>Pointer to function which is called whenever the server
        has stopped listening for incoming connections. This function pointer
        has a default value of NULL in which case it isn't called.
        When the server operates as an NT service this handler is called
        whenever the service is stopped.
       </para></listitem></varlistentry>

     <varlistentry><term>
       <literal>void *handle</literal></term>
      <listitem><para>User defined pointer (default value NULL).
        This is a per-server handle that can be used to specify "user-data".
        Do not confuse this with the session-handle as returned by bend_init.
       </para></listitem></varlistentry>

    </variablelist>
   </para>

   <para>
    The pointer returned by <literal>statserv_getcontrol</literal> points to
    a static area. You are allowed to change the contents of the structure,
    but the changes will not take effect before you call
   </para>
   
   <synopsis>
void statserv_setcontrol(statserv_options_block *block);
   </synopsis>

   <note>
    <para>
     that you should generally update this structure before calling
     <function>statserv_main()</function>.
    </para>
   </note>
  </sect1>

  <sect1 id="server.backendfunctions"><title>The Backend Functions</title>

   <para>
    For each service of the protocol, the backend interface declares one or
    two functions. You are required to provide implementations of the
    functions representing the services that you wish to implement.
   </para>

   <sect2 id="server.init"><title>Init</title>

    <synopsis>
bend_initresult (*bend_init)(bend_initrequest *r);
    </synopsis>

    <para>
     This handler is called once for each new connection request, after
     a new process/thread has been created, and an Initialize Request has
     been received from the client. The pointer to the
     <function>bend_init</function> handler is passed in the call to
     <function>statserv_start</function>.
    </para>

    <para>
     This handler is also called when operating in SRU mode - when
     a connection has been made (even though SRU does not offer
     this service).
    </para>

    <para>
     Unlike previous versions of YAZ, the <function>bend_init</function> also
     serves as a handler that defines the Z39.50 services that the backend
     wish to support. Pointers to <emphasis>all</emphasis> service handlers,
     including search - and fetch must be specified here in this handler.
    </para>
    <para>
     The request  - and result structures are defined as
    </para>

    <synopsis>
typedef struct bend_initrequest
{
    /** \brief user/name/password to be read */
    Z_IdAuthentication *auth; 
    /** \brief encoding stream (for results) */
    ODR stream;
    /** \brief printing stream */
    ODR print;
    /** \brief decoding stream (use stream for results) */
    ODR decode; 
    /** \brief reference ID */
    Z_ReferenceId *referenceId;
    /** \brief peer address of client */
    char *peer_name;           
    
    /** \brief character set and language negotiation 

    see include/yaz/z-charneg.h 
    */
    Z_CharSetandLanguageNegotiation *charneg_request;

    /** \brief character negotiation response */
    Z_External *charneg_response;

    /** \brief character set (encoding) for query terms 
        
    This is NULL by default. It should be set to the native character
    set that the backend assumes for query terms */
    char *query_charset;      

    /** \brief whehter query_charset also applies to recors 
    
    Is 0 (No) by default. Set to 1 (yes) if records is in the same
    character set as queries. If in doubt, use 0 (No).
    */
    int records_in_same_charset;

    char *implementation_id;
    char *implementation_name;
    char *implementation_version;

    /** \brief Z39.50 sort handler */
    int (*bend_sort)(void *handle, bend_sort_rr *rr);
    /** \brief SRU/Z39.50 search handler */
    int (*bend_search)(void *handle, bend_search_rr *rr);
    /** \brief SRU/Z39.50 fetch handler */
    int (*bend_fetch)(void *handle, bend_fetch_rr *rr);
    /** \brief SRU/Z39.50 present handler */
    int (*bend_present)(void *handle, bend_present_rr *rr);
    /** \brief Z39.50 extended services handler */
    int (*bend_esrequest) (void *handle, bend_esrequest_rr *rr);
    /** \brief Z39.50 delete result set handler */
    int (*bend_delete)(void *handle, bend_delete_rr *rr);
    /** \brief Z39.50 scan handler */
    int (*bend_scan)(void *handle, bend_scan_rr *rr);
    /** \brief Z39.50 segment facility handler */
    int (*bend_segment)(void *handle, bend_segment_rr *rr);
    /** \brief SRU explain handler */
    int (*bend_explain)(void *handle, bend_explain_rr *rr);
    /** \brief SRU scan handler */
    int (*bend_srw_scan)(void *handle, bend_scan_rr *rr);
    /** \brief SRU record update handler */
    int (*bend_srw_update)(void *handle, bend_update_rr *rr);

} bend_initrequest;

typedef struct bend_initresult
{
    int errcode;               /* 0==OK */
    char *errstring;           /* system error string or NULL */
    void *handle;              /* private handle to the backend module */
} bend_initresult;
    </synopsis>

    <para>
     In general, the server frontend expects that the
     <literal>bend_*result</literal> pointer that you return is valid at
     least until the next call to a <literal>bend_* function</literal>.
     This applies to all of the functions described herein. The parameter
     structure passed to you in the call belongs to the server frontend, and
     you should not make assumptions about its contents after the current
     function call has completed. In other words, if you want to retain any
     of the contents of a request structure, you should copy them.
    </para>

    <para>
     The <literal>errcode</literal> should be zero if the initialization of
     the backend went well. Any other value will be interpreted as an error.
     The <literal>errstring</literal> isn't used in the current version, but
     one option would be to stick it in the initResponse as a VisibleString.
     The <literal>handle</literal> is the most important parameter. It should
     be set to some value that uniquely identifies the current session to
     the backend implementation. It is used by the frontend server in any
     future calls to a backend function.
     The typical use is to set it to point to a dynamically allocated state
     structure that is private to your backend module.
    </para>

    <para>
     The <literal>auth</literal> member holds the authentication information
     part of the Z39.50 Initialize Request. Interpret this if your serves
     requires authentication. 
    </para>

    <para>
     The members <literal>peer_name</literal>,
     <literal>implementation_id</literal>,
     <literal>implementation_name</literal> and
     <literal>implementation_version</literal> holds
     DNS of client, ID of implementor, name
     of client (Z39.50) implementation - and version.
    </para>

    <para>
     The <literal>bend_</literal> - members are set to NULL when
     <function>bend_init</function> is called. Modify the pointers by
     setting them to point to backend functions.
    </para>

   </sect2>

   <sect2 id="server.search.retrieve"><title>Search and Retrieve</title>

    <para>We now describe the handlers that are required to support search -
     and retrieve. You must support two functions - one for search - and one
     for fetch (retrieval of one record). If desirable you can provide a
     third handler which is called when a present request is received which
     allows you to optimize retrieval of multiple-records.
    </para>

    <synopsis>
int (*bend_search) (void *handle, bend_search_rr *rr);

typedef struct {
    char *setname;             /* name to give to this set */
    int replace_set;           /* replace set, if it already exists */
    int num_bases;             /* number of databases in list */
    char **basenames;          /* databases to search */
    Z_ReferenceId *referenceId;/* reference ID */
    Z_Query *query;            /* query structure */
    ODR stream;                /* encode stream */
    ODR decode;                /* decode stream */
    ODR print;                 /* print stream */

    bend_request request;
    bend_association association;
    int *fd;
    int hits;                  /* number of hits */
    int errcode;               /* 0==OK */
    char *errstring;           /* system error string or NULL */
    Z_OtherInformation *search_info; /* additional search info */
    char *srw_sortKeys;        /* holds SRU/SRW sortKeys info */
    char *srw_setname;         /* holds SRU/SRW generated resultsetID */
    int *srw_setnameIdleTime;  /* holds SRU/SRW life-time */
    int estimated_hit_count;   /* if hit count is estimated */
    int partial_resultset;     /* if result set is partial */
} bend_search_rr;
    </synopsis>

    <para>
     The <function>bend_search</function> handler is a fairly close
     approximation of a protocol Z39.50 Search Request - and Response PDUs
     The <literal>setname</literal> is the resultSetName from the protocol.
     You are required to establish a mapping between the set name and whatever
     your backend database likes to use.
     Similarly, the <literal>replace_set</literal> is a boolean value
     corresponding to the resultSetIndicator field in the protocol.
     <literal>num_bases/basenames</literal> is a length of/array of character
     pointers to the database names provided by the client.
     The <literal>query</literal> is the full query structure as defined in
     the protocol ASN.1 specification.
     It can be either of the possible query types, and it's up to you to
     determine if you can handle the provided query type.
     Rather than reproduce the C interface here, we'll refer you to the
     structure definitions in the file
     <filename>include/yaz/z-core.h</filename>. If you want to look at the
     attributeSetId OID of the RPN query, you can either match it against
     your own internal tables, or you can use the <link linkend="tools.oid">
     OID tools</link>.
    </para>

    <para>
     The structure contains a number of hits, and an
     <literal>errcode/errstring</literal> pair. If an error occurs
     during the search, or if you're unhappy with the request, you should
     set the errcode to a value from the BIB-1 diagnostic set. The value
     will then be returned to the user in a nonsurrogate diagnostic record
     in the response. The <literal>errstring</literal>, if provided, will
     go in the addinfo field. Look at the protocol definition for the
     defined error codes, and the suggested uses of the addinfo field.
    </para>

    <para>
     The <function>bend_search</function> handler is also called when
     the frontend server receives a SRU SearchRetrieveRequest.
     For SRU, a CQL query is usually provided by the client.
     The CQL query is available as part of <literal>Z_Query</literal>
     structure (note that CQL is now part of Z39.50 via an external).
     To support CQL in existing implementations that only do Type-1,
     we refer to the CQL-to-PQF tool described
     <link linkend="cql.to.pqf">here</link>.
    </para>

    <para>
     To maintain backwards compatibility, the frontend server
     of yaz always assume that error codes are BIB-1 diagnostics.
     For SRU operation, a Bib-1 diagnostic code is mapped to
     SRU diagnostic.
    </para>
    
    <synopsis>
int (*bend_fetch) (void *handle, bend_fetch_rr *rr);

typedef struct bend_fetch_rr {
    char *setname;             /* set name */
    int number;                /* record number */
    Z_ReferenceId *referenceId;/* reference ID */
    Odr_oid *request_format;        /* format, transfer syntax (OID) */
    Z_RecordComposition *comp; /* Formatting instructions */
    ODR stream;                /* encoding stream - memory source if req */
    ODR print;                 /* printing stream */

    char *basename;            /* name of database that provided record */
    int len;                   /* length of record or -1 if structured */
    char *record;              /* record */
    int last_in_set;           /* is it?  */
    Odr_oid *output_format;        /* response format/syntax (OID) */
    int errcode;               /* 0==success */
    char *errstring;           /* system error string or NULL */
    int surrogate_flag;        /* surrogate diagnostic */
    char *schema;              /* string record schema input/output */
} bend_fetch_rr;
    </synopsis>

    <para>
     The frontend server calls the <function>bend_fetch</function> handler
     when it needs database records to fulfill a Z39.50 Search Request, a
     Z39.50 Present Request or a SRU SearchRetrieveRequest.
     The <literal>setname</literal> is simply the name of the result set
     that holds the reference to the desired record.
     The <literal>number</literal> is the offset into the set (with 1
     being the first record in the set). The <literal>format</literal> field
     is the record format requested by the client (See
     <xref linkend="tools.oid"/>).
     A value of NULL for <literal>format</literal> indicates that the
     client did not request a specific format.
     The <literal>stream</literal> argument is an &odr; stream which
     should be used for allocating space for structured data records.
     The stream will be reset when all records have been assembled, and
     the response package has been transmitted.
     For unstructured data, the backend is responsible for maintaining a
     static or dynamic buffer for the record between calls.
    </para>

    <para>
     If a SRU SearchRetrieveRequest is received by the frontend server,
     the <literal>referenceId</literal> is NULL and the
     <literal>format</literal> (transfer syntax) is the OID for XML.
     The schema for SRU is stored in both the
     <literal>Z_RecordComposition</literal>
     structure and <literal>schema</literal> (simple string).
    </para>

    <para>
     In the structure, the <literal>basename</literal> is the name of the
     database that holds the
     record. <literal>len</literal> is the length of the record returned, in
     bytes, and <literal>record</literal> is a pointer to the record.
     <literal>last_in_set</literal> should be nonzero only if the record
     returned is the last one in the given result set.
     <literal>errcode</literal> and <literal>errstring</literal>, if
     given, will be interpreted as a global error pertaining to the
     set, and will be returned in a non-surrogate-diagnostic.
     If you wish to return the error as a surrogate-diagnostic
     (local error) you can do this by setting
     <literal>surrogate_flag</literal> to 1 also.
    </para>

    <para>
     If the <literal>len</literal> field has the value -1, then
     <literal>record</literal> is assumed to point to a constructed data
     type. The <literal>format</literal> field will be used to determine
     which encoder should be used to serialize the data.
    </para>

    <note>
     <para>
      If your backend generates structured records, it should use
      <function>odr_malloc()</function> on the provided stream for allocating
      data: This allows the frontend server to keep track of the record sizes.
     </para>
    </note>

    <para>
     The <literal>format</literal> field is mapped to an object identifier
     in the direct reference of the resulting EXTERNAL representation
     of the record.
    </para>

    <note>
     <para>
      The current version of &yaz; only supports the direct reference mode.
     </para>
    </note>

    <synopsis>
int (*bend_present) (void *handle, bend_present_rr *rr);

typedef struct {
    char *setname;             /* set name */
    int start;
    int number;                /* record number */
    Odr_oid *format;           /* format, transfer syntax (OID) */
    Z_ReferenceId *referenceId;/* reference ID */
    Z_RecordComposition *comp; /* Formatting instructions */
    ODR stream;                /* encoding stream - memory source if required */
    ODR print;                 /* printing stream */
    bend_request request;
    bend_association association;

    int hits;                  /* number of hits */
    int errcode;               /* 0==OK */
    char *errstring;           /* system error string or NULL */
} bend_present_rr;
    </synopsis>

    <para>
     The <function>bend_present</function> handler is called when
     the server receives a Z39.50 Present Request.
     The <literal>setname</literal>,
     <literal>start</literal> and <literal>number</literal> is the
     name of the result set - start position - and number of records to
     be retrieved respectively. <literal>format</literal> and
     <literal>comp</literal> is the preferred transfer syntax and element
     specifications of the present request.
    </para>
    <para>
     Note that this is handler serves as a supplement for
     <function>bend_fetch</function> and need not to be defined in order to
     support search - and retrieve. 
    </para>

   </sect2>

   <sect2 id="server.delete"><title>Delete</title>

    <para>
     For back-ends that supports delete of a result set only one handler
     must be defined.
    </para>

    <synopsis>
int (*bend_delete)(void *handle, bend_delete_rr *rr);

typedef struct bend_delete_rr {
    int function;
    int num_setnames;
    char **setnames;
    Z_ReferenceId *referenceId;
    int delete_status;      /* status for the whole operation */
    int *statuses;          /* status each set - indexed as setnames */
    ODR stream;
    ODR print; 
} bend_delete_rr;
    </synopsis>

    <note>
     <para>
      The delete set function definition is rather primitive, mostly because
      we have had no practical need for it as of yet. If someone wants
      to provide a full delete service, we'd be happy to add the
      extra parameters that are required. Are there clients out there
      that will actually delete sets they no longer need?
     </para>
    </note>

   </sect2>

   <sect2 id="server.scan"><title>Scan</title>

    <para>
     For servers that wish to offer the scan service one handler
     must be defined.
    </para>

    <synopsis>
int (*bend_scan)(void *handle, bend_scan_rr *rr);

typedef enum {
    BEND_SCAN_SUCCESS,  /* ok */
    BEND_SCAN_PARTIAL   /* not all entries could be found */
} bend_scan_status;

typedef struct bend_scan_rr {
    int num_bases;      /* number of elements in databaselist */
    char **basenames;   /* databases to search */
    Odr_oid *attributeset;
    Z_ReferenceId *referenceId; /* reference ID */
    Z_AttributesPlusTerm *term;
    ODR stream;         /* encoding stream - memory source if required */
    ODR print;          /* printing stream */

    int *step_size;     /* step size */
    int term_position;  /* desired index of term in result list/returned */
    int num_entries;    /* number of entries requested/returned */

    /* scan term entries. The called handler does not have
       to allocate this. Size of entries is num_entries (see above) */
    struct scan_entry *entries;
    bend_scan_status status;
    int errcode;
    char *errstring;
    char *scanClause;   /* CQL scan clause */
    char *setname;      /* Scan in result set (NULL if omitted) */
} bend_scan_rr;
    </synopsis>
   <para>
    This backend server handles both Z39.50 scan 
    and SRU scan. In order for a handler to distinguish between SRU (CQL) scan 
    Z39.50 Scan , it must check for a non-NULL value of 
    <literal>scanClause</literal>.
   </para>
   <note>
    <para>
     if designed today, it would be a choice using a union or similar,
     but that would break binary compatibility with existing servers.
    </para>
    </note>
   </sect2>
  </sect1>

  <sect1 id="server.invocation"><title>Application Invocation</title>

   <para>
    The finished application has the following
    invocation syntax (by way of <function>statserv_main()</function>):
   </para>

   &gfs-synopsis;
   
   <para>
    The options are:

    &gfs-options;

   </para>
   
   <para>
    A listener specification consists of a transport mode followed by a
    colon (:) followed by a listener address. The transport mode is
    either <literal>tcp</literal>, <literal>unix:</literal> or
    <literal>ssl</literal>.
   </para>
   
   <para>
    For TCP and SSL, an address has the form
   </para>

   <synopsis>
    hostname | IP-number [: portnumber]
   </synopsis>
   
   <para>
    The port number defaults to 210 (standard Z39.50 port).
   </para>

   <para>
    For UNIX, the address is the filename of socket.
   </para>

   <para>
    For TCP/IP and SSL, the special hostname <literal>@</literal> 
    (at sign) is mapped to the address <literal>INADDR_ANY</literal>,
    which causes the server to listen on any local interface. 
   </para>

   <example id="server.example.running.unix"><title>Running the GFS on Unix</title>
    <para>
     Assuming the server application <replaceable>appname</replaceable> is
     started as root, the following will make it listen on port 210.
     The server will change identity to <literal>nobody</literal>
     and write its log to <filename>/var/log/app.log</filename>.
     <screen>
      <replaceable>appname</replaceable> -l /var/log/app.log -u nobody tcp:@:210
     </screen>
    </para>
    <para>
     The server will accept Z39.50 requests and offer SRU service on port 210.
    </para>
   </example>
   <example id="server.example.apache.sru"><title>Setting up Apache as SRU Frontend</title>
    <para>
     If you use <ulink url="&url.apache;">Apache</ulink>
     as your public web server and want to offer HTTP port 80
     access to the YAZ server on 210, you can use the
     <ulink url="&url.apache.directive.proxypass;">
      <literal>ProxyPass</literal></ulink> 
     directive.
     If you have virtual host
     <literal>srw.mydomain</literal> you can use the following directives
     in Apache's httpd.conf:
     <screen>
      &lt;VirtualHost *>
       ErrorLog /home/srw/logs/error_log
       TransferLog /home/srw/logs/access_log
       ProxyPass / http://srw.mydomain:210/
      &lt;/VirtualHost>
     </screen>
    </para>
    <para>
     The above for the Apache 1.3 series.
    </para>
   </example>
   <example id="server.example.local.access">
    <title>Running a server with local access only</title>
    <para>
     Servers that is only being accessed from the local host should listen
     on UNIX file socket rather than a Internet socket. To listen on
     <filename>/tmp/mysocket</filename> start the server as follows:
     <screen>
      <replaceable>appname</replaceable> tcp:/tmp/mysocket
     </screen>
    </para>
   </example>
  </sect1>
  <sect1 id="server.vhosts"><title>GFS Configuration and Virtual Hosts</title>
   &gfs-virtual;
  </sect1>
 </chapter>
 
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