plucky (3) Net::Z3950::SimpleServer.3pm.gz

Provided by: libnet-z3950-simpleserver-perl_1.29-1build1_amd64 bug

NAME

       Net::Z3950::SimpleServer - Simple Perl API for building Z39.50 servers.

SYNOPSIS

         use Net::Z3950::SimpleServer;

         sub my_search_handler {
               my $args = shift;

               my $set_id = $args->{SETNAME};
               my @database_list = @{ $args->{DATABASES} };
               my $query = $args->{QUERY};

               ## Perform the query on the specified set of databases
               ## and return the number of hits:

               $args->{HITS} = $hits;
         }

         sub my_fetch_handler {        # Get a record for the user
               my $args = shift;

               my $set_id = $args->{SETNAME};

               my $record = fetch_a_record($args->{OFFSET});

               $args->{RECORD} = $record;
               if (number_of_hits() == $args->{OFFSET}) {      ## Last record in set?
                       $args->{LAST} = 1;
               } else {
                       $args->{LAST} = 0;
               }
         }

         ## Register custom event handlers:
         my $z = new Net::Z3950::SimpleServer(GHANDLE = $someObject,
                                              INIT   =>  "main::my_init_handler",
                                              CLOSE  =>  "main::my_close_handler",
                                              SEARCH =>  "main::my_search_handler",
                                              FETCH  =>  "main::my_fetch_handler");

         ## Launch server:
         $z->launch_server("ztest.pl", @ARGV);

DESCRIPTION

       The SimpleServer module is a tool for constructing Z39.50 "Information Retrieval" servers in Perl. The
       module is easy to use, but it does help to have an understanding of the Z39.50 query structure and the
       construction of structured retrieval records.

       Z39.50 is a network protocol for searching remote databases and retrieving the results in the form of
       structured "records". It is widely used in libraries around the world, as well as in the US Federal
       Government.  In addition, it is generally useful whenever you wish to integrate a number of different
       database systems around a shared, abstract data model.

       The model of the module is simple: It implements a "generic" Z39.50 server, which invokes callback
       functions supplied by you to search for content in your database. You can use any tools available in Perl
       to supply the content, including modules like DBI and WWW::Search.

       The server will take care of managing the network connections for you, and it will spawn a new process
       (or thread, in some environments) whenever a new connection is received.

       The programmer can specify subroutines to take care of the following type of events:

         - Start service (called once).
         - Initialize request
         - Search request
         - Present request
         - Fetching of records
         - Scan request (browsing)
         - Closing down connection

       Note that only the Search and Fetch handler functions are required.  The module can supply default
       responses to the other on its own.

       After the launching of the server, all control is given away from the Perl script to the server. The
       server calls the registered subroutines to field incoming requests from Z39.50 clients.

       A reference to an anonymous hash is passed to each handler. Some of the entries of these hashes are to be
       considered input and others output parameters.

       The Perl programmer specifies the event handlers for the server by means of the SimpleServer object
       constructor

         my $z = new Net::Z3950::SimpleServer(
                               START   =>      "main::my_start_handler",
                               INIT    =>      "main::my_init_handler",
                               CLOSE   =>      "main::my_close_handler",
                               SEARCH  =>      "main::my_search_handler",
                               PRESENT =>      "main::my_present_handler",
                               SCAN    =>      "main::my_scan_handler",
                               FETCH   =>      "main::my_fetch_handler",
                               EXPLAIN =>      "main::my_explain_handler",
                               DELETE  =>      "main::my_delete_handler",
                               ESREQUEST =>    "main::my_esrequest_handler",
                               SORT    =>      "main::my_sort_handler");

       In addition, the arguments to the constructor may include GHANDLE, a global handle which is made
       available to each invocation of every callback function.  This is typically a reference to either a hash
       or an object. (replace main with your package, if not main).

       After the custom event handlers are declared, the server is launched by means of the method

         $z->launch_server("MyServer.pl", @ARGV);

       Notice, the first argument should be the name of your server script (for logging purposes), while the
       rest of the arguments are documented in the YAZ toolkit manual: The section on application invocation:
       <http://www.indexdata.com/yaz/doc/server.invocation.html>

       In particular, you need to use the -T switch to start your SimpleServer in threaded mode.

   Start handler
       The start handler is called when service is started. The argument hash passed to the start handler has
       the form

         $args = {
                    CONFIG =>  "default-config" ## GFS config (as given by -c)
                 };

       The purpose of the start handler is to read the configuration file for the Generic Frontend Server . This
       is specified by option -c.  If -c is omitted, the configuration file is set to "default-config".

       The start handler is optional. It is supported in Simpleserver 1.16 and later.

   Init handler
       The init handler is called whenever a Z39.50 client is attempting to logon to the server. The exchange of
       parameters between the server and the handler is carried out via an anonymous hash reached by a
       reference, i.e.

         $args = shift;

       The argument hash passed to the init handler has the form

         $args = {
                                           ## Response parameters:

                    PEER_NAME =>  "",      ## Name or IP address of connecting client
                    IMP_ID    =>  "",      ## Z39.50 Implementation ID
                    IMP_NAME  =>  "",      ## Z39.50 Implementation name
                    IMP_VER   =>  "",      ## Z39.50 Implementation version
                    ERR_CODE  =>  0,       ## Error code, cnf. Z39.50 manual
                    ERR_STR   =>  "",      ## Error string (additional info.)
                    USER      =>  "xxx"    ## If Z39.50 authentication is used,
                                           ## this member contains user name
                    PASS      =>  "yyy"    ## Under same conditions, this member
                                           ## contains the password in clear text
                    GROUP     =>  "zzz"    ## Under same conditions, this member
                                           ## contains the group in clear text
                    GHANDLE   =>  $obj     ## Global handle specified at creation
                    HANDLE    =>  undef    ## Handler of Perl data structure
                 };

       The HANDLE member can be used to store any scalar value which will then be provided as input to all
       subsequent calls (i.e. for searching, record retrieval, etc.). A common use of the handle is to store a
       reference to a hash which may then be used to store session-specific parameters.  If you have any
       session-specific information (such as a list of result sets or a handle to a back-end search engine of
       some sort), it is always best to store them in a private session structure - rather than leaving them in
       global variables in your script.

       The Implementation ID, name and version are only really used by Z39.50 client developers to see what kind
       of server they're dealing with.  Filling these in is optional.

       The ERR_CODE should be left at 0 (the default value) if you wish to accept the connection. Any other
       value is interpreted as a failure and the client will be shown the door, with the code and the associated
       additional information, ERR_STR returned.

   Search handler
       Similarly, the search handler is called with a reference to an anonymous hash. The structure is the
       following:

         $args = {
                                            ## Request parameters:

           GHANDLE             =>  $obj     # Global handle specified at creation
           HANDLE              =>  ref,     # Your session reference.
           SETNAME             =>  "id",    # ID of the result set
           REPL_SET            =>  0,       # Replace set if already existing?
           DATABASES           =>  ["xxx"], # Reference to a list of databases to search
           QUERY               =>  "query", # The query expression as a PQF string
           RPN                 =>  $obj,    # Reference to a Net::Z3950::APDU::Query
           CQL                 =>  $x,      # A CQL query, if this is provided instead of Type-1
           SRW_SORTKEYS        =>  $x,      # XXX to be described
           PID                 =>  $x,      # XXX to be described
           PRESENT_NUMBER      =>  $x,      # XXX to be described
           EXTRA_ARGS          =>  $x,      # XXX to be described
           INPUTFACETS         =>  $x,      # Specification of facets required: see below.

                                            ## Response parameters:

           ERR_CODE            =>  0,       # Error code (0=Successful search)
           ERR_STR             =>  "",      # Error string
           HITS                =>  0,       # Number of matches
           ESTIMATED_HIT_COUNT =>  $x,      # XXX to be described
           EXTRA_RESPONSE_DATA =>  $x,      # XXX to be described
           OUTPUTFACETS        =>  $x       # Facets returned: see below.
         };

       Note that a search which finds 0 hits is considered successful in Z39.50 terms - you should only set the
       ERR_CODE to a non-zero value if there was a problem processing the request. The Z39.50 standard provides
       a comprehensive list of standard diagnostic codes, and you should use these whenever possible.

       Query structures

       In Z39.50, the most comment kind of query is the so-called Type-1 _query, a tree-structure of terms
       combined by operators, the terms being qualified by lists of attributes.

       The QUERY parameter presented this tree to the search function in the Prefix Query Format (PQF) which is
       used in many applications based on the YAZ toolkit. The full grammar is described in the YAZ manual.

       The following are all examples of valid queries in the PQF.

               dylan

               "bob dylan"

               @or "dylan" "zimmerman"

               @set Result-1

               @or @and bob dylan @set Result-1

               @and @attr 1=1 "bob dylan" @attr 1=4 "slow train coming"

               @attrset @attr 4=1 @attr 1=4 "self portrait"

       You will need to write a recursive function or something similar to parse incoming query expressions, and
       this is usually where a lot of the work in writing a database-backend happens. Fortunately, you don't
       need to support any more functionality than you want to. For instance, it is perfectly legal to not
       accept boolean operators, but you should try to return good error codes if you run into something you
       can't or won't support.

       A more convenient alternative to the QUERY member is the RPN member, which is a reference to a
       Net::Z3950::APDU::Query object representing the RPN query tree.  The structure of that object is supposed
       to be self-documenting, but here's a brief summary of what you get:

       •   "Net::Z3950::APDU::Query" is a hash with two fields:

           "attributeSet"
               Optional.  If present, it is a reference to a "Net::Z3950::APDU::OID".  This is a string of dot-
               separated integers representing the OID of the query's top-level attribute set.

           "query"
               Mandatory: a reference to the RPN tree itself.

       •   Each node of the tree is an object of one of the following types:

           "Net::Z3950::RPN::And"
           "Net::Z3950::RPN::Or"
           "Net::Z3950::RPN::AndNot"
               These three classes are all arrays of two elements, each of which is a node.

           "Net::Z3950::RPN::Term"
               A query term. See below for details.

           "Net::Z3950::RPN::RSID"
               A reference to a result-set ID indicating a previous search.  The ID of the result-set is in the
               "id" element.

       •   "Net::Z3950::RPN::Term" is a hash with two fields:

           "term"
               A string containing the search term itself.

           "attributes"
               A reference to a "Net::Z3950::RPN::Attributes" object.

       •   "Net::Z3950::RPN::Attributes" is an array of references to "Net::Z3950::RPN::Attribute" objects.
           (Note the plural/singular distinction.)

       •   "Net::Z3950::RPN::Attribute" is a hash with three elements:

           "attributeSet"
               Optional.  If present, it is dot-separated OID string, as above.

           "attributeType"
               An integer indicating the type of the attribute - for example, under the BIB-1 attribute set,
               type 1 indicates a ``use'' attribute, type 2 a ``relation'' attribute, etc.

           "attributeValue"
               An integer or string indicating the value of the attribute - for example, under BIB-1, if the
               attribute type is 1, then value 4 indicates a title search and 7 indicates an ISBN search; but if
               the attribute type is 2, then value 4 indicates a ``greater than or equal'' search, and 102
               indicates a relevance match.

       All of these classes except "Attributes" and "Attribute" are subclasses of the abstract class
       "Net::Z3950::RPN::Node".  That class has a single method, toPQF(), which may be used to turn an RPN tree,
       or part of one, back into a textual prefix query.

       Note that, apart to toPQF(), none of these classes have any methods at all: the blessing into classes is
       largely just a documentation thing so that, for example, if you do

               { use Data::Dumper; print Dumper($args->{RPN}) }

       you get something fairly human-readable.  But of course, the type distinction between the three different
       kinds of boolean node is important.

       By adding your own methods to these classes (building what I call ``augmented classes''), you can easily
       build code that walks the tree of the incoming RPN.  Take a look at "samples/render-search.pl" for a
       sample implementation of such an augmented classes technique.

       Finally, when SimpleServer is invoked using SRU/SRW (and indeed using Z39.50 if the unusual type-104
       query is used), the query that is _passed is expressed in CQL, the Contextual Query Language. In this
       case, the query string is made available in the CQL argument.

       Facets

       Servers may support the provision of facets -- counted lists of field values which may subsequently be be
       used as query terms to narrow the search.

       In SRU, facets may be requested by the "facetLimit" parameter, as documented in the OASIS standard that
       formalises the SRU specification <http://docs.oasis-open.org/search-
       ws/searchRetrieve/v1.0/os/part3-sru2.0/searchRetrieve-v1.0-os-part3-sru2.0.html#_Toc324162453>.  Its
       value is a string consisting of a comma-separated list of facet specifications. Each facet specification
       consists of of a count, a colon and a fieldname. For example, "facetLimit=10:title,5:author" asks for ten
       title facets and five author facets.

       Request format

       The facet request is passed to the search-handler function in the INPUTFACETS parameter. Its value is
       rather complex, due to backwards compatibility with Z39.50:

       •   The top-level value is a "Net::Z3950::FacetList" array.

       •   This is an array of "Net::Z3950::FacetField" objects.

       •   Each of these is an object with two members, "attributes" and "terms".

       •   "attributes" has type "Net::Z3950::RPN::Attributes" and is a list of objects of type
           "Net::Z3950::RPN::Attribute".

       •   Each attribute has two elements, "attributeType" and "attributeValue". Each value is interpreted
           according to its type. The meanings of the types are as follows:

           1.  The name of the field to provide values of the facets.

           2.  The order in which to sort the values. (But it's not clear how this is to be interpreted: it may
               be implementation dependent.)

           3.  The number of facets to include for the specified field.

           4.  The first facet to include in the response: for example, if this is 11, then the first ten facts
               should be skipped.

       So for example, the SRU facet specification "facetLimit=10:title,5:author" would be translated as a
       "Net::Z3950::FacetList" list of two "Net::Z3950::FacetField"s. The "attributes" of the first would be
       [1="title", 3=10], and those of the second would be [1="author", 3=5].

       It is not clear what the purpose of "terms" is, but for the record, this is how it is represented:

       •   "terms" is a "Net::Z3950::FacetTerms" array.

       •   This is an array of "Net::Z3950::FacetTerm" objects.

       •   Each of these is an object with two members, "term" and "count". The first of these is an integer,
           the second a string.

       Response format

       Having generated facets corresponding to the request, the search handler should return them in the
       "OUTPUTFACETS" argument. The structure of this response is similar to that of the request:

       •   The top-level value is a "Net::Z3950::FacetList" array.

       •   This is an array of "Net::Z3950::FacetField" objects.

       •   Each of these is an object with two members, "attributes" and "terms".

       •   "attributes" has type "Net::Z3950::RPN::Attributes" and is a list of objects of type
           "Net::Z3950::RPN::Attribute".

       •   Each attribute has two elements, "attributeType" and "attributeValue". Each value is interpreted
           according to its type. The meanings of the types are as follows:

           1.  The name of the field for which terms are provided.

           (That is the only type used.)

       •   "terms" is a "Net::Z3950::FacetTerms" array.

       •   This is an array of "Net::Z3950::FacetTerm" objects.

       •   Each of these is an object with two members, "term" and "count". The first of these is a string
           containing one of the facet terms, and the second is an integer indicating how many times it occurs
           in the records that were found by the search.

       The example SimpleServer applicaation server "ztest.pl" includes code that shows how to examine the
       INPUTFACETS data structure and create the OUTPUTFACETS structure.

   Present handler
       The presence of a present handler in a SimpleServer front-end is optional.  Each time a client wishes to
       retrieve records, the present service is called. The present service allows the origin to request a
       certain number of records retrieved from a given result set.  When the present handler is called, the
       front-end server should prepare a result set for fetching. In practice, this means to get access to the
       data from the backend database and store the data in a temporary fashion for fast and efficient fetching.
       The present handler does *not* fetch anything. This task is taken care of by the fetch handler, which
       will be called the correct number of times by the YAZ library. More about this below.  If no present
       handler is implemented in the front-end, the YAZ toolkit will take care of a minimum of preparations
       itself. This default present handler is sufficient in many situations, where only a small amount of
       records are expected to be retrieved. If on the other hand, large result sets are likely to occur, the
       implementation of a reasonable present handler can gain performance significantly.

       The information exchanged between client and present handle is:

         $args = {
                                           ## Client/server request:

                    GHANDLE   =>  $obj     ## Global handle specified at creation
                    HANDLE    =>  ref,     ## Reference to datastructure
                    SETNAME   =>  "id",    ## Result set ID
                    START     =>  xxx,     ## Start position
                    COMP      =>  "",      ## Desired record composition
                    SCHEMA_OID => "",      ## Z39.50 schema (OID), if any
                    NUMBER    =>  yyy,     ## Number of requested records

                                           ## Response parameters:

                    HITS      =>  zzz,     ## Number of returned records
                    ERR_CODE  =>  0,       ## Error code
                    ERR_STR   =>  ""       ## Error message
                 };

   Fetch handler
       The fetch handler is asked to retrieve a SINGLE record from a given result set (the front-end server will
       automatically call the fetch handler as many times as required).

       The parameters exchanged between the server and the fetch handler are:

         $args = {
                                           ## Client/server request:

                    GHANDLE   =>  $obj     ## Global handle specified at creation
                    HANDLE    =>  ref      ## Reference to data structure
                    SETNAME   =>  "id"     ## ID of the requested result set
                    OFFSET    =>  nnn      ## Record offset number
                    REQ_FORM  =>  "n.m.k.l"## Client requested format OID
                    COMP      =>  "xyz"    ## Formatting instructions
                    SCHEMA_OID => "",      ## Z39.50 schema (OID), if any
                    SCHEMA    =>  "abc"    ## Requested schema (string), if any

                                           ## Handler response:

                    RECORD    =>  ""       ## Record string
                    BASENAME  =>  ""       ## Origin of returned record
                    LAST      =>  0        ## Last record in set?
                    ERR_CODE  =>  0        ## Error code
                    ERR_STR   =>  ""       ## Error string
                    SUR_FLAG  =>  0        ## Surrogate diagnostic flag
                    REP_FORM  =>  "n.m.k.l"## Provided format OID
                    SCHEMA    =>  "abc"    ## Provided schema, if any
                 };

       The REP_FORM value has by default the REQ_FORM value, but can be set to something different if the
       handler desires. The BASENAME value should contain the name of the database from where the returned
       record originates.  The ERR_CODE and ERR_STR works the same way they do in the search handler. If there
       is an error condition, the SUR_FLAG is used to indicate whether the error condition pertains to the
       record currently being retrieved, or whether it pertains to the operation as a whole (e.g. the client has
       specified a result set which does not exist.)

       If you need to return USMARC records, you might want to have a look at the MARC module on CPAN, if you
       don't already have a way of generating these.

       NOTE: The record offset is 1-indexed, so 1 is the offset of the first record in the set.

   Scan handler
       A full featured Z39.50 server should support scan (or in some literature browse). The client specifies a
       starting term of the scan, and the server should return an ordered list of specified length consisting of
       terms actually occurring in the data base. Each of these terms should be close to or equal to the term
       originally specified. The quality of scan compared to simple search is a guarantee of hits. It is simply
       like browsing through an index of a book, you always find something! The parameters exchanged are:

         $args = {
                                                       ## Client request

                       GHANDLE         => $obj,        ## Global handle specified at creation
                       HANDLE          => $ref,        ## Reference to data structure
                       DATABASES       => ["xxx"],     ## Reference to a list of data-
                                                       ## bases to search
                       TERM            => 'start',     ## The start term
                       RPN             =>  $obj,       ## Reference to a Net::Z3950::RPN::Term
                       attributeSet    => OID,         ## OID String (optional)

                       NUMBER          => xx,          ## Number of requested terms
                       POS             => yy,          ## Position of starting point
                                                       ## within returned list
                       STEP            => 0,           ## Step size

                                                       ## Server response

                       ERR_CODE        => 0,           ## Error code
                       ERR_STR         => '',          ## Diagnostic message
                       NUMBER          => zz,          ## Number of returned terms
                       STATUS          => $status,     ## ScanSuccess/ScanFailure
                       ENTRIES         => $entries     ## Referenced list of terms
               };

       where the term list is returned by reference in the scalar $entries, which should point at a data
       structure of this kind,

         my $entries = [
                               {       TERM            => 'energy',
                                       OCCURRENCE      => 5            },

                               {       TERM            => 'energy density',
                                       OCCURRENCE      => 6,           },

                               {       TERM            => 'energy flow',
                                       OCCURRENCE      => 3            },

                                       ...

                                       ...
               ];

       The $status flag is only meaningful after a successful scan, and should be assigned one of two values:

         Net::Z3950::SimpleServer::ScanSuccess  Full success (default)
         Net::Z3950::SimpleServer::ScanPartial  Fewer terms returned than requested

       The STEP member contains the requested number of entries in the term-list between two adjacent entries in
       the response.

       A better alternative to the TERM member is the the RPN member, which is a reference to a
       Net::Z3950::RPN::Term object representing the scan clause.  The structure of that object is the same as
       for Term objects included as part of the RPN tree passed to search handlers.  This is more useful than
       the simple TERM because it includes attributes (e.g. access points associated with the term), which are
       discarded by the TERM element.

   Close handler
       The argument hash received by the close handler has two elements only:

         $args = {
                                           ## Server provides:

                    GHANDLE   =>  $obj     ## Global handle specified at creation
                    HANDLE    =>  ref      ## Reference to data structure
                 };

       What ever data structure the HANDLE value points at goes out of scope after this call. If you need to
       close down a connection to your server or something similar, this is the place to do it.

   Explain handler
       The argument hash received by the explain handler has the following elements:

         $args = {
                                   ## Request parameters:
            GHANDLE   =>  $obj,    # Global handle specified at creation
            HANDLE    =>  ref,     # Reference to data structure
            DATABASE  =>  $dbname, # Name of database to be explained

                                   ## Response parameters:
            EXPLAIN   =>  $zeerex  # ZeeRex record for specified database
         };

       The handler should return a string containing the ZeeRex XML that describes that nominated database.

   Delete handler
       The argument hash received by the delete handler has the following elements:

         $args = {
                                           ## Client request:
                    GHANDLE   =>  $obj,    ## Global handle specified at creation
                    HANDLE    =>  ref,     ## Reference to data structure
                    SETNAME   =>  "id",    ## Result set ID

                                           ## Server response:
                    STATUS    => 0         ## Deletion status
                 };

       The SETNAME element of the argument hash may or may not be defined.  If it is, then SETNAME is the name
       of a result set to be deleted; if not, then all result-sets associated with the current session should be
       deleted.  In either case, the callback function should report on success or failure by setting the STATUS
       element either to zero, on success, or to an integer from 1 to 10, to indicate one of the ten possible
       failure codes described in section 3.2.4.1.4 of the Z39.50 standard -- see
       http://www.loc.gov/z3950/agency/markup/05.html#Delete-list-statuses1

   Sort handler
       The argument hash received by the sort handler has the following elements:

               $args = {
                                               ## Client request:
                       GHANDLE => $obj,        ## Global handle specified at creation
                       HANDLE => ref,          ## Reference to data structure
                       INPUT => [ a, b ... ],  ## Names of result-sets to sort
                       OUTPUT => "name",       ## Name of result-set to sort into
                       SEQUENCE                ## Sort specification: see below

                                               ## Server response:
                       STATUS => 0,            ## Success, Partial or Failure
                       ERR_CODE => 0,          ## Error code
                       ERR_STR => '',          ## Diagnostic message

               };

       The SEQUENCE element is a reference to an array, each element of which is a hash representing a sort key.
       Each hash contains the following elements:

       RELATION
           0 for an ascending sort, 1 for descending, 3 for ascending by frequency, or 4 for descending by
           frequency.

       CASE
           0 for a case-sensitive sort, 1 for case-insensitive

       MISSING
           How to respond if one or more records in the set to be sorted are missing the fields indicated in the
           sort specification.  1 to abort the sort, 2 to use a "null value", 3 if a value is provided to use in
           place of the missing data (although in the latter case, the actual value to use is currently not made
           available, so this is useless).

       And one or other of the following:

       SORTFIELD
           A string indicating the field to be sorted, which the server may interpret as it sees fit (presumably
           by an out-of-band agreement with the client).

       ELEMENTSPEC_TYPE and ELEMENTSPEC_VALUE
           I have no idea what this is.

       ATTRSET and SORT_ATTR
           ATTRSET is the attribute set from which the attributes are taken, and SORT_ATTR is a reference to an
           array containing the attributes themselves.  Each attribute is represented by (are you following this
           carefully?) yet another hash, this one containing the elements ATTR_TYPE and ATTR_VALUE: for example,
           type=1 and value=4 in the BIB-1 attribute set would indicate access-point 4 which is title, so that a
           sort of title is requested.

       Precisely why all of the above is so is not clear, but goes some way to explain why, in the Z39.50 world,
       the developers of the standard are not so much worshiped as blamed.

       The backend function should set STATUS to 0 on success, 1 for "partial success" (don't ask) or 2 on
       failure, in which case ERR_CODE and ERR_STR should be set.

   Support for SRU and SRW
       Since release 1.0, SimpleServer includes support for serving the SRU and SRW protocols as well as Z39.50.
       These ``web-friendly'' protocols enable similar functionality to that of Z39.50, but by means of rich
       URLs in the case of SRU, and a SOAP-based web-service in the case of SRW.  These protocols are described
       at http://www.loc.gov/standards/sru/

       In order to serve these protocols from a SimpleServer-based application, it is necessary to launch the
       application with a YAZ Generic Frontend Server (GFS) configuration file, which can be specified using the
       command-line argument "-f" filename.  A minimal configuration file looks like this:

         <yazgfs>
           <server>
             <cql2rpn>pqf.properties</cql2rpn>
           </server>
         </yazgfs>

       This file specifies only that "pqf.properties" should be used to translate the CQL queries of SRU and SRW
       into corresponding Z39.50 Type-1 queries.  For more information about YAZ GFS configuration, including
       how to specify an Explain record, see the Virtual Hosts section of the YAZ manual at
       http://www.indexdata.com/yaz/doc/server.vhosts.html

       The mapping of CQL queries into Z39.50 Type-1 queries is specified by a file that indicates which BIB-1
       attributes should be generated for each CQL index, relation, modifiers, etc.  A typical section of this
       file looks like this:

         index.dc.title                        = 1=4
         index.dc.subject                      = 1=21
         index.dc.creator                      = 1=1003
         relation.<                            = 2=1
         relation.le                           = 2=2

       This file specifies the BIB-1 access points (type=1) for the Dublin Core indexes "title", "subject" and
       "creator", and the BIB-1 relations (type=2) corresponding to the CQL relations "<" and "<=".  For more
       information about the format of this file, see the CQL section of the YAZ manual at
       http://www.indexdata.com/yaz/doc/tools.html#cql

       The YAZ distribution includes a sample CQL-to-PQF mapping configuration file called "pqf.properties";
       this is sufficient for many applications, and a good base to work from for most others.

       If a SimpleServer-based application is run without this SRU-specific configuration, it can still serve
       SRU; however, CQL queries will not be translated, but passed straight through to the search-handler
       function, as the "CQL" member of the parameters hash.  It is then the responsibility of the back-end
       application to parse and handle the CQL query, which is most easily done using Ed Summers' fine
       "CQL::Parser" module, available from CPAN at http://search.cpan.org/~esummers/CQL-Parser/

AUTHORS

       Anders Sønderberg (sondberg@indexdata.dk), Sebastian Hammer (quinn@indexdata.com), Mike Taylor
       (indexdata.com).

       Copyright (C) 2000-2017 by Index Data.

       This library is free software; you can redistribute it and/or modify it under the same terms as Perl
       itself, either Perl version 5.8.4 or, at your option, any later version of Perl 5 you may have available.

SEE ALSO

       Any Perl module which is useful for accessing the data source of your choice.