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 => \&my_init_handler,
CLOSE => \&my_close_handler,
SEARCH => \&my_search_handler,
FETCH => \&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:
- 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(
INIT => \&my_init_handler,
CLOSE => \&my_close_handler,
SEARCH => \&my_search_handler,
PRESENT => \&my_present_handler,
SCAN => \&my_scan_handler,
FETCH => \&my_fetch_handler,
EXPLAIN => \&my_explain_handler,
DELETE => \&my_delete_handler,
SORT => \&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.
If you want your SimpleServer to start a thread (threaded mode) to handle each
incoming Z39.50 request instead of forking a process (forking mode), you need
to register the handlers by symbol rather than by code reference. Thus, in
threaded mode, you will need to register your handlers this way:
my $z = new Net::Z3950::SimpleServer(
INIT => "my_package::my_init_handler",
CLOSE => "my_package::my_close_handler",
....
.... );
where my_package is the Perl package in which your handler is located.
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://indexdata.com/yaz/doc/server.invocation.tkl>
In particular, you need to use the -T switch to start your SimpleServer in
threaded mode.
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
GHANDLE => $obj ## Global handler 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 (ie. 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 anony- mous hash.
The structure is the following:
$args = {
## Request parameters:
GHANDLE => $obj ## Global handler 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 data-
## bases to search
QUERY => "query", ## The query expression
RPN => $obj, ## Reference to a Net::Z3950::APDU::Query
## Response parameters:
ERR_CODE => 0, ## Error code (0=Successful search)
ERR_STR => "", ## Error string
HITS => 0 ## Number of matches
};
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.
The QUERY is a tree-structure of terms combined by operators, the terms being
qualified by lists of attributes. The query is presented 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
anymore 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 may be 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 of one of the above types.
- "Net::Z3950::RPN::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.
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 handler specified at creation
HANDLE => ref, ## Reference to datastructure
SETNAME => "id", ## Result set ID
START => xxx, ## Start position
COMP => "", ## Desired record composition
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 handler 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 => "abc" ## Requested schema, 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 (eg. 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 - 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 handler 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
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 handler 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.
Delete handler¶
The argument hash received by the delete handler has the following elements:
$args = {
## Client request:
GHANDLE => $obj, ## Global handler 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 handler 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/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://indexdata.com/yaz/doc/server.vhosts.tkl
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://indexdata.com/yaz/doc/tools.tkl#tools.cql
The YAZ distribution include 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 Sonderberg (sondberg@indexdata.dk), Sebastian Hammer
(quinn@indexdata.dk), Mike Taylor (indexdata.com).
COPYRIGHT AND LICENCE¶
Copyright (C) 2000-2011 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.