Ulrich Pfeifer > WAIT > WAIT

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NAME ^

WAIT - a rewrite of the freeWAIS-sf engine in Perl and XS

SYNOPSIS ^

A Synopsis is not yet available.

Status of this document ^

I started writing down some information about the implementation before I forget them in my spare time. The stuff is incomplete at least. Any additions, corrections, ... welcome.

PURPOSE ^

As you might know, I developed and maintained freeWAIS-sf (with the help of many people in The Net). FreeWAIS-sf is based on freeWAIS maintained by the Clearing House for Network Information Retrieval (CNIDR) which in turn is based on wais-8-b5 implemented by Thinking Machine et al. During this long history - implementation started about 1989 - many people contributed to the distribution and added features not foreseen by the original design. While the system fulfills its task now, the code has reached a state where adding new features is nearly impossible and even fixing longstanding bugs and removing limitations has become a very time consuming task.

Therefore I decided to pass the maintenance to WSC Inc. and built a new system from scratch. For obvious reasons I choosed Perl as implementation language.

DESCRIPTION ^

The central idea of the system is to provide a framework and the building blocks for any indexing and search system the users might want to build. Obviously the framework limits the class of system which can be build.

       +------+     +-----+     +------+
   ==> |Access| ==> |Parse| ==> |      |
       +------+     +-----+     |      |
                       ||       |      |     +-----+
                       ||       |Filter| ==> |Index|
                       \/       |      |     +-----+
      +-------+     +-----+     |      |
   <= |Display| <== |Query| <-> |      |
      +-------+     +-----+     +------+

A collection (aka table) is defined by the instances of the access and parse module together with the filter definitions. At query time in addition a query and a display module must be choosen.

Access

The access module defines which documents are members of a database. Usually an access module is a tied hash, whose keys are the Ids of the documents (did = document id) and whose values are the documents themselves. The indexing process loops over the keys using FIRSTKEY and NEXTKEY. Documents are retrieved with FETCH.

By convention access modules should be members of the WAIT::Document hierarchy. Have a look at the WAIT::Document::Split module to get the idea.

Parse

The task of the parse module is to split the documents into logical parts via the split method. E.g. the WAIT::Parse::Nroff splits manuals piped through nroff(1) into the sections name, synopsis, options, description, author, example, bugs, text, see, and environment. Here is the implementation of WAIT::Parse::Base which handles documents with a pretty simple tagged format:

  AU: Pfeifer, U.; Fuhr, N.; Huynh, T.
  TI: Searching Structured Documents with the Enhanced Retrieval
      Functionality of freeWAIS-sf and SFgate
  ER: D. Kroemker
  BT: Computer Networks and ISDN Systems; Proceedings of the third
      International World-Wide Web Conference
  PN: Elsevier
  PA: Amsterdam - Lausanne - New York - Oxford - Shannon - Tokyo
  PP: 1027-1036
  PY: 1995

  sub split {                     # called as method
    my %result;
    my $fld;

    for (split /\n/, $_[1]) {
      if (s/^(\S+):\s*//) {
        $fld = lc $1;
      }
      $result{$fld} .= $_ if defined $fld;
    }
    return \%result;
  }

Since the original document cannot be reconstructed from its attributes, we need a second method (tag) which marks the regions of the document with tags for the different attributes. This tagged form is used by the display module to hilight search terms in the documents. Besides the tags for the attributes, the method might assign the special tags _b and _i for indicating bold and italic regions.

  sub tag {
    my @result;
    my $tag;

    for (split /\n/, $_[1]) {
      next if /^\w\w:\s*$/;
      if (s/^(\S+)://) {
        push @result, {_b => 1}, "$1:";
        $tag = lc $1;
      }
      if (defined $tag) {
        push @result, {$tag => 1}, "$_\n";
      } else {
        push @result, {}, "$_\n";
      }
    }
    return @result;               # we don't go for speed
  }

Obviously one could implement split via tag. The reason for having two functions is speed. We need to call split for each document when indexing a collection. Therefore speed is essential. On the other hand, tag is called in order to display a single document and may be a little slower. It may care about tagging bold and italic regions. See WAIT::Parse::Nroff how this might decrease performance.

Filter definition

From the Information Retrieval perspective, the hardest part of the system is the filter module. The database administrator defines for each attribute, how the contents should be processed before it is stored in the index. Usually the processing contains steps to restrict the character set, case transformation, splitting to words and transforming to word stems. In WAIT these steps are defined naturally as a pipeline of processing steps. The pipelines are made up by functions in the package WAIT::Filter which is pre-populated by the most common functions but may be extended any time.

The equivalent for a typical freeWAIS-sf processing would be this pipeline:

        [ 'isotr', 'isolc', 'split2', 'stop', 'Stem']

The function isotr replaces unknown characters by blanks. isolc transforms to lower case. split2 splits into words and removes words shorter than two characters. stop removes the freeWAIS-sf stopwords and Stem applies the Porter algorithm for computing the stem of the words.

The filter definition for a collection defines a set of pipelines for the attributes and modifies the pipelines which should be used for prefix and interval searches.

Several complete working examples come with WAIT in the script directory. It is recommended to follow the pattern of the scripts smakewhatis and sman.

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