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## -*- mode: html; coding: utf-8; -*-
## This file is part of Invenio.
## Copyright (C) 2011 CERN.
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<!-- WebDoc-Page-Title: BibAuthority Internals -->
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Here you will find a few explanations to the inner workings of BibAuthority.
<H2 CLASS="western">Indexing</H2>
<H3 CLASS="western">Introduction</H3>
<P>There are two cases that need special attention when idexing bibliographic
data that contains references to authority records.
The first case is relatively simple and requires the
enriching of bibliographic data with data from authority records
whenever a bibliographic record is being indexed. The second is a bit
more complex, for it requires detecting which bibliographic records
should be re-indexed, based on referenced authority records having
been updated within a given date range.</P>
<H3 CLASS="western">Indexing by record ID, by modification date
or by index type</H3>
<P>First of all, we need to say something about how INVENIO let's the
admin index the data. INVENIO's indexer (BibIndex) is always run as a
task that is executed by INVENIO's scheduler (BibSched). Typically,
this is done either by scheduling a bibindex task from the command
line (manually), or it is part of a periodic task (BibTask) run
directly from BibSched, typically ever 5 minutes. In case it is run
manually, the user has the option of specifying certain record IDs to
be re-indexed, e.g. by specifying ranges of IDs or collections to be
re-indexed. In this case, the selected records are re-indexed whether
or not there were any modifications to the data. Alternatively, the
user can specify a date range, in which case the indexer will search
all the record IDs that have been modified in the selected date range
(by default, the date range would specify all IDs modified since the
last time the indexer was run) and update the index only for those
records. As a third option, the user can specify specific types of
indexes. INVENIO lets you search by different criteria (e.g. 'any
field', 'title', 'author', 'abstract', 'keyword', 'journal', 'year',
'fulltext', …), and each of these criteria corresponds to a
separate index, indexing only the data from the relevant MARC
subfields. Normally, the indexer would update all index types for any
given record ID, but with this third option, the user can limit the
re-indexing to only specific types of indexes if desired.
</P>
<P>Note: In reality, INVENIO creates not only 1 but 6 different
indexes per index type. 3 are forward indexes (mapping words, pairs
or phrases to record IDs), 3 are reverse indexes (mapping record IDs
to words, pairs or phrases). The word, pair and phrase indexes are
used for optimizing the searching speed depending on whether the user
searches for words, sub-phrases or entire phrases. These details are
however not relevant for BibAuthority. It simply finds the values to
be indexed and passes them on to the indexer which indexes them as if
it was data coming directly from the bibliographic record.</P>
<H3 CLASS="western">Enriching the index data – simple case</H3>
<P>Once the indexer knows which record ID (and optionally, which
index type) to re-index, including authority data is simply a
question of checking whether the MARC subfields currently being
indexed are under authority control (as specified in the BibAuthority
configuration file). If they are, the indexer must follow the
following (pseudo-)algorithm which will fetch the necessary data from
the referenced authority records:</P>
<P STYLE="margin-left: 1.25cm"><FONT COLOR="#000000"><B>For</B> each
subfield and each record ID currently being re-indexed:</FONT></P>
<P STYLE="margin-left: 2.5cm"><FONT COLOR="#000000"><B>If</B> the
subfield is under authority control (→ config file):</FONT></P>
<P STYLE="margin-left: 3.75cm"><FONT COLOR="#000000"><B>Get</B> the
<I>type</I> of referenced authority record expected for this field</FONT></P>
<P STYLE="margin-left: 3.75cm"><FONT COLOR="#000000"><B>For</B> each
authority record control number found in the corresponding 'XXX__0'
subfields and matching the expected authority record <I>type</I>
(control number prefix):</FONT></P>
<P STYLE="margin-left: 5cm"><FONT COLOR="#000000"><B>Find</B> the
authority record ID (MARC field '001' control number) corresponding
to the authority record control number (as contained in MARC field
'035' of the authority record)</FONT></P>
<P STYLE="margin-left: 5cm"><FONT COLOR="#000000"><B>For</B> each
authority record subfield marked as index relevant for the given
$type (→ config file)</FONT></P>
<P STYLE="margin-left: 6.25cm"><FONT COLOR="#000000"><B>Add</B> the
values of these subfields to the list of values to be returned and
used for enriching the indexed strings.</FONT></P>
<P>The strings collected with this algorithm are simply added to the
strings already found by the indexer in the regular bibliographic
record MARC data. Once all the strings are collected, the indexer
goes on with the usual operation, parsing them 3 different times,
once for phrases, once for word-pairs, once for words, which are used
to populate the 6 forward and reverse index tables in the database.</P>
<H3 CLASS="western">Updating the index by date range</H3>
<P>When a bibindex task is created by date range, we are presented
with a more tricky situation which requires a more complex treatment
for it to work properly. As long as the bibindex task is configured
to index by record ID, the simple algorithm described above is enough
to properly index the authority data along with the data from
bibliographic records. This is true also if we use the third option
described above, specifying the particular index type to re-index
with the bibindex task. However, if we launch a bibindex task based
on a date range (by default the date range covers the time since the
last time bibindex task was run on for each of the index types),
bibindex would have no way to know that it must update the index for
a specific bibliographic record if one of the authority records it
references was modified in the specified date range. This would lead
to incomplete indexes.</P>
<P>A first idea was to modify the time-stamp for any bibliographic
records as soon as an authority record is modified. Every MARC record
in INVENIO has a 'modification_date' time-stamp which indicates to
the indexer when this record was last modified. If we search for
dependent bibliographic records every time we modify an authority
record, and if we then update the 'modification_date' time-stamp for
each of these dependent bibliographic records, then we can be sure
that the indexer would find and re-index these bibliographic records
as well when indexing by a specified date-range. The problem with
this is a performance problem. If we update the time-stamp for the
bibliographic record, this record will be re-indexed for all of the
mentioned index-types ('author', 'abstract', 'fulltext', etc.), even
though many of them may not cover MARC subfields that are under
authority control, and hence re-indexing them because of a change in
an authority record would be quite useless. In an INVENIO
installation there would typically be 15-30 index-types. Imagine if
you make a change to a 'journal' authority record and only 1 out of
the 20+ index-types is for 'journal'. INVENIO would be re-indexing
20+ index types in stead of only the 1 index type which is relevant
to the the type of the changed authority record.</P>
<P>There are two approaches that could solve this problem equally
well. The first approach would require checking – for each
authority record ID which is to be re-indexed – whether there are
any dependent bibliographic records that need to be re-indexed as
well. If done in the right manner, this approach would only re-index
the necessary index types that can contain information from
referenced authority records, and the user could specify the index
type to be re-indexed and the right bibliographic records would still
be found. The second approach works the other way around. In stead of
waiting until we find a recently modified authority record, and then
looking for dependent bibliographic records, we directly launch a
search for bibliographic records containing links to recently updated
authority records and add the record IDs found in this way to the
list of record IDs that need to be re-indexed.
</P>
<P>Of the two approaches, the second one was choses based solely upon
considerations of integration into existing INVENIO code. As indexing
in INVENIO currently works, it is more natural and easily readable to
apply the second method than the first.</P>
<P>According to the second method, the pseudo-algorithm for finding
the bibliographic record IDs that need to be updated based upon
recently modified authority records in a given date range looks like
this:</P>
<P STYLE="margin-left: 1.25cm"><B>For</B> each index-type to
re-index:</P>
<P STYLE="margin-left: 2.5cm"><B>For</B> each <I>subfield</I>
concerned by the index-type:</P>
<P STYLE="margin-left: 3.75cm"><B>If</B> the <I>subfield</I> is under
authority control (→ config file):</P>
<P STYLE="margin-left: 5cm"><B>Get </B>the <I>type</I> of authority
record associated with this field</P>
<P STYLE="margin-left: 5cm"><B>Get </B> all of the record IDs for
authority records updated in the specified date range.</P>
<P STYLE="margin-left: 5cm"><B>For</B> each record ID</P>
<P STYLE="margin-left: 6.25cm"><B>Get </B> the authority record
control numbers of this record ID</P>
<P STYLE="margin-left: 6.25cm"><B>For</B> each authority record
control number</P>
<P STYLE="margin-left: 7.5cm"><B>Search </B>for and <B>add</B> the
<SPAN STYLE="font-style: normal">record IDs</SPAN> of bibliographic
records containing this control number (with <I>type</I> in the
prefix) in the 'XXX__0' field of the current <I>subfield</I><SPAN STYLE="font-style: normal">
to the list of record IDs to be returned to the caller to be marked
as needing re-indexing</SPAN>.</P>
<P STYLE="margin-left: 7.5cm"><BR><BR>
</P>
<P>The record IDs returned in this way are added to the record IDs
that need to be re-indexed (by date range) and then the rest of the
indexing can run as usual.</P>
<H3 CLASS="western">Implementation specifics</H3>
<P>The pseudo-algorithms described above were used as described in
this document, but were not each implemented in a single function. In
order for parts of them to be reusable and also for the various parts
to be properly integrated into existing python modules with similar
functionality (e.g auxiliary search functions were added to INVENIO's
search_engine.py code), the pseudo-algorithms were split up into
multiple nested function calls and integrated where it seemed to best
fit the existing code base of INVENIO. In the case of the
pseudo-algorithm described in “Updating the index by date range”,
the very choice of the algorithm had already depended on how to best
integrate it into the existing code for date-range related indexing.</P>
<H2 CLASS="western">Cross-referencing between MARC records</H2>
<P>In order to reference authority records, we use alphanumeric strings
stored in the $0 subfields of fields that contain other, authority-controlled
subfields as well. The format of these alphanumeric strings for INVENIO is
in part determined by the MARC standard itself, which states that:</P>
<blockquote>
<i>
<P CLASS="code-western">Subfield $0 contains the system control
number of the related authority record, or a standard identifier such
as an International Standard Name Identifier (ISNI). The control
number or identifier is preceded by the appropriate MARC Organization
code (for a related authority record) or the Standard Identifier
source code (for a standard identifier scheme), enclosed in
parentheses. See MARC Code List for Organizations for a listing of
organization codes and Standard Identifier Source Codes for code
systems for standard identifiers. Subfield $0 is repeatable for
different control numbers or identifiers.</P>
</i>
</blockquote>
<P STYLE="margin-bottom: 0cm">An example of such a string could be
“(SzGeCERN)abc1234”, where “SzGeCERN” would be the MARC
organization code,
and abc1234 would be the unique identifier for this authority record
within the given organization.</P>
<P>Since it is possible for a single field (e.g. field '100') to have
multiple $0 subfields for the same field entry, we need a way to
specify which $0 subfield reference is associated with which other
subfield of the same field entry.</P>
<P>For example, imagine that in bibliographic records both '700__a'
('other author' <I>name</I>) as well as '700__u' ('other author'
<I>affiliation</I>) are under authority control. In this case we
would have two '700__0' subfields. Of of them would reference the
<I>author</I> authority record (for the <I>name</I><SPAN STYLE="font-style: normal">)</SPAN>,
the other one would reference an <I>institution</I> authority record
(for the <I>affiliation</I><SPAN STYLE="font-style: normal">).
INVENIO needs some way to know which $0 subfield is associated with
the $a subfield and which one with the $u subfield.</SPAN></P>
<P STYLE="font-style: normal">We have chosen to solve this in the
following way. Every $0 subfield value will not only contain the
authority record control number, but in addition will be prefixed by
the type of authority record (e.g. 'AUTHOR', 'INSTITUTION', 'JOURNAL'
or 'SUBJECT), separated from the control number by a separator, e.g. ':' (configurable). A possible
$0 subfield value could therefore be: “author:(SzGeCERN)abc1234”.
This will allow INVENIO to know that the $0 subfield containing
“author:(SzGeCERN)abc1234” is associated with the $a subfield
(author's name), containing e.g. “Ellis, John”, whereas the $0
subfield containing “institution:(SzGeCERN)xyz4321” is associated
with the $u subfield (author's affiliation/institution) of the same
field entry, containing e.g. “CERN”.</P>
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