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KinoSearch1::Docs::FilUsermContributed Perl DoKinoSearch1::Docs::FileFormat(3)

NAME
       KinoSearch1::Docs::FileFormat - overview	of invindex file format

OVERVIEW
       It is not necessary to understand the guts of the Lucene-derived
       "invindex" file format in order to use KinoSearch1, but it may be
       helpful if you are interested in	tweaking for high performance, exotic
       usage, or debugging and development.

       On a file system, all the files in an invindex exist in one, flat
       directory.  Conceptually, the files have	a hierarchical relationship:
       an invindex is made up of "segments", each of which is an independent
       inverted	index, and each	segment	is made	up of several subsections.

	   [invindex]--|
		       |-"segments" file
		       |
		       |-[segments]------|
					 |--[seg _0]--|
					 |	      |--[postings]
					 |	      |--[stored fields]
					 |	      |--[deletions]
					 |
					 |--[seg _1]--|
					 |	      |--[postings]
					 |	      |--[stored fields]
					 |	      |--[deletions]
					 |
					 |--[ ... ]---|

       The "segments" file keeps a list	of the segments	that make up an
       invindex.  When a new segment is	being written, KinoSearch1 may put
       files into the directory, but until the segments	file is	updated, a
       Searcher	reading	the index won't	know about them.

       Each segment is an independent inverted index.  All the files which
       belong to a given segment share a common	prefix which consists of an
       underscore followed by 1	or more	decimal	digits:	_0, _67, _1058.	 A
       fully optimized index has only a	single segment.

       In theory there are many	files which make up each segment.  However,
       when you	look inside an invindex	not in the process of being updated,
       you'll probably see only	the segments file and files with either	a .cfs
       or .del extension.  The .cfs file, a "compound" file which is
       consolidated when a segment is finalized, "contains" all	the other per-
       segment files.

       Segments	are written once, and with the exception of the	deletions
       file, are never modified	once written.  They are	deleted	when their
       data is written to new segments during the process of optimization.

A segment's component parts
       Each segment can	be said	to have	four logical parts: postings, stored
       fields, the deletions file, and the term	vectors	data.

   Stored fields
       The stored fields are organized into two	files.

       o   [seg_name].fdx - Field inDeX	- pointers to field data

       o   [seg_name].fdt - Field DaTa - the actual stored fields

       When a document turns up	as a hit in a search and must be retrieved,
       KinoSearch1 looks at the	Field inDeX file to see	where in the data file
       the document's stored fields start, then	retrieves all of them from the
       .fdt file in one	lump.

	   _1.fdx--|
		   |--[doc#0  =>   0]----->_1.fdt--|
		   |				   |--[bodytext]
		   |				   |--[title]
		   |				   |--[url]
		   |--[doc#1  => 305]----->_1.fdt--|		 # byte	305
		   |				   |--[bodytext]
		   |				   |--[title]
		   |				   |--[url]
		   |--[...]--------------->_1.fdt--|--[...]

       If a field is marked as "vectorized", its "term vectors"	are also
       stored in the .fdx file.

   Postings
       "Posting" is a technical	term from the field of Information Retrieval
       which refers to an single instance of a one term	indexing one document.
       If you are looking at the index in the back of a	book, and you see that
       "freedom" is referenced on pages	8, 86, and 240,	that would be three
       postings, which taken together form a "posting list".  The same
       terminology applies to an index in electronic form.

       The postings data is spread out over 4 main files (not including	field
       normalization data, which we'll get to in a moment).  From lowest to
       highest in the hierarchy, they are...

       [seg_name].prx -	PRoXimity data.	A list of the positions	at which terms
       appear in any given document.  The .prx file is just a raw stream of
       VInts; the document numbers and terms are implicitly indicated by files
       higher up the hierarchy.

       [seg_name].frq -	FReQuency data for terms.  If a	term has a frequency
       of 5 in a given document, that implies that there will be 5 entries in
       the .prx	file.  The terms themselves are	implicitly specified by	the
       .tis file.

	   _1.frq--|
		   |--[doc#40 => 2]----->_1.prx--|--[54,107]
		   |--[doc#0  => 1]----->_1.prx--|--[6]
		   |--[doc#6  => 1]----->_1.prx--|--[504]
		   |--[doc#36 => 3]----->_1.prx--|--[2,33,747]
		   |--[...]------------->_1.frq--|--[...]

       [seg_name].tis -	TermInfoS.  Among the items stored here	is the term's
       doc_freq, which is the number of	documents the term appears in.	If a
       term has	a doc_freq of 22 in a given collection,	that implies that
       there will be 22	corresponding entries in the .frq file.	 Terms are
       ordered lexically, first	by field, then by term text.

	   _1.tis--|
		   |--[...]----------------------->_1.frq--|--[...]
		   |--[bodytext:mule	  =>  1]-->_1.frq--|--[doc#40 => 2]
		   |--[bodytext:multitude =>  3]-->_1.frq--|--[doc#0  => 1]
		   |					   |--[doc#6  => 1]
		   |					   |--[doc#36 => 3]
		   |--[bodytext:navigate  =>  1]-->_1.frq--|--[doc#21 => 1]
		   |--[...]----------------------->_1.frq--|--[...]
		   |--[title:amendment	  => 27]-->_1.frq--|--[doc#21 => 1]
		   |					   |--[doc#22 => 1]
		   |--[...]----------------------->_1.frq--|--[...]

       [seg_name].tii -	TermInfos Index.  This file, which is decompressed and
       loaded into RAM as soon as the IndexReader is initialized, contains a
       small subset of the .tis	data, with pointers to locations in the	.tis
       file.  It is used to locate the right general vicinity in the .tis file
       as quickly as possible.

	   _1.tii--|
		   |--[bodytext:a => 20]---------->_1.tis--|--[bodytext:a] # byte 20
		   |					   |--[bodytext:about]
		   |					   |--[bodytext:absolute]
		   |					   |--[...]
		   |--[bodytext:mule =>	27065]---->_1.tis--|--[bodytext:mule]
		   |					   |--[bodytext:multitude]
		   |					   |--[...]
		   |--[title:amendment => 56992]-->_1.tis--|--[title:amendment]
							   |--[...]

       Here's a	simplified version of how a search for "freedom" against a
       given segment plays out:

       1.  The searcher	asks the .tii file, "Do	you know anything about
	   'freedom'?"	The .tii file replies, "Can't say for sure, but	if the
	   .tis	file does, 'freedom' is	probably somewhere around byte 21008".

       2.  The .tis file tells the searcher "Yes, we have 2 documents which
	   contain 'freedom'.  You'll find them	in the .frq file starting at
	   byte	66991."

       3.  The .frq file says "document	number 40 has 1	'freedom', and
	   document 44 has 8.  If you need to know more, like if any 'freedom'
	   is part of the phrase 'freedom of speech', take a look at the .prx
	   file	starting at..."

       4.  If the searcher is only looking for 'freedom' in isolation, that's
	   where it stops.  It already knows enough to assign the documents
	   scores against "freedom", with the 8-freedom	document scoring
	   higher than the single-freedom document.

   Deletions
       When a document is "deleted" from a segment, it is not actually purged
       from the	postings data and the stored fields data right away; it	is
       merely marked as	"deleted", via the .del	file.  The .del	file contains
       a bit vector with one bit for each document in the segment; if bit #254
       is set then document 254	is deleted, and	if it turns up in a search it
       will be masked out.

       It is only when a segment's contents are	rewritten to a new segment
       during the segment-merging process that deleted documents truly go
       away.

   Field Normalization Files
       For the sake of simplicity, the example search scenario above omits the
       role played the field normalization files, or "fieldnorms" for short.
       These files have	the (theoretical) suffix of .f followed	by an integer
       -- .f0, .f1, etc.  Each segment contains	one such file for every
       indexed field.

       By default, the fieldnorms' job is to make sure that a field which is
       100 terms long and contains 10 mentions of the word 'freedom' scores
       higher than a field which also contains 10 mentions of the word
       'freedom', but is 1000 terms in length.	The idea is that the higher
       the density of the desired term,	the more relevant the document.

       The fieldnorms files contain one	byte per document per indexed field,
       and all of them must be loaded into RAM before a	search can be
       executed.

Document Numbers
       Document	numbers	are ephemeral.	 They change every time	a document
       gets moved from one segment to a	new one	during optimization.  If you
       need to assign a	primary	key to each document, you need to create a
       field and populate it with an externally	generated unique identifier.

Not compatible with Java Lucene
       The file	format used by KinoSearch1 is closely related to the Lucene
       compound	index format. (The technical specification for Lucene's	file
       format is distributed along with	Lucene.)  However, indexes generated
       by Lucene and KinoSearch1 are not compatible.

COPYRIGHT
       Copyright 2005-2010 Marvin Humphrey

LICENSE, DISCLAIMER, BUGS, etc.
       See KinoSearch1 version 1.01.

perl v5.32.1			  2021-08-26  KinoSearch1::Docs::FileFormat(3)

NAME | OVERVIEW | A segment's component parts | Document Numbers | Not compatible with Java Lucene | COPYRIGHT | LICENSE, DISCLAIMER, BUGS, etc.

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