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1 INF 2914 Information Retrieval and Web Search Lecture 3: Parsing/Tokenization/Storage These slides are adapted from Stanford’s class CS276 / LING 286.

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Presentation on theme: "1 INF 2914 Information Retrieval and Web Search Lecture 3: Parsing/Tokenization/Storage These slides are adapted from Stanford’s class CS276 / LING 286."— Presentation transcript:

1 1 INF 2914 Information Retrieval and Web Search Lecture 3: Parsing/Tokenization/Storage These slides are adapted from Stanford’s class CS276 / LING 286 Information Retrieval and Web Mining

2 2 (Offline) Search Engine Data Flow - Parse - Tokenize - Per page analysis tokenized web pages dup table Parse & Tokenize Global Analysis 2 inverted text index 1 Crawler web page - Scan tokenized web pages, anchor text, etc - Generate text index Index Build - Dup detection - Static rank comp - Anchor text - … 34 rank table anchor text in background

3 3 Inverted index Brutus Calpurnia Caesar Dictionary Postings lists Sorted by docID (more later on why). Posting

4 4 Inverted index construction Tokenizer Token stream. Friends RomansCountrymen Linguistic modules Modified tokens. friend romancountryman Indexer Inverted index. friend roman countryman Documents to be indexed. Friends, Romans, countrymen.

5 5 Plan for this lecture The Dictionary Parsing Tokenization What terms do we put in the index? Storage Log structured file systems XML Introduction

6 6 Parsing a document What format is it in? pdf/word/excel/html? What language is it in? What character set is in use? Each of these is a classification problem. But these tasks are often done heuristically …

7 7 Complications: Format/language Documents being indexed can include docs from many different languages A single index may have to contain terms of several languages. Sometimes a document or its components can contain multiple languages/formats French with a German pdf attachment. What is a unit document? A file? An ? (Perhaps one of many in an mbox.) An with 5 attachments? A group of files (PPT or LaTeX in HTML)

8 8 Tokenization

9 9 Input: “Friends, Romans and Countrymen” Output: Tokens Friends Romans Countrymen Each such token is now a candidate for an index entry, after further processing Described below But what are valid tokens to emit?

10 10 Tokenization Issues in tokenization: Finland’s capital  Finland? Finlands? Finland’s? Hewlett-Packard  Hewlett and Packard as two tokens? State-of-the-art: break up hyphenated sequence. co-education ? the hold-him-back-and-drag-him-away-maneuver ? It’s effective to get the user to put in possible hyphens San Francisco: one token or two? How do you decide it is one token?

11 11 Numbers 3/12/91 Mar. 12, B.C. B-52 My PGP key is 324a3df234cb23e Often, don’t index as text. But often very useful: think about things like looking up error codes/stacktraces on the web (One answer is using n-grams, lectures 6 and 7) Will often index “meta-data” separately Creation date, format, etc.

12 12 Tokenization: Language issues L'ensemble  one token or two? L ? L’ ? Le ? Want l’ensemble to match with un ensemble German noun compounds are not segmented Lebensversicherungsgesellschaftsangestellter ‘life insurance company employee’

13 13 Tokenization: language issues Chinese and Japanese have no spaces between words: 莎拉波娃现在居住在美国东南部的佛罗里达。 Not always guaranteed a unique tokenization Further complicated in Japanese, with multiple alphabets intermingled Dates/amounts in multiple formats フォーチュン 500 社は情報不足のため時間あた $500K( 約 6,000 万円 ) KatakanaHiraganaKanjiRomaji End-user can express query entirely in hiragana!

14 14 Tokenization: language issues Arabic (or Hebrew) is basically written right to left, but with certain items like numbers written left to right Words are separated, but letter forms within a word form complex ligatures استقلت الجزائر في سنة 1962 بعد 132 عاما من الاحتلال الفرنسي. ← → ← → ← start ‘Algeria achieved its independence in 1962 after 132 years of French occupation.’ With Unicode, the surface presentation is complex, but the stored form is straightforward

15 15 Normalization Need to “normalize” terms in indexed text as well as query terms into the same form We want to match U.S.A. and USA We most commonly implicitly define equivalence classes of terms e.g., by deleting periods in a term Alternative is to do asymmetric expansion: Enter: windowSearch: window, windows Enter: windowsSearch: windows Potentially more powerful, but less efficient Execute queries in parallel or do a second pass over the index

16 16 Normalization: other languages Accents: résumé vs. resume. Most important criterion: How are your users like to write their queries for these words? Even in languages that have accents, users often may not type them German: Tuebingen vs. T ü bingen Should be equivalent

17 17 Normalization: other languages Need to “normalize” indexed text as well as query terms into the same form Character-level alphabet detection and conversion Tokenization not separable from this. Sometimes ambiguous: 7 月 30 日 vs. 7/30 Morgen will ich in MIT … Is this German “mit”?

18 18 Case folding Reduce all letters to lower case exception: upper case (in mid-sentence?) e.g., General Motors Fed vs. fed SAIL vs. sail Often best to lower case everything, since users will use lowercase regardless of ‘correct’ capitalization…

19 19 Stop words With a stop list, you exclude from dictionary entirely the commonest words. Intuition: They have little semantic content: the, a, and, to, be They take a lot of space: ~30% of postings for top 30 But the trend is away from doing this: Good compression techniques means the space for including stopwords in a system is very small Good query optimization techniques mean you pay little at query time for including stop words. You need them for: Phrase queries: “King of Denmark” Various song titles, etc.: “Let it be”, “To be or not to be” “Relational” queries: “flights to London”

20 20 Thesauri and soundex Handle synonyms and homonyms Hand-constructed equivalence classes e.g., car = automobile color = colour Rewrite to form equivalence classes Index such equivalences When the document contains automobile, index it under car as well (usually, also vice-versa) Or expand query? When the query contains automobile, look under car as well

21 21 Soundex Traditional class of heuristics to expand a query into phonetic equivalents Language specific – mainly for names E.g., chebyshev  tchebycheff

22 22 Lemmatization Reduce inflectional/variant forms to base form E.g., am, are, is  be car, cars, car's, cars'  car the boy's cars are different colors  the boy car be different color Lemmatization implies doing “proper” reduction to dictionary headword form

23 23 Stemming Reduce terms to their “roots” before indexing “Stemming” suggest crude affix chopping language dependent e.g., automate(s), automatic, automation all reduced to automat. for example compressed and compression are both accepted as equivalent to compress. for exampl compress and compress ar both accept as equival to compress

24 24 Porter’s algorithm Commonest algorithm for stemming English Results suggest at least as good as other stemming options Conventions + 5 phases of reductions phases applied sequentially each phase consists of a set of commands sample convention: Of the rules in a compound command, select the one that applies to the longest suffix.

25 25 Typical rules in Porter sses  ss ies  i ational  ate tional  tion

26 26 Other stemmers Other stemmers exist, e.g., Lovins stemmer Single-pass, longest suffix removal (about 250 rules) Motivated by linguistics as well as IR Full morphological analysis – at most modest benefits for retrieval Do stemming and other normalizations help? Often very mixed results: really help recall for some queries but harm precision on others

27 27 Language-specificity Many of the above features embody transformations that are Language-specific and Often, application-specific These are “plug-in” addenda to the indexing process Both open source and commercial plug-ins available for handling these

28 28 Index Build Flow - Overview Crawled documents Indexing Search indexes Per document analysis Global analysis

29 29 Per document analysis Multi-format parsing Handles different files types (HTML, PDF, PowerPoint, etc) Multi-language tokenization, stemming, synonyms, user-defined annotations, etc. Per document analysis is tipically the bottleneck of the index build process 50 times slower than I/O Indexing can be done at I/O speed

30 30 Incorporating per document analysis Per document analysis is much slower than indexing Store tokenized documents in a scalable document store Crawled documents Per document analysis Document store

31 31 Storage

32 32 Document store Log-structured file system Only the most recent version of each document is accessible No in place updates Documents are grouped into bundles to optimize I/O Typically built over the file system 3 basic operation modes Document insertion (during per-document analysis) Sequential access for index build Random access during query processing

33 33 data timestamp # docs hash(URL)offset timestamphash(URL)offset # fields lengthfield ID lengthfield ID lengthfield ID data # fields offsetfield ID offset length Header Doc 2 Doc 1 # fields # docs attributes Store design (1/5) Bundle disk layout Fixed bundle size (for instance 8MB) All fields are 64-bit aligned All fields are binary Store does not know how to interpret fields Compression Fields are tokens, anchor text, URL, shingle, statistics, etc.

34 34 Store design (2/5) Document insertion uses a double buffering algorithm and asynchronous I/O Try to fit as many documents as it can in a bundle Schedule write for bundle Start writing the next bundle

35 35 Bundle# 1053 Bundle# 1052 currentBuffer nextBuffer Store design (3/5) Store is sequentially scanned during index build and global analysis A double buffering algorithm with asynchronous I/O is also used here Return only the newest version of each document Store is accessed in reverse order LFS semantics

36 36 Store design (4/5) Store cleanup algorithm “Smarter” algorithms can be used if we are not I/O bound Avoid seeks D1’ D5’ D6 New documents bundle D1 D3 D4 bundle D5 D2 bundle Store i Store i+1 D1’ D5’ D6 bundle D3 D4 D2 bundle * garbage collected * * Bloom filter probe setcopy

37 37 Bloom Filters (1/2) Compact data structures for a probabilistic representation of a set Appropriate to answer membership queries False positives!

38 38 Bloom Filters (2/2) Query for b: check the bits at positions H 1 (b), H 2 (b),..., H 4 (b).

39 39 Store design (5/5) During runtime the summarizer uses the store to fetch the tokens (random access) Store provides an API call for retrieving a set of documents (e.g. 20) given its bundle number and offset in the file Internally the store uses a buffer pool for documents Asynchronous I/O is used for exploiting parallelism from the storage subsystem Summarizer releases the documents after it is done

40 40 Storage Issues Performance Fault tolerance Distribution Redundancy Field compression Google File System tries to address these issues

41 41 XML Introduction

42 42 Preliminaries: XML PODS Josifovski 1 Fagin 3 conference name speaker name paper_cnt root speaker name paper_cnt PODS Josifovski Fagin 1 3 x0x0 x1x1 x2x2 x3x3 x6x6 x4x4 x5x5 x7x7 x8x8

43 43 Preliminaries: XPath 1.0 /conference[name = PODS]/speaker[paper_cnt > 1]/name conference name root Document Query Result: { x 7 } speaker name paper_cnt = PODS > 1 conference name speaker name paper_cnt root speaker name paper_cnt PODS Josifovski Fagin 1 3 x0x0 x1x1 x2x2 x3x3 x6x6 x4x4 x5x5 x7x7 x8x8

44 44 XML Indexing //article//section[ //title contains(‘Query Processing’) AND //figure//caption contains(‘XML’)] In an index-based method, 8 tags and text elements need to be verified to process this query (lessons 6 and 7) “Query Processing” article section titlefigure caption “XML”

45 45 Position Encoding Scheme #1: Begin/End/Level Begin: preorder position of tag/text End: preorder position of last descendent Level: depth Containment: X contains Y iff X.begin < Y.begin <= X.end (assuming well-formed) A1A1 B1B1 B2B2 C1C1 D1D1 B3B3 C2C2 R (0,7,0) (1,5,1) (2,2,2) (4,4,3) (5,5,3) (6,7,1) (7,7,2) (3,5,2)

46 46 Position Encoding Scheme #2: Dewey Position of element E = {position of parent}.n, where E is the nth child of its parent Containment: X contains Y iff X is a prefix of Y A1A1 B1B1 B2B2 C1C1 D1D1 B3B3 C2C2 R (1) (1.1) (1.1.1) ( ) ( ) (1.2) (1.2.1) (1.1.2)

47 47 Position Encoding Begin/End/Level Typically more compact Fewer implementation issues Dewey Encodes positions of all ancestors

48 48 Path Index A1A1 B1B1 B2B2 C1C1 D1D1 B3B3 C2C2 R PathID /R1 /R/A2 /R/A/B3 /R/A/B/C4 /R/A/B/D5 /R/B6 /R/B/C7 Path Pattern->Set of matching path IDs /R/B->{6} //R//C->{4, 7}

49 49 Basic Access Path Inverted posting lists Posting: Token = Location = Exercise: Create the posting list representation for the following XML document A1A1 B1B1 B2B2 C1C1 D1D1 B3B3 C2C2 R

50 50 Inverted index Brutus Calpurnia Caesar Dictionary Postings lists Sorted by docID (Why on lessons 6/7). Posting

51 51 Joins in XML Structural (Containment) Joins Twig Joins A || B A || B || C D B || C B || D A || B || C

52 52 Resources for today’s lecture IIR 2 Porter’s stemmer: Rosenblum, Mendel and Ousterhout, John K. (February 1992). "The Design and Implementation of a Log-Structured Filesystem." ACM Transactions on Computer Systems. 10(1) XML Introduction (IIR 10)

53 53 Trabalho 4 - Proposta Google File System Map Reduce

54 54 Trabalho 5 - Proposta XML Parsing, Tokenization, and Indexing JuruXML - an XML retrieval system at INEX'02 Optimizing cursor movement in holistic twig joins. CIKM 2005:

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