May 11, 2005 WWW 2005 -- Chiba, Japan 1

Thresher: Automating the Unwrapping of

Semantic Content from the World Wide Web

Andrew HogueGoogle MIT CSAIL

May 11, 2005 WWW 2005 -- Chiba, Japan 2

Acknowledgments

• David Karger (karger@csail.mit.edu)

• Haystack Group(http://haystack.csail.mit.edu)

May 11, 2005 WWW 2005 -- Chiba, Japan 3

Agenda

• Overview• Demo• Details

– Induction– Matching– Semantics– Heuristics

May 11, 2005 WWW 2005 -- Chiba, Japan 4

Agenda

• Overview• Demo• Details

– Induction– Matching– Semantics– Heuristics

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Unwrapping the Web

• Majority of semantic content in “deep web”• Transformed into human-readable HTML

by scripts• HTML is difficult for automated agents to

understand• Little incentive for content providers to

provide RDF markup• How to “unwrap” this content?

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Thresher

• Simple UI for wrapper induction on structured web content

• “Demonstrate” examples of objects• Induce wrapper, or pattern, based on

DOM• User may also label properties with RDF

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Thresher

• Built on Haystack Semantic Web client• Everything is RDF• Everything has context menus• Thresher brings RDF into the web browser• Wrappers reify web objects for full

interaction

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Thresher

• Underlying wrapper algorithm based on tree edit distance

• Align user’s examples• Keep aligned nodes (layout elements)• Wildcard non-aligned nodes (content)• Pattern matching is also alignment

May 11, 2005 WWW 2005 -- Chiba, Japan 9

Agenda

• Overview• Demo• Details

– Induction– Matching– Semantics– Heuristics

May 11, 2005 WWW 2005 -- Chiba, Japan 10

Agenda

• Overview• Demo• Details

– Induction– Matching– Semantics– Heuristics

May 11, 2005 WWW 2005 -- Chiba, Japan 11

Wrapper Induction

• Wrapper: pattern created from examples• User provides positive examples• Generalize examples into reusable pattern• Existing techniques:

– head-left-right-tail (HLRT) descriptors– Hidden Markov models– Support Vector Machines– Other Machine Learning

May 11, 2005 WWW 2005 -- Chiba, Japan 12

Wrapper Induction

• Our approach: take advantage of hierarchical structure of HTML

• Each example picks out a subtree of DOM• Calculate tree edit distance between

examples• Least-cost edit distance gives best

mapping• Remove unmapped nodes to make pattern

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Tree Edit Distance

• Calculate cost ( ) of sequence of operations to transform one tree into the other

• Operations: insert, delete, change a node• Cost of an operation = size of subtree it

affects• Least-cost set of operations gives best

mapping between elements

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Mapping Examples

May 11, 2005 WWW 2005 -- Chiba, Japan 15

Mapping Examples

May 11, 2005 WWW 2005 -- Chiba, Japan 16

Mapping Examples

May 11, 2005 WWW 2005 -- Chiba, Japan 17

Agenda

• Overview• Demo• Details

– Induction– Matching– Semantics– Heuristics

May 11, 2005 WWW 2005 -- Chiba, Japan 18

Pattern Matching

• Look for document subtrees with similar structure

• Find alignments of wrapper in tree• Require every node in wrapper be mapped

to some node in document subtree• Wildcards match zero or more times• Each valid alignment is a match

May 11, 2005 WWW 2005 -- Chiba, Japan 19

Matching Example

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Agenda

• Overview• Demo• Details

– Induction– Matching– Semantics– Heuristics

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Adding Semantics

• How to tie wrappers to semantic content?• Assert RDF statements about unwrapped

objects• Tied to wrapper structure• Classes bound to wrappers• Properties bound to wildcards

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Semantic Labels

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Semantic Matching

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Semantic Matching

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Semantic Matching

[ <rdf:type> <TalkAnnouncement> ; <series> “Dertouzos Lect…” ; <dc:title> “Distributed Hash…” ; <time> “3:30 PM”]

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Agenda

• Overview• Demo• Details

– Induction– Matching– Semantics– Heuristics

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• Find additional examples automatically • Consider nodes neighboring the example• Require low normalized cost:

• Often allows us to create wrappers with a single example

Automatically Adding Examples

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Automatically Adding Examples

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List Collapse

• Current wrappers generalize well for single elements

• Will not recognize variable length lists• Collapse neighboring nodes with low

normalized cost• For matching, allow nodes to match more

than once

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Wrapper Wrap-up

• Gather user example(s)• Automatically find additional examples• Generalize examples using best mapping• Add semantic labels• Match by finding alignments• Overlay objects on the page for interaction

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Additional Tools

• Wrapper Sharing• RSS• Web Operations

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Our Contributions

• End-user wrapper induction

• Few examples required

• Bring object interaction into the browser

• Wrappers bridge syntactic-semantic gap

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Future Work and Applications

• Document-level classes• Page reformatting• Autonomous agent interaction• Negative examples• Automatic wrapper induction

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ahogue@google.com

http://haystack.csail.mit.edu