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1.2 RDFa, Microdata and Microformats Extraction Framework

In order to support web applications to understand the content of HTML pages, an increasing number of websites have started to semantically markup their pages, that is, embed structured data describing products, people, organizations, places, events, etc. into HTML pages using such markup standards as Microformats[1], RDFa[2] and Microdata[3]. Microformats use style definitions to annotate HTML text with terms from a fixed set of vocabularies, RDFa allows embedding any kind of RDF data into HTML pages, and Microdata is part of the HTML5 standardization effort allowing the use of arbitrary vocabularies for structured data.

The embedded data is crawled together with the HTML pages by search engines, such as Google, Yahoo! and Bing, which use these data to enrich their search results. Up to now, only these companies were capable of providing insights [15] into the amount as well as the types of data that are published on the web using different markup standards as they were the only ones possessing large-scale web crawls. However, the situation changed with the advent of the Common Crawl[4], a non-profit foundation that crawls the web and regularly publishes the resulting corpora for public usage on Amazon S3.

For the purpose of extracting structured data from these large-scale web corpora we have developed the RDFa, Microdata and Microformats extraction framework that is available online[5].

The extraction consists of the following steps. Firstly, a file with the crawled data, in the form of ARC or WARC archive, is downloaded from the storage. The archives usually contain up to several thousands of archived web pages. The framework relies on the Anything To Triples (Any23)[6] parser library for extracting RDFa, Microdata, and Microformats from HTML content. Any23 outputs RDF quads, consisting of subject, predicate, object, and a URL which identifies the HTML page from which the triple was extracted. Any23 parses web pages for structured data by building a DOM tree and then evaluates XPath expressions to extract the structured data. As we have found that the tree generation accounts for much of the parsing cost, we have introduced the filtering step: We run regular expressions against each archived HTML page prior to extraction to detect the presence of structured data, and only run the Any23 extractor when potential matches are found. The output of the extraction process is in NQ (RDF quads) format.

We have made available two implementations of the extraction framework, one based on the Amazon Web Services, and the second one being a Map/Reduce implementation that can be run over any Hadoop cluster. Additionally, we provide a plugin to the Apache Nutch crawler allowing the user to configure the crawl and then extract structured data from the resulting page corpus.

To verify the framework, three large scale RDFa, Microformats and Microdata extractions have been performed, corresponding to the Common Crawl data from 2009/2010, August 2012 and November 2013. The results of the 2012 and 2009/2010 are published in [2] and [16], respectively. Table 1 presents the comparative summary of the three extracted datasets. The table reports the number and the percentage of URLs in each crawl containing structured data, and gives the percentage of these data represented using Microformats, RDFa and Microdata, respectively.

Table 1. Large-scale RDF datasets extracted from Common Crawl (CC): summary

CC 2009/2010

CC August 2012

CC November 2013

Size(TB), compressed

28.9

40.1

44

Size, URLs

2,565,741,671

3,005,629,093

2,224,829,946

Size, Domains

19,113,929

40,600,000

12,831,509

Parsing cost, USD

576

398

263

Structured data, URLs with triples

147,871,837

369,254,196

585,792,337

Structured data, in %

5.76

12.28

26.32

Microformats, in %

96.99

70.98

47.48

RDFa, in %

2.81

22.71

26.48

Microdata, in %

0.2

6.31

26.04

Average num. of triples per URL

3.35

4.05

4.04

The numbers illustrate the trends very clearly: in the recent years, the amount of structured data embedded into HTML pages keeps increasing. The use of Microformats is decreasing rapidly, while the use of RDFa and especially Microdata standards has increased a lot, which is not surprising as the adoption of the latter is strongly encouraged by the biggest search engines. On the other hand, the average number of triples per web page (only pages containing structured data are considered) stays the same through the different version of the crawl, which means that the data completeness has not changed much.

Concerning the topical domains of the published data, the dominant ones are: persons and organizations (for all three formats), blogand CMS-related metadata (RDFa and Microdata), navigational metadata (RDFa and Microdata), product data (all three formats), and event data (Microformats). Additional topical domains with smaller adoption include job postings (Microdata) and recipes (Microformats). The data types, formats and vocabularies seem to be largely determined by the major consumers the data is targeted at. For instance, the RDFa portion of the corpora is dominated by the vocabulary promoted by Facebook, while the Microdata subset is dominated by the vocabularies promoted by Google, Yahoo! and Bing via schema.org.

More detailed statistics on the three corpora are available at the Web Data Commons page[7].

By publishing the data extracted from RDFa, Microdata and Microformats annotations, we hope on the one hand to initialize further domain-specific studies by third parties. On the other hand, we hope to lay the foundation for enlarging the number of applications that consume structured data from the web.

  • [1] microformats.org/
  • [2] w3.org/TR/xhtml-rdfa-primer/
  • [3] w3.org/TR/microdata/
  • [4] commoncrawl.org/
  • [5] https://subversion.assembla.com/svn/commondata/
  • [6] https://any23.apache.org/
  • [7] webdatacommons.org
 
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