1 Introduction

We implemented netSifter within IBM's WebFountain [3] to counter the challenges faced by current crawler methodologies. netSifter consists of a scalable, flexible architecture that approaches the Web as a collection of websites, so as to avoid evaluating web pages one by one. The unique concept of the solution is to rank the URL frontier by making page-level judgements based on knowledge of the originating site. This site knowledge is created by performing various analyses on a sample of pages from the site, and subsequently formulating an overall site score. Future pages from the site can then be prioritized in relation to other pages via the site score. The rationale for this idea is that pages from a common site are related, and share characteristics indicative of quality.

netSifter is extensible to varied needs, allowing it to take advantage of both modern focused crawling techiques [1] and full-web crawling strategies [2]. For example, netSifter can make use of relatively expensive content-based analyses while remaining scalable, since only a sample of pages from a site are examined. Additionally, netSifter can make simplified use of popularity measures by, for example, examining outlinks from the sample set of web pages, or by incorporating link-based ranking results into the site scores periodically. Since netSifter employs a plurality of analysis techniques, a website is not excluded or included based on any one metric.

2 System Details

1 Architecture

• Site Score Database The site score database serves as the link between the online and offline analysis stages, where online analysis queries for site scores in order to prioritize the URL frontier, and offline analysis generates or updates them. Site scores represent the relative value of a site compared to any other site.
• Scheduler The crawler perpetually iterates over a list of all URLs that have been discovered or seeded. The list is retrieved in batches, and each URL in the batch is given a score based on its website's score. The batch is sorted by score in descending order, and then sent to the fetcher. The fetcher is allocated a time period in which to fetch as many URLs as possible, though it is generally not able to exhaust a given list. If a site score does not exist, a slightly higher than neutral score is assigned, as unscored sites are favored for their potential to contain novel content.
• Online Analysis Manager In addition to being sent to the main system for storage and indexing, all newly fetched pages are routed to the online analysis manager, which determines whether or not a page should be sent to the offline analysis stage. If a page is empty, contains any ``soft'' errors, or fails other basic data-validity tests, it is discarded. A page is sent offline if no site score for the page exists, or if a sufficient period of time has elapsed since a site score was produced.
• Offline Analytics Chain The offline analytics chain passes a page through various annotators, each of which scores the page based on various algorithms. The final annotator in the chain combines the individual scores into a weighted average and applies optional heuristics. The final page score is then stored temporarily in a database. Once a sufficient sample of pages for a site have been collected, those page scores are aggregated and submitted to the site score database.

2 Sampling Method

We require a website to pass the t-test for three sequential observations in order to provide an opportunity for the crawl to move past locally consistent content, and find a better estimate of the mean for the entire site. A maximum sample size is set to avoid analyzing too many pages for websites which are unable to produce a consistent site score within a reasonable sample size.

3 Results and Conclusion

Figure 1: netSifter and SiteRank score correlation

Among the top 1000 SiteRank scores, 148 sites were scored negatively by netSifter. There were many Asian language, spam, link farm, and adult content sites. The presence of Asian sites indicates that some annotators improperly handle non-English content. netSifter correctly scored spam and adult content sites, even though these sites were rated well by SiteRank. Among the bottom 1000 SiteRank scores, 373 sites were scored positively by netSifter. Manual examination of these sites rated a large majority as postive. This shows that netSifter was able to identify interesting sites which were not well-connected.

The second experiment compares a netSifter crawler against an unbiased crawler. The utility of the crawlers was measured using ODP Utility (the count of pages crawled from sites which appear in the ODP listings, ODP Utility (Weighted) (the sum of the number of pages crawled from a given site multiplied by the number of site appearances in the ODP, and SiteRank Utility (the number of pages crawled from a given site multipled by the SiteRank of that site. The results are shown in Table . Though netSifter crawled fewer pages than the unbiased crawl, it outperformed it on ODP Utility (Weighted) and SiteRank Utility.

netSifter demonstrates that by exploiting the logical association of a web page to a website, and then forming an estimate of the overall quality of a website, the URL frontier of a web-scale crawler can be effectively prioritized to bias the crawler towards higher-quality content.

Bibliography

1

S. Chakrabarti, M. van den Berg, and B. Dom.
Focused crawling: a new approach to topic-specific Web resource discovery.
Computer Networks (Amsterdam, Netherlands: 1999), 1999.

2

J. Cho, H. García-Molina, and L. Page.
Efficient crawling through URL ordering.
Computer Networks and ISDN Systems, 1998.

3

D. Gruhl, L. Chavet, D. Gibson, J. Meyer, P. Pattanayak, A. Tomkins, and J. Zien.
How to build a webfountain: An architecture for very large-scale text analytics.
IBM Systems Journal, 43(1):64-77, 2004.

4

International Business Machines Company.
http:/www.research.ibm.com/UIMA.

Footnotes

... González^1#1

Work done at IBM Almaden Research Center.