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Thursday, 14 June 2018

AN EFFICIENT METHOD FOR DEEP WEB CRAWLER BASED ON ACCURACY -A REVIEW

ABSTRACT:-

As the deep web grows at a very fast pace, there has been increased interest in techniques that help efficiently locate deep-web interfaces. However, due to the large volume of web resources and the dynamic nature of deep web, achieving wide coverage and high efficiency is a challenging issue. We propose a three-stage framework, for efficient harvesting deep web interfaces. Project experimental results on a set of representative domains show the agility and accuracy of our proposed crawler framework, which efficiently retrieves deep-web interfaces from large-scale sites and achieves higher harvest rates than other crawlers using Naïve Bayes algorithm. In this paper we have made a survey on how web crawler works and what are the methodologies available in existing system from different researchers.

INTRODUCTION:-

The deep (or hidden) web refers to the contents lie behind searchable web interfaces that cannot be indexed by searching engines. Based on extrapolations from a study done at University of California, Berkeley, it is estimated that the deep web contains approximately 91,850 terabytes and the surface web is only about 167 terabytes in 2003. More recent studies estimated that 1.9 petabytes were reached and 0.3 petabytes were consumed worldwide in 2007. An IDC report estimates that the total of all digital data created, replicated, and consumed will reach 6 petabytes in 2014. A significant portion of this huge amount of data is estimated to be stored as structured or relational data in web databases deep web makes up about 96% of all the content on the Internet, which is 500- 550 times larger than the surface web. These data contain a vast amount of valuable information and entities such as Infomine, Clusty, Books In Print may be interested in building an index of the deep web sources in a given domain (such as book). Because these entities cannot access the proprietary web indices of search engines (e.g., Google and Baidu), there is a need for an efficient crawler that is able to accurately and quickly explore the deep web databases.

 It is challenging to locate the deep web databases, because they are not registered with any search engines, are usually sparsely distributed, and keep constantly changing. To address this problem, previous work has proposed two types of crawlers, generic crawlers and focused crawlers. Generic crawlers, fetch all searchable forms and cannot focus on a specific topic. Focused crawlers such as Form-Focused Crawler (FFC) and Adaptive Crawler for Hidden-web Entries (ACHE) can automatically search online databases on a specific topic. FFC is designed with link, page, and form classifiers for focused crawling of web forms, and is extended by ACHE with additional components for form filtering and adaptive link learner. The link classifiers in these crawlers play a pivotal role in achieving higher crawling efficiency than the best-first crawler. However, these link classifiers are used to predict the distance to the page containing searchable forms, which is difficult to estimate, especially for the delayed benefit links (links eventually lead to pages with forms). As a result, the crawler can be inefficiently led to pages without targeted forms. Besides efficiency, quality and coverage on relevant deep web sources are also challenging. When selecting a relevant subset from the available content sources, FFC and ACHE prioritize links that bring immediate return (links directly point to pages containing searchable forms) and delayed benefit links. But the set of retrieved forms is very heterogeneous. For example, from a set of representative domains, on average only 16%   of forms retrieved by FFC are relevant. Furthermore, little work has been done on the source selection problem when crawling more content sources. Thus it is crucial to develop smart crawling strategies that are able to quickly discover relevant content sources from the deep web as much as possible. The propose work, achieve both wide coverage and high efficiency for a focused crawler. Our main contributions are:We propose a novel three-stage framework to address the problem of searching for hidden-web resources. Our site locating technique employs a reverse searching technique (e.g., using Google’s”link:” facility to get pages pointing to a given link) and incremental three-level site prioritizing technique for unearthing relevant sites, achieving more data sources. During the in-site exploring stage, design a link tree for balanced link prioritizing, eliminating bias toward web pages in popular directories. In the propose work an adaptive learning algorithm that performs online feature selection and uses these features to automatically construct link rankers. In the site locating stage, high relevant sites are prioritized and the crawling is focused on a topic using the contents of the root page of sites, achieving more accurate results. During the insite exploring stage, relevant links are prioritized for fast in-site searching. Project experimental results on a set of representative domains show the agility and accuracy of our proposed crawler framework, which efficiently retrieves deep-web interfaces from large-scale sites and achieves higher harvest rates than other crawlers using Naïve Bayes algorithm.

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