## 7 Related Work  
Since 1996, several projects have pursued Web archiving (e.g., [45]). The Heritrix crawler [43], jointly developed by several Scandinavian national libraries and the Internet Archive through the International Internet Preservation Consortium (IIPC) [46], is a mature and efficient tool for large-scale, archival-qualitycrawling. The method of choice for memory institutions is client-side archiving based on crawling. This method is derived from a search engine crawl and has been evolved by the archiving community to achieve a greater completeness of capture and an increase in the temporal coherence of crawls. These two requirements follow from the fact that, for Web archiving, crawlers are used to build collections and not only to index [47]. These issues were addressed in the European project, LiWA (Living Web Archives) [48]. The task of crawl prioritization and focusing is the step in the crawl processing chain that combines the different analysis results and the crawl specification for filtering and ranking the URLs of a seed list. The filtering of URLs is necessary to avoid unrelated content in the archive. For content that is partly relevant, URLs need to be prioritised to focus the crawler tasks in order to perform a crawl according to relevance. A number of strategies and therefore URL ordering metrics exist for this, such as breadth-first, back link count and PageRank. PageRank and breadth-first are good strategies to crawl \"important\" content on the Web [49, 50], but since these generic approaches do not cover specific information needs, focused or topical crawls have been developed [51, 52]. Recently, [PERSON] _et al._[53] published a comparative study about the quality of different crawling strategies for gathering comparable corpora on specific topics. According to this stud,y variants of the best-N-first strategy perform best depending on quality of the seed list. The best-N-first (BFS) strategy prioritizes links according to the relevance of their parent page and fetches the first \(n\) pages of the queue. In contrast, the ARCOMEM crawler uses entities instead of n-grams to determine the relevance of a page [44]. Beside domain or topical crawl strategies \"user-centric\" [54] harvesting strategies also exist, such as Advscape [55]. The aim of Advscape is to maximize the number of harvested unique advertisements on the Web. This is achieved by dynamically generating user profiles to collect personalized advertisements from Web pages. Recently, the collection of Social Web content has gained more interest. Many services, such as Twitter, YouTube or Flickr, provide through their APIs access to structured information about users, user networks and created content. Data collection from these services is not supported by standard Web crawlers. Usually it is conducted in an _ad hoc_ manner, although some structured approaches exist [56, 57, 58]. However, these platforms focus on API access and do not archive Web pages linked from Social Web platforms. With the success of the semantic Web and linked data, also the harvesting of these resources has gained more interest. LDSpider [59] is a crawler to traverse and harvest the Linked Data Web. A similar approach is followed by the Slug [60]. A limited set of tools exist for accessing Web archives, like NutchWAX and Wayback. NutchWAX [61] is based on Apache Nutch [62]--an open source Web-search software project that supports URL- and keyword-based access. It adapts Nutch by searching against Web archives rather than crawling the Web. The Portuguese Web Archive information retrieval system [63] adapted NutchWAX to support versions of the same URL across time and to improve the ranking of results. A URL-based access to Web archives is provided by Wayback [64]. Wayback is an open source implementation of the Internet Archive Wayback Machine. It allows browsing the history of a page or domain over time. Overall, the possibilities to explore Web archives are limited to basic functionalities.