外文翻译--网络爬虫(共23页).doc

精选优质文档-倾情为你奉上 Crawling the web is deceptively simple: the basic algorithm is (a)Fetch a page (b) Parse it to extract all linked URLs (c) For all the URLs not seen before, repeat (a)(c). However, the size of the web (estimated at over 4 billion pages) and its rate of change (estimated at 7% per week) move this plan from a trivial programming exercise to a serious algorithmic and system design challenge. Indeed, these two factors alone imply that for a reasonably fresh and complete crawl of the web, step (a) must be executed about a thousand times per second, and thus the membership test (c) must be done well over ten thousand times per second against a set too large to store in main memory. This requires a distributed architecture, which further complicates the membership test. A crucial way to speed up the test is to cache, that is, to store in main memory a (dynamic) subset of the “seen” URLs. The main goal of this paper is to carefully investigate several URL caching techniques for web crawling. We consider both practical algorithms: random replacement, static cache, LRU, and CLOCK, and theoretical limits: clairvoyant caching and infinite cache. We performed about 1,800 simulations using these algorithms with various cache sizes, using actual log data extracted from a massive 33 day web crawl that issued over one billion HTTP requests. Our main conclusion is that caching is very effective in our setup, a cache of roughly 50,000 entries can achieve a hit rate of almost 80%. Interestingly, this cache size falls at a critical point: a substantially smaller cache is much less effective while a substantially larger cache brings little additional benefit. We conjecture that such critical points are inherent to our problem and venture an explanation for this phenomenon. 1. INTRODUCTION A recent Pew Foundation study 31 states that “Search engines have become an indispensable utility for Internet users” and estimates that as of mid-2002, slightly over 50% of all Americans have used web search to find information. Hence, the technology that powers web search is of enormous practical interest. In this paper, we concentrate on one aspect of the search technology, namely the process of collecting web pages that eventually constitute the search engine corpus. Search engines collect pages in many ways, among them direct URL submission, paid inclusion, and URL extraction from nonweb sources, but the bulk of the corpus is obtained by recursively exploring the web, a process known as crawling or SPIDERing. The basic algorithm is (a) Fetch a page (b) Parse it to extract all linked URLs (c) For all the URLs not seen before, repeat (a)(c) Crawling typically starts from a set of seed URLs, made up of URLs obtained by other means as described above and/or made up of URLs collected during previous crawls. Sometimes crawls are started from a single well connected page, or a directory such as , but in this case a relatively large portion of the web (estimated at over 20%) is never reached. See 9 for a discussion of the graph structure of the web that leads to this phenomenon. If we view web pages as nodes in a graph, and hyperlinks as directed edges among these nodes, then crawling becomes a process known in mathematical circles as graph traversal. Various strategies for graph traversal differ in their choice of which node among the nodes not yet explored to explore next. Two standard strategies for graph traversal are Depth First Search (DFS) and Breadth First Search (BFS) they are easy to implement and taught in many introductory algorithms classes. (See for instance 34). However, crawling the web is not a trivial programming exercise but a serious algorithmic and system design challenge because of the following two factors. 1. The web is very large. Currently, Google 20 claims to have indexed over 3 billion pages. Various studies 3, 27, 28 have indicated that, historically, the web has doubled every 9-12 months. 2. Web pages are changing rapidly. If “change” means “any change”, then about 40% of all web pages change weekly 12. Even if we consider only pages that change by a third or more, about 7% of all web pages change weekly 17. These two factors imply that to obtain a reasonably fresh and 679 complete snapshot of the web, a search engine must crawl at least 100 million pages per day. Therefore, step (a) must be executed about 1,000 times per second, and the membership test in step (c) must be done well over ten thousand times per second, against a set of URLs that is too large to store in main memory. In addition, crawlers typically use a distributed architecture to crawl more pages in parallel, which further complicates the membership test: it is possible that the membership question can only be answered by a peer node, not locally. A crucial way to speed up the membership test is to cache a (dynamic) subset of the “seen” URLs in main memory. The main goal of this paper is to investigate in depth several URL caching techniques for web crawling. We examined four practical techniques: random replacement, static cache, LRU, and CLOCK, and compared them against two theoretical limits: clairvoyant caching and infinite cache when run against a trace of a web crawl that issued over one billion HTTP requests. We found that simple caching techniques are extremely effective even at relatively small cache sizes such as 50,000 entries and show how these caches can be implemented very efficiently. The paper is organized as follows: Section 2 discusses the various crawling solutions proposed in the literature and how caching fits in their model. Section 3 presents an introduction to caching techniques and describes several theoretical and practical algorithms for caching. We implemented these algorithms under the experimental setup described in Section 4. The results of our simulations are depicted and discussed in Section 5, and our recommendations for practical algorithms and data structures for URL caching are presented in Section 6. Section 7 contains our conclusions and directions for further research.2. CRAWLINGWeb crawlers are almost as old as the web itself, and numerous crawling systems have been described in the literature. In this section, we present a brief survey of these crawlers (in historical order) and then discuss why most of these crawlers could benefit from URL caching. The crawler used by the Internet Archive 10 employs multiple crawling processes, each of which performs an exhaustive crawl of 64 hosts at a time. The crawling processes save non-local URLs to disk; at the end of a crawl, a batch job adds these URLs to the per-host seed sets of the next crawl. The original Google crawler, described in 7, implements the different crawler components as different processes. A single URL server process maintains the set of URLs to download; crawling processes fetch pages; indexing processes extract words and links; and URL resolver processes convert relative into absolute URLs, which are then fed to the URL Server. The various processes communicate via the file system. For the experiments described in this paper, we used the Mercator web crawler 22, 29. Mercator uses a set of independent, communicating web crawler processes. Each crawler process is responsible for a subset of all web servers; the assignment of URLs to crawler processes is based on a hash of the URLs host component. A crawler that discovers an URL for which it is not responsible sends this URL via TCP to the crawler that is responsible for it, batching URLs together to minimize TCP overhead. We describe Mercator in more detail in Section 4. Cho and Garcia-Molinas crawler 13 is similar to Mercator. The system is composed of multiple independent, communicating web crawler processes (called “C-procs”). Cho and Garcia-Molina consider different schemes for partitioning the URL space, including URL-based (assigning an URL to a C-proc based on a hash of the entire URL), site-based (assigning an URL to a C-proc based on a hash of the URLs host part), and hierarchical (assigning an URL to a C-proc based on some property of the URL, such as its top-level domain). The WebFountain crawler 16 is also composed of a set of independent, communicating crawling processes (the “ants”). An ant that discovers an URL for which it is not responsible, sends this URL to a dedicated process (the “controller”), which forwards the URL to the appropriate ant. UbiCrawler (formerly known as Trovatore) 4, 5 is again composed of multiple independent, communicating web crawler processes. It also employs a controller process which oversees the crawling processes, detects process failures, and initiates fail-over to other crawling processes. Shkapenyuk and Suels crawler 35 is similar to Googles; the different crawler components are implemented as different processes. A “crawling application” maintains the set of URLs to be downloaded, and schedules the order in which to download them. It sends download requests to a “crawl manager”, which forwards them to a pool of “downloader” processes. The downloader processes fetch the pages and save them to an NFS-mounted file system. The crawling application reads those saved pages, extracts any links contained within them, and adds them to the set of URLs to be downloaded. Any web crawler must maintain a collection of URLs that are to be downloaded. Moreover, since it would be unacceptable to download the same URL over and over, it must have a way to avoid adding URLs to the collection more than once. Typically, avoidance is achieved by maintaining a set of discovered URLs, covering the URLs in the frontier as well as those that have already been downloaded. If this set is too large to fit in memory (which it often is, given that there are billions of valid URLs), it is stored on disk and caching popular URLs in memory is a win: Caching allows the crawler to discard a large fraction of the URLs without having to consult the disk-based set. Many of the distributed web crawlers described above, namely Mercator 29, WebFountain 16, UbiCrawler4, and Cho and Molinas crawler 13, are comprised of cooperating crawling processes, each of which downloads web pages, extracts their links, and sends these links to the peer crawling process responsible for it. However, there is no need to send a URL to a peer crawling process more than once. Maintaining a cache of URLs and consulting that cache before sending a URL to a peer crawler goes a long way toward reducing transmissions to peer crawlers, as we show in the remainder of this paper.3. CACHINGIn most computer systems, memory is hierarchical, that is, there exist two or more levels of memory, representing different tradeoffs between size and speed. For instance, in a typical workstation there is a very small but very fast on-chip memory, a larger but slower RAM memory, and a very large and much slower disk memory. In a network environment, the hierarchy continues with network accessible storage and so on. Caching is the idea of storing frequently used items from a slower memory in a faster memory. In the right circumstances, caching greatly improves the performance of the overall system and hence it is a fundamental technique in the design of operating systems, discussed at length in any standard textbook 21, 37. In the web context, caching is often mentionedin the context of a web proxy caching web pages 26, Chapter 11. In our web crawler context, since the number of visited URLs becomes too large to store in main memory, we store the collection of visited URLs on disk, and cache a small portion in main memory. Caching terminology is as follows: the cache is memory used to store equal sized atomic items. A cache has size k if it can store at most k items.1 At each unit of time, the cache receives a request for an item. If the requested item is in the cache, the situation is called a hit and no further action is needed. Otherwise, the situation is called a miss or a fault. If the cache has fewer than k items, the missed item is added to the cache. Otherwise, the algorithm must choose either to evict an item from the cache to make room for the missed item, or not to add the missed item. The caching policy or caching algorithm decides which item to evict. The goal of the caching algorithm is to minimize the number of misses. Clearly, the larger the cache, the easier it is to avoid misses. Therefore, the performance of a caching algorithm is characterized by the miss ratio for a given size cache. In general, caching is successful for two reasons: _ Non-uniformity of requests. Some requests are much more popular than others. In our context, for instance, a link to is a much more common occurrence than a link to the authors home pages. _ Temporal correlation or locality of reference. Current requests are more likely to duplicate requests made in the recent past than requests made long ago. The latter terminology comes from the computer memory model data needed now is likely to be close in the address space to data recently needed. In our context, temporal correlation occurs first because links tend to be repeated on the same page we found that on average about 30% are duplicates, cf. Section 4.2, and second, because pages on a given host tend to be explored sequentially and they tend to share many links. For example, many pages on a Computer Science department server are likely to share links to other Computer Science departments in the world, notorious papers, etc. Because of these two factors, a cache that contains popular requests and recent requests is likely to perform better than an arbitrary cache. Caching algorithms try to capture this intuition in various ways. We now describe some standard caching algorithms, whose performance we evaluate in Section 5.3.1 Infinite cache (INFINITE) This is a theoretical algorithm that assumes that the size of the cache is larger than the number of distinct requests.3.2 Clairvoyant caching (MIN) More than 35 years ago, L´aszl´o Belady 2 showed that if the entire sequence of requests is known in advance (in other words, the algorithm is clairvoyant), then the best strategy is to evict the item whose next request is farthest away in time. This theoretical algorithm is denoted MIN because it achieves the minimum number of misses on any sequence and thus it provides a tight bound on performance.3.3 Least recently used (LRU) The LRU algorithm evicts the item in the cache that has not been requested for the longest time. The intuition for LRU is that an item that has not been needed for a long time in the past will likely not be needed for a long time in the future, and therefore the number of misses will be minimized in the spirit of Belad