Overview of working principles and key technologies of crawlers:
Web crawler is an automatic extracting program which downloads Web pages from Internet and is an important part of search engine. The conventional crawler starts from one or several urls of initial web pages and obtains the urls of initial web pages. In the process of crawling web pages, it continuously extracts new urls from the current page until it meets some stop conditions of the system.
Compared with ordinary web crawlers, a focused crawler needs to solve three main problems:
1. Describe or define the fetching target.
2. Analyze and filter web pages or data.
3. Search for URL policies.
How to make web page analysis algorithm and URL search strategy is the basis to determine the target of capture. Among them, Web analysis algorithm and candidate URL sorting algorithm are the key to determine the service form and crawl behavior provided by search engine. There is a close relationship between the two algorithms.
With the popularity of big data, web crawler has become the mainstream technology today. Not only programmers, but also ordinary users have simple knowledge of crawler and know how to use proxy IP to crawler. As we all know, crawlers can obtain website information, so what are the benefits for focus web crawlers? Is this a crawler technique? Next, we'll take a look at how to focus attention on reptiles.
The work flow of focus crawler is quite complex. It is necessary to filter the links irrelevant to the topic according to certain analysis algorithm, reserve the useful links, and then put them into the URL queue waiting to be captured. It then selects the next web URL it wants to grab from the queue according to a specific search strategy, and repeats the above steps until it reaches a certain standard of the system.
In addition, all pages captured by crawlers will be stored in the system for some analysis. Filter and index for later query and retrieval; For the focused crawler, the analysis results obtained through this process can also provide feedback and guidance for the subsequent grasping process.
The above mainly introduces the focus on the content of the crawler, crawler is similar to it, but there are also differences, naturally will also be restricted by the crawler. In this case, we need to use crawler techniques such as proxy IP to help us.
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