In the process of data analysis, a large amount of data is required first, and crawlers are usually used to collect information of peers for analysis and dig out useful information. However, there are a lot of such materials, which only rely on manual collection and take a long time. There is also a lazy method, that is, using crawlers for data crawling. How do crawlers collect data? The essence of Web crawler is an HTTP request, the browser is the user's active operation, and then complete the HTTP request, crawler needs a complete set of architecture to complete, crawler needs a complete set of architecture to complete.
1. Website management.
The urlManager starts by adding a new URL to the background collection, determining whether the url to be added is in the container or the url to be crawled, then retrieving the url to be crawled, and moving the URL from the set of urls to be crawled.
2. Download web pages.
The downloader passes the received URL to the Internet, which returns the HTML file to the downloader, which stores it locally and typically deploys it distributed, one as a submission and the other as a request broker.
3. Extract content. The main task of the page parser is to obtain valuable data and a new LIST of urls from the obtained HTML page string.
Common data extraction methods include CSS selectors, regex, and xpath rule extraction. Usually after the extraction is complete, there is some cleaning or customization of the data, so that the requested unstructured data can be transformed into the required structured data.
4. Keep data.
Data is stored in relevant databases, queues, and files for data calculation and application interconnection.
How do crawlers collect data? As can be seen from the above introduction, a complete crawler life cycle includes: web site management, web page download, content extraction and preservation.
Many times of crawler will affect the server of the other party, so the other party will take anti-crawler measures, such as IP restriction, verification code detection, etc. In order to successfully complete the data acquisition task, it is necessary to further study how to break through the anti-crawler mechanism.
At the same time, it is necessary to reduce the acquisition speed, abide by the crawler agreement and act in accordance with the law!
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