​Web crawler: definition and data crawling process

Web crawler (Web Crawler), as an automated data collection tool, is gradually playing an irreplaceable role in scientific research, business analysis, data mining and other fields. This article aims to explore the definition of web crawlers and the basic process of how to crawl data.

1. Definition of web crawler

Web crawler, also known as web spider, web robot, is a program or script that automatically crawls World Wide Web information according to certain rules. They are widely used in search engines, data analysis, information monitoring and other fields. Simply put, web crawlers simulate the operation of crawling data on browsers, automatically access web pages on the Internet, and crawl data on the pages.

2. How web crawlers crawl data

Determine the target website and crawling rules

Before starting to crawl data, you first need to determine the target website and crawling rules to be crawled. This includes determining the URL of the web page to be crawled, which data on the page needs to be crawled, and the storage format of the data.

Send HTTP request

Web crawlers access target web pages by sending HTTP requests. HTTP request contains information such as the requested URL, request method (such as GET, POST), request header (such as User-proxy, Cookie, etc.). When the crawler sends an HTTP request, the target server will return the corresponding HTTP response, which contains the HTML code of the web page.

Parse HTML code

After receiving the HTTP response, the crawler needs to parse the returned HTML code to extract the required data. This usually requires the use of HTML parsing libraries such as BeautifulSoup, lxml, etc. Parsing libraries can help crawlers identify elements, attributes, and text in HTML documents to extract the required data.

Store and process data

After extracting the data, the crawler needs to store the data in local files, databases, or cloud storage. At the same time, the data needs to be cleaned, deduplicated, formatted, etc. for subsequent analysis and use.

Comply with anti-crawler mechanisms

In the process of crawling data, the crawler needs to comply with the anti-crawler mechanisms of the target website. These mechanisms include limiting access frequency, verification code verification, user login, etc. If the crawler does not comply with these mechanisms, it may be blocked or restricted from access by the target website.

Iterative crawling and updating

For scenarios where data needs to be updated regularly, crawlers need to implement iterative crawling. This usually involves maintaining a queue of URLs to be crawled and taking URLs from the queue for crawling according to certain strategies. At the same time, crawled data needs to be updated regularly to ensure the timeliness and accuracy of the data.