The principle and steps of web crawlers

<p style="line-height: 2;"><span style="font-size: 16px;">In today's era of information explosion, whether it's search engines crawling web pages to build indexes, companies collecting competitor pricing data, or researchers </span><a href="https://www.b2proxy.com/use-case/web" target="_blank"><span style="color: rgb(9, 109, 217); font-size: 16px;">gathering public datasets</span></a><span style="font-size: 16px;">, a key technology is indispensable — the web crawler. Web crawlers can automatically traverse web pages and extract required information, providing a continuous stream of raw data for applications such as data analysis, public opinion monitoring, and business intelligence. However, as websites implement increasingly strict anti-crawling mechanisms, how to efficiently and stably obtain data while complying with rules has become a challenge developers must face. This article will systematically introduce the basic principles and implementation steps of web crawlers, with a focus on the critical role and integration methods of proxy technology.</span></p><p style="line-height: 2;"><br></p><p style="line-height: 2;"><span style="font-size: 24px;"><strong>I. Basic Principles</strong></span></p><p style="line-height: 2;"><span style="font-size: 16px;">A web crawler is a program that automatically extracts information from the internet. Its core principle is to simulate human browsing behavior — sending requests to target servers via the </span><a href="https://www.b2proxy.com/use-case/web" target="_blank"><span style="color: rgb(9, 109, 217); font-size: 16px;">HTTP/HTTPS</span></a><span style="font-size: 16px;"> protocol, obtaining web page source code, and then parsing out the required data. The entire process is like a tireless "spider" on the World Wide Web, continuously crawling and following links.</span></p><p style="line-height: 2;"><br></p><p style="line-height: 2;"><span style="font-size: 24px;"><strong>II. Main Steps</strong></span></p><p style="line-height: 2;"><span style="font-size: 16px;">The workflow of a standard web crawler typically includes the following six steps:</span></p><p style="line-height: 2;"><br></p><p style="line-height: 2;"><span style="font-size: 19px;"><strong>1. Define targets and seed URLs</strong></span></p><p style="line-height: 2;"><span style="font-size: 16px;">Identify the websites and data fields to be scraped, and collect initial URLs (seed links).</span></p><p style="line-height: 2;"><br></p><p style="line-height: 2;"><span style="font-size: 19px;"><strong>2. Send requests</strong></span></p><p style="line-height: 2;"><span style="font-size: 16px;">The crawler sends HTTP requests to the target server, commonly using the GET method. The request headers must include fields such as User-Agent (browser identifier) to disguise as a real user and avoid being rejected by the server.</span></p><p style="line-height: 2;"><br></p><p style="line-height: 2;"><span style="font-size: 19px;"><strong>3. Receive responses</strong></span></p><p style="line-height: 2;"><span style="font-size: 16px;">The server returns a status code (e.g., 200 for success) and the web page content (usually in HTML, JSON, etc.). If the status code is 4xx or 5xx, errors must be handled or retries attempted.</span></p><p style="line-height: 2;"><br></p><p style="line-height: 2;"><span style="font-size: 19px;"><strong>4. Parse data</strong></span></p><p style="line-height: 2;"><span style="font-size: 16px;">Use tools such as regular expressions, XPath, or BeautifulSoup to extract target information from the HTML, such as text, links, image URLs, etc.</span></p><p style="line-height: 2;"><br></p><p style="line-height: 2;"><span style="font-size: 19px;"><strong>5. Store data</strong></span></p><p style="line-height: 2;"><span style="font-size: 16px;">Save the parsed structured data into files (CSV, JSON) or databases (MySQL, MongoDB).</span></p><p style="line-height: 2;"><br></p><p style="line-height: 2;"><span style="font-size: 19px;"><strong>6. Control crawl depth and deduplication</strong></span></p><p style="line-height: 2;"><span style="font-size: 16px;">Extract new links from the current page, deduplicate them (using sets or Bloom filters), add them to the pending crawl queue, and loop back to step 2.</span></p><p style="line-height: 2;"><br></p><p style="line-height: 2;"><span style="font-size: 24px;"><strong>III. Role and Integration of Proxies</strong></span></p><p style="line-height: 2;"><span style="font-size: 16px;">During actual scraping, many websites monitor IP access frequency and ban crawlers. The introduction of proxy servers is precisely to circumvent such restrictions. A </span><a href="https://www.b2proxy.com/" target="_blank"><span style="color: rgb(9, 109, 217); font-size: 16px;">proxy</span></a><span style="font-size: 16px;"> acts as an "intermediary" between the client and the target server — the crawler sends requests to the proxy first, which then forwards them to the target website. The target website sees the proxy's IP address, not the crawler's real IP.</span></p><p style="line-height: 2;"><br></p><p style="line-height: 2;"><span style="font-size: 16px;">Key steps for using proxies include:</span></p><p style="line-height: 2;"><span style="font-size: 16px;">Obtain proxy IPs: Use paid proxy pools or free proxies.</span></p><p style="line-height: 2;"><span style="font-size: 16px;">Configure proxy: Set the proxy address in request parameters (e.g., the proxies parameter in the requests library).</span></p><p style="line-height: 2;"><span style="font-size: 16px;">Rotate proxies: Switch to a different proxy IP after every few requests or when a ban is detected.</span></p><p style="line-height: 2;"><span style="font-size: 16px;">Handle failures: Remove invalid proxies promptly to ensure crawling stability.</span></p><p style="line-height: 2;"><br></p><p style="line-height: 2;"><span style="font-size: 16px;">A well-designed proxy strategy not only reduces the risk of being blocked but also improves crawling concurrency and stability. By combining the basic crawler workflow with proxy technology, you can build a robust and efficient data collection system.</span></p>