What is a Web Crawler?
Speaking of web crawlers, what comes to your mind? A spider crawling on spiderwebs? That's what actually a web crawler does. It crawls over the web like a spider.
To give you a precise definition of a web crawler, it is an Internet bot, also known as web spider, automatic indexer, web robot, that automatically scans information through the Web to create an index of the data. This process is called web crawling. It is called a web crawler because crawling is a term to describe the behavior of automatically accessing websites and acquiring data through scraping tools.
Web crawlers are operated frequently by search engines, such as Google and Yahoo. The most famous web crawler is Googlebot. Do you ever wonder what makes search engines go around and work? With web crawlers, search engines can present relevant web pages results responding to users' search inputs.
How Does a Web Crawler Work?
Now, you have a basic idea about what a web crawler is. You may also wonder how a web crawler works. Generally, a web crawler is like an online librarian who indexes websites to update web information and evaluate the quality of the web page content.
Let's take the search engine crawler as an example. The crawler will go through many web pages to check the words on the page and where the words are used in other places. Crawlers will build a big index to include all the findings. To put it simply, the index is a list of words as well as the web pages related to those words. When you search "big data" on a certain search engine, the search will check its index and return findings for you.
Here are the major steps executed by a web crawler:
- Select a URL from a group of candidates
- Download other related and associated web pages
- Extract the URLs among the associated web pages
- Add those new URLs to the related candidate
To elaborate, a web crawler starts its work by visiting a list of websites that it has visited before. During the visits, it will also look for other related websites that worth a visit. By constant visiting, web crawlers can discover new pages or URLs, update changes to existing pages, as well as mark those dead links. When web crawlers visit a certain page, it goes through all the content of the page and then conveys it to its database. After the data on the page is captured, the words on the page will be placed into the index of a search engine. You can take the index as a huge database of words and where they appear on different pages.
Crawlers won't stop completely once they have indexed web pages. They will check if there are any changes made to the web pages from time to time. If there is something new, the index created will also be updated.
You know that there are countless web pages existing and there are also many new created, updated pages every minute and every day, so you can imagine what an exhausting job web crawlers are doing. Therefore, search engines have made some policies on the content to crawl, the order to crawl and the frequency to crawl, etc. For example, a web page that updates regularly may be crawled more frequently than those seldom make any changes. All these rules are made to help this whole process be more efficient and be more selected on the content they crawl.
Examples of Web Crawlers
Each search engine has its own web crawlers to help them update web page data. Here are some common examples:
- Bingbot for Bing
- Baiduspider for Baidu
- Slurp Bot for Yahoo!
- DuckDuckBot for DuckDuckGo
- Yandex Bot for Yandex
In such a data-driven and fast developing world, people have a great demand for data. However, not everyone has good knowledge about crawling a certain website to get their desired data. In this section, I'd like to introduce some useful and powerful web crawling tools to help you get through of it.
If you're a programmer or you're familiar with web crawling or web scraping, open-source web crawlers could be more suitable for you to manipulate. For example, Scrapy, one of the most famous open-source web crawlers available on the Web, is a free web crawling framework written in Python.
However, if you're very new to web crawling and have no knowledge of coding, let me introduce you to a powerful web crawling tool which is Octoparse.
Octoparse can quickly scrape web data from different websites. Without coding, you can turn web pages into structured spreadsheets by very simple steps. The most prominent features of Octoparse are task templates and the Cloud Service.
Octoparse has many built-in task templates for many popular and common websites, such as Amazon, Instagram, Twitter, Walmart, and YouTube, etc. With the templates, there is no need for you to configure a crawler to get your desired data. You just need to enter URLs or keywords you want to search for. Then, you just need to wait for the data to come out.
In addition, we know that some websites may apply strict anti-scraping techniques to block web crawling or web scraping behavior. Octoparse's Cloud Service is a good solution then. With Octoparse Cloud Service, you can run the task with our auto-IP rotation function to minimize the chance of being blocked. Also, you can schedule the crawler to run at your expected time so you don't need to keep eyes on the whole scraping process. Octoparse is a good tool, so if you have the web scraping needs, you should click here to give it a try.
To sum up, web crawlers play a huge role in the Internet era. Without web crawlers, you can't imagine how difficult it is to find the information you want among such an information ocean.
Artículo en español: ¿Qué Es Web Scraping (Web Crawler) y Cómo Funciona?
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