A web crawler (also known as a web spider, spider bot, web bot, or simply a crawler) is a computer software program that is used by a search engine to index web pages and content across the World Wide Web.
Indexing is quite an essential process as it helps users find relevant queries within seconds. The search indexing can be compared to the book indexing. For instance, if you open the last pages of a textbook, you will find an index with a list of queries in alphabetical order and pages where they are mentioned in the textbook. The same principle underlines the search index, but instead of page numbering, a search engine shows you some links where you can look for answers to your inquiry.
The significant difference between the search and book indices is that the former is dynamic, therefore, it can be changed, and the latter is always static.
How Does a Web Search Work?
Before plunging into the details of how a crawler robot works, let’s see how the whole search process is executed before you get an answer to your search query.
For instance, if you type “What is the distance between Earth and Moon” and hit enter, a search engine will show you a list of relevant pages. Usually, it takes three major steps to provide users with the required information to their searches:
- A web spider crawls content on websites
- It builds an index for a search engine
- Search algorithms rank the most relevant pages
Also, one needs to bear in mind two essential points:
- You do not do your searches in real-time as it is impossible
There are plenty of websites on the World Wide Web, and many more are being created even now when you are reading this article. That is why it could take eons for a search engine to come up with a list of pages that would be relevant to your query. To speed up the process of searching, a search engine crawls the pages before showing them to the world.
- You do not do your searches in the World Wide Web
Indeed, you do not perform searches in the World Wide Web but in a search index and this is when a web crawler enters the battlefield.
What Is a Web Crawler? How Does a Web Crawler Work?
There are many search engines out there − Google, Bing, Yahoo!, DuckDuckGo, Baidu, Yandex, and many others. Each of them uses its spider bot to index pages.
They start their crawling process from the most popular websites. Their primary purpose of web bots is to convey the gist of what each page content is all about. Thus, web spiders seek words on these pages and then build a practical list of these words that will be used by a search engine next time when you want to find information about your query.
All pages on the Internet are connected by hyperlinks, so site spiders can discover those links and follow them to the next pages. Web bots only stop when they locate all content and connected websites. Then they send the recorded information a search index, which is stored on servers around the globe. The whole process resembles a real-life spider web where everything is intertwined.
Crawling does not stop immediately once pages have been indexed. Search engines periodically use web spiders to see if any changes have been made to pages. If there is a change, the index of a search engine will be updated accordingly.
What Are the Main Web Crawler Types?
Web crawlers are not limited to search engine spiders. There are other types of web crawling out there.
Email crawling
Email crawling is especially useful in outbound lead generation as this type of crawling helps extract email addresses. It is worth mentioning that this kind of crawling is illegal as it violates personal privacy and can’t be used without user permission.
News crawling
With the advent of the Internet, news from all over the world can be spread rapidly around the Web, and to extract data from various websites can be quite unmanageable.
There are many web crawlers that can cope with this task. Such crawlers are able to retrieve data from new, old, and archived news content and read RSS feeds. They extract the following information: date of publishing, the author’s name, headlines, lead paragraphs, main text, and publishing language.
Image crawling
As the name implies, this type of crawling is applied to images. The Internet is full of visual representations. Thus, such bots help people find relevant pictures in a plethora of images across the Web.
Social media crawling
Social media crawling is quite an interesting matter as not all social media platforms allow to be crawled. You should also bear in mind that such type of crawling can be illegal if it violates data privacy compliance. Still, there are many social media platform providers that are fine with crawling. For instance, Pinterest and Twitter allow spider bots to scan their pages if they are not user-sensitive and do not disclose any personal information. Facebook, LinkedIn are strict regarding this matter.
