What is a Web Crawler?

: Posted By Monika Ehrenreich; 14 March 2019; Glossary

Have you ever wondered how search engines get their results in their search index?

The answer: Through web crawlers.

In this articel you'll find out what web crawlers are and how they work.

What is a web crawler?

A web crawler is a computer program that searches the Internet (also www or world wide web) and examines websites. Other terms for web crawlers are:

Spiders (because they wander figuratively through the worldwide web),
Robot (because the machine works automatically) or
Searchbot (because the robot searches websites).

Search engines use web crawlers to automatically analyze pages and add them to their index. Analyzing a page is called crawling (because the small spiders crawl from one URL to another across the wide web).

Some of the best-known web crawlers and their operators

GoogleBot		The Googlebot is one of the most popular web crawlers on the Internet because it is used to index content for the Google search engine and because it gives us many tools (webmaster tools, analytics, etc.) and control over the process.
Bingbot		Bingbot is a web crawler that was released in 2010 to replace Microsoft's earlier MSN bot to deliver information to its Bing search engine.
Slurp Bot		Yahoo's search results come from the Yahoo web crawler Slurp and the Bing web crawler. Slurp also collects content from partner sites for inclusion on websites such as Yahoo News, Yahoo Finance and Yahoo Sports and accesses pages from websites on the Internet to verify accuracy and improve Yahoo's personalized content for its users.
DuckDuckBot		DuckDuckBot is the web crawler for DuckDuckGo, a search engine that has become very popular lately because it is known for privacy and does not spy on its users. Today, more than 12 million queries are processed every day.
Baiduspider		Baiduspider is the web crawler of the Chinese search engine Baidu. It crawls websites and delivers updates to the Baidu index. Baidu is the leading Chinese search engine with a market share of 80% of the total search engine market in China.
Yandex Bot		YandexBot is the web crawler of Yandex, one of the largest Russian search engines. The search engine is the clear market leader in the field of Internet search in Russia with 64% market share. Yandex also has a strong presence in some other Eastern European countries.

Is a web crawler a Search Engine?

In 1993, Matthew Gray at MIT developed the World Wide Web Wanderer as the first web crawler to measure the size of the internet. This was based on the programming language Perl.

The first publicly accessible full-text index search engine was developed in 1994 by CSE student Brian Pinkerton in his spare time.

webcrawler

From its name WebCrawler comes the term Web Crawler for a program that searches the Internet.

Today there are many search engines and many different web crawlers. Search engines need web crawlers to be able to search pages.

How does a web crawler work?

A web crawler is software based on the client-server model. That is, it is not a desktop application, but web crawlers get from one website to another via links, just like when surfing through a browser.

Therefore, a good link building is important for search engines and SEO.

At the beginning of the process, one or more URLs are entered from which the Web crawlers start. The new links are added to the list of known URLs. This process is programmed as an algorithm.

With an algorithm, a calculation process is specified that repeats itself according to a certain scheme. Ada Lovelace recorded the first computer algorithm. The programming language Ada was named after her.

Can a web crawler search the entire Internet?

Theoretically, web crawlers can search all linked sites. However, some search engine operators, such as Google, Yahoo and Bing, have agreed on the 1994 Robots Exclusion Standard protocol to control the behavior of web crawlers on web pages.

The web crawlers must first search the root directory of a domain, the root directory, for the https://www.domain-beispiel.com/robots.txt file. This is where the Web crawlers read out whether they can follow the links on the Web page and for which crawlers this applies.

User-agent: * means that this section applies to all Web crawlers.
Disallow: / tells the web crawlers that it is NOT allowed to follow the links.

However, this does not prevent access by malicious software. In addition, anyone can see which pages they want to block for Web crawlers.

What do I need to keep in mind when doing search engine optimization for web crawlers?

For a website to be displayed in search results, it must first be included in the search engine's search index. SEO experts ensure that the web pages are optimized for the web crawlers of the search engines.

Sometimes it makes sense to block individual pages for certain web crawlers. This can be set using the meta tags of the page.

With noindex you make the search engines understand that the respective page should not be included in the index.
With nofollow you show the web crawlers that the links on the page should not be followed.
For SEO you can also use your own bots to detect and correct errors.

How can I program web crawlers?

Of course, you can write the software for your own web crawlers yourself. There are instructions and tutorials for different programming languages.

Here are some examples:

and a web crawler tutorial (video) in 7 parts.

The first part: Make your Own Web Crawler - Part 1 - The Basics

Are web crawler tools also available online or as open source?

Yes, you can try the Web Crawler online. Here is a small list of tools - online or for download.

20 Web Crawling Tools (aktualisiert am 22.1.2019)
Top 50 Open Source Web Crawlers (12.9.2018)
WebCrawler

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