I’ll bet that if you asked everyone in the country to list their topmost fears, spiders would sit comfortably in the top five (after public speaking and death, naturally).
Creepy, crawly, and quick, even small spiders can make a grown man jump. But when it comes to the Internet, spiders do more than spin their webs. Search engines use spiders (also known as web crawlers) to explore the web, not spin their own. If you have a website, a spider has creeped onto it at some point, but unusually, this is something for which you should be thankful. Without the spiders, no one could find your website on a search engine. Turns out, spiders aren’t so bad after all! Keep reading if you’re wondering, how do web crawlers work? We’ve got all the facts you need.
What Is a Web Crawler?
A web crawler is a robot that lives and works on the Internet. It is known by a variety of different names including a web spider, an ant, an automatic indexer, and a web scutter, but its purpose remains the same. A web crawler is created and employed by a search engine to update their web content or index the web content of other web sites. It copies the pages so that they can be processed later by the search engine, which indexes the downloaded pages. This allows users of the search engine to find webpages quickly. The web crawler also validates links and HTML code, and sometimes it extracts other information from the website.
How Do Web Crawlers Work?
Once a web crawler is given a list of URLs (also called seeds) to check out, it begins visiting each website and downloading the content. It also identifies all of the hyperlinks on the website and adds them to the list of URLs to visit (also known as the crawl frontier).
It’s an exhausting job when you think about how many web pages exist and how many more are being created everyday. Plus, it’s difficult for web crawlers to abstain from downloading duplicate content. To avoid wasting time on duplicates and irrelevant websites, crawlers choose which pages they should visit and use many techniques to guide their behavior. So as you wonder how do web crawlers work, keep in mind that these are very complicated creatures with a wide variety of techniques . . .
- If a crawler only wants to find certain types of MIME webpages, it can make a HEAD request to be sure that all of the found resources will be the needed type.
- A crawler can avoid some spider traps (which cause the web crawler to download an infinite number of URLs from a website) by steering clear of URLs with specific characters.
- Web crawlers sometimes modify and standardize URLs so that they can avoid crawling the same resource multiple times.
- If a crawler needs to download all of the resources from a given website, a path-ascending crawler can be used. It attempts to crawl every path in every URL on the list.
- Focused web crawlers are only interested in downloading webpages that are similar to each other. For example, academic crawlers only search for and download academic papers (they use filters to find PDF, postscript, and Word files and then use algorithms to determine if the pages are academic or not).
- Things move fast on the Internet. By the time a web crawler is finished with a long crawl, the pages it downloaded might have updated or deleted. To keep content up to date, crawlers use equations to determine websites’ freshness and age.
- Administrators can use exclusionary protocol (robots.txt) to tell crawlers which parts of their web servers should not be accessed by crawlers. This helps prevent server overloads and disrupted networks, and it gives administrators control over which parts of their website they want to open to web crawlers.
So . . . How Do Web Crawlers Work?
Using these techniques, spiders are able to explore the web and report back to their search engine bosses. Most search engines run many occurrences of their crawling programs simultaneously on multiple servers. All of the downloaded content is put into a database and the text and images are loaded onto the search engine’s index, which is a huge database of web content and the content’s location. However, while we know the general answer to “how do web crawlers work?”, the details behind of the data they report back to their search engine bosses gets used to rank websites is a topic for another day. Search engines (especially Google) try to keep the algorithms and architecture behind their web crawlers a secret. When they talk about how things work, they keep things pretty general and avoid giving away any secrets that might lead the SEO community into an exploit. This reduces spamming and prevents other search engines from stealing their secrets. An SEO can, at times, live or die based on their ability to understand these algorithms well enough to give their client an advantage.
See? Spiders aren’t so scary after all. A little secretive, perhaps, but perfectly harmless! Well…at least search engine spiders. We should probably collect all living, breathing spiders into a single pile and burn it as soon as possible.