Home / Articles / Security / Web Crawlers Explained

Web Crawlers Explained

web crawler

What are Web Crawlers?

Web Crawlers are Internet Robots (bots) designed to move across websites and index all available content. Often simply referred to as Crawlers or Spiders, their actions help search engines to gather data. This data in turn helps improve search results.

The Internet is growing every day. As more people get access to the web, so too is the number of websites increasing. Today there are over 2 billion websites available. This amount of data takes immense effort for search engines to watch over.

As with every other technology, Crawlers are simply tools and can be used for good and bad. Not all Crawlers are useful and too many bad Crawlers can impact your website performance and in worst-case scenarios even bring down your website.

How do Web Crawlers Work?

Because of the massive amount of information online, search engines use crawlers to organize the information for more efficient performance. The work that Crawlers do helps them to index and serve information much more quickly.

Think of the process in a similar way as how books are organized. Without a contents page and structure, the book will be a readable but messy collection of words. The Crawler scans the available content then lists it in an organized form, creating a table of content.

This way, when someone looks for something, a quick scan of the table of content will be sufficient. Compared to that, looking over the entire collection of pages will be much more time consuming each time you want to find something.

To handle this difficult task, Crawlers are typically given a few enhanced directives to help them in their decision making. For example;

  • Relativity of importance – With so much information viable, Crawlers are given the ability to judge the importance of content from one page to another. They do this based on certain factors like the number of links and volume of web traffic.
  • Recrawling – Web content changes frequently. Crawlers are also able to estimate how often pages need to be scanned against or re-assessment in indexing. This helps to keep search results up to date.

Dealing With Crawlers

Given how important Crawlers are in helping website owners get their content listed in search, you need to handle them correctly. Helping make the Crawler’s job easier is beneficial to site owners.

Build a Site Map

There are various ways you can do this, such as with the inclusion of a site map. By creating a site map, you’re essentially helping crawlers create indexes and listing the most crucial information out for them. 

More importantly, you can help clarify the relationships between your pages. This is far more effective than relying on the Crawler’s directives to do a good job in figuring out how your site is structured. Thankfully, sitemaps can be relatively easy to generate.

Use Robots.txt

You should also always include a Robots.txt file. Websites often contain many files, not all of which are important to your search profile. Spelling out what should or should not be crawled in your robots.txt file for the Crawler is very helpful for both parties.

The robots.txt file also helps you stop some Crawlers from indexing your site. Not all Crawlers work for search engines – some may be there simply to steal data.

Relevant Reads

Know Your Crawlers

Knowing what common and useful Crawlers are is the key to keeping your side clean of bad actors. It is best to allow the most well-known search engines index your site, but for others it is really a personal choice.

The main Crawlers you should be aware of (and allow) are Googlebot (there are a few variants such as Googlebot Desktop, Googlebot Mobile, and Mediabot), Bing with Bingbot, Baidu with Baidu Spider, and Yandex with Yandex Bot.

Avoiding bad Crawlers with a robots.txt file can be difficult since many are created on-the-fly. This means that you need to create a series of defenses against them instead. Some ways of avoiding these Crawlers are by taking a challenge-based or behavioral approach.

Alternatively, you can simply use a bot management service such as that provided by Cloudflare and Imperva (among others).

Building a Web Crawler

For the curious, aside from helping search engines index pages, Crawlers are also built and used to scrape data. Crawlers like these are more specific in their purpose than search engine crawlers. Their primary goal is to gather specific types of data – not always for benevolent use.

Building a Crawler might not be the easiest thing to do, but possible if you have some technical skills. Simple Crawlers can be built with relatively little code in programming languages such as Python.

Technically, your code only needs to do three things; Send and wait for a HTTP response, parse the pages on the site, then search the parse tree. Using Python to build a web crawler is much simpler than other methods such as Java. For real world application, a web scrapping proxy like ScraperAPI may be a good idea for easy JS rendering and bypassing anti bots technology.

Final Thoughts

It is important to manage how you handle web crawlers well since they affect two important areas of your website operations. The first is search indexing, and the second is when it comes to performance.

The best way to handle them is by taking a balanced approach, since a little bit of flexibility can go a long way.

Photo of author

Article by Jerry Low

Keep Reading