Daniel - September 14, 2019

How to Build a Web Crawler

Do you intend to build your own kind of web crawler for diverse web scraping purpose? In this post, we will reveal how you can do so.

The web crawling starts with a mapping of the web and how websites are connected. The web crawlers were used by search engines to discover new pages and index them. Web crawlers were also used in other cases, such as security, to test whether a site was vulnerable or not.

Besides, Crawlers are used to collect the content of a page and then process, classify, and provide information.

However, creating your crawler is not a difficult task for anyone who knows how to code at the minimum. On the other hand, if you want to develop a much more efficient crawler, it becomes more technical.

How does a web crawler work?

To crawl a site or the whole Internet, you need an entry point. Robots( Web crawlers) need to know that a website exists to come and analyze it. A few years ago, you still had to manually submit your site to a search engine to tell them that your site was online. Now build a few links, and your website will be spotted after a while.

Once a crawler arrives on a site, it analyzes its content line by line and follows each link it finds, whether internal or external. It continues this way until it arrives on a page without a relationship or if it encounters an error such as a 404, 403, or 500, for example.

From a technical point of view, a web crawler works with an initial list of URLs called “seed”. This list is then passed to a Fetcher that retrieves all the content from each URL it analyzes. This content is redirected to a link extractor that extracts each link on the page. These URLs are stored on the one hand and the other hand, subjected to a filter that sends the useful URLs back to a URL-Seen module. This module then detects whether the URL sent has already been seen or not. If this is not the case, it is sent to the Fetcher, which retrieves the content of the page.

Not all contents can be ‘crawled.’ This is particularly the case for contents containing a flash, and sometimes Javascript. Images cannot be analyzed either, so there is no need to insert text in them. If no instructions are given to the crawler robot, it will analyze all the content of a site, without distinction. However, there is no point in wasting your “budget crawl” on some pages. It is better to focus the robot’s attention on news and current events.

This is the interest of the robots.txt file, which specifies the crawl instructions, which are the only ones that have a real added value. You can also indicate that you do not want the robot to follow links to certain pages, with the NoFollow option.

You can also specify to web crawlers that you do not want them to follow links to specific pages by using rel=”nofollow”. However, some tests have shown that GoogleBot still follows these links.

Web Crawling vs. Web scraping: What are the Similarities and Differences?

Have you ever wondered what the difference between Web crawling and web scraping is?

Web scraping is a process of using bots to extract content and data from a particular website. This is how HTML code is extracted. And, with it, the data stored in the database. This means that you can duplicate or copy all the content of the website elsewhere.

Web crawlers, on the other hand, are software, i.e. bots programmed to examine web pages or even databases to extract information. A wide variety of bot types are used, many of them are fully customizable for:

  • Identifying unique HTML site structures.

  • Extract and transform content.

  • Store data

  • Extract data from APIs.

On the other hand, bots utilize similar system resources to access websites data. Therefore, detecting either malicious or legitimate bots is a complex task.

Alternatively, Web scraping is implemented by many digital companies to collect databases. To better clarify what web scraping is, here are some use cases:

  • Search engine robots crawl a site, analyze its content, and then classify it.

  • Price comparison sites that implement bots to automatically get prices and product descriptions for allied vendors’ websites.

  • Market research companies then use it to extract data from forums and social networks.

Conversely, web scraper and web crawlers do more or less the same thing; however, web crawlers crawl through/search through the website/ database to view the available data elements whereas web scrapers go further after the crawling process to collect/retrieve the information that was crawled, which are then stored/indexed in its database.

Recommended Tools for building Web Crawler

Web crawling is a technique used for many years. Over time the technologies for carrying out automated analyses have changed, not minding the logic behind the extraction.

Here are tools which you can use to build your own web crawler:

1 .Octoparse

Octoparse is a powerful and useful scraping tool that allows you to extract different types of data from online sources. Thanks to a simple and visual interface, it is possible to configure the tool in a few steps and set the web crawler without having to write a single line of code.

In addition, Octoparse offers a premium version with a proxy with automatic rotation of the IP, access to the API, and management of the extracted data in the cloud.

  • Pros: very simple to use but also sturdy. The free version allows you to extract up to 10,000 records with the use of 10 different crawlers.

  • Cons: unfortunately does not offer a web version, but you need to download the stand-alone software that is only compatible with Windows operating systems.

2 .ParseHub

Parsehub is a desktop software available for Windows, Mac, and Linux. Its advanced features include the ability to take advantage of different IP (to avoid server crashes), integration with storage systems (such as dropbox), and scanning sites built with technologies such as Javascript and Ajax (challenging to scan from other tools).

In the free version, Parsehub allows the management of 5 projects and the crawling/scraping of 200 pages in 40 minutes.

  • Pros: instrument with very advanced functions

  • Cons: It only has a desktop software and does not have a web version.

3 .Data-Miner.io

Data Miner is a scraping tool that integrates with Google Chrome and consists of two components, the performer (Data Miner) and a creator of “recipes” (Data Miner Beta).

Through the extension, you can create scraping recipes by visually selecting the data to be extracted in a single page. Once the recipe has been created, you can visit the site and launch the tool that extracts and then downloads the resources.

In the free version, you can extract up to 500 pages per month.

  • Pros: The tool is straightforward to use and allows the extraction of data in pages not visible through a navigation system in the background

  • Cons: 500 pages/month limits in the free version may not be sufficient for some projects.

4 .Webscraper.io

Web Scraper is a Google Chrome extension that integrates with the Developer’s Console. Once launched, the extension allows you to create a sitemap of the site you want to “crawl/scrape” by selecting the various elements and providing a preview of the result.

After creating the sitemap, launch the extraction, and the tool provides you with a table with the downloaded data exportable to CSV.

  • Pros: completely free and easy to use

  • Cons: the system is fundamental and does not allow advanced extractions.

5 .Google Spreadsheets

Google Spreadsheets is the Google tool dedicated to spreadsheets (the Google version of Excel). The tool is not built as a scraping system. But thanks to the IMPORT XML function which allows the import of various types of structured data, including XML, HTML, CSV, TSV and XML RSS and ATOM feeds.

In the spreadsheet file, you have to insert the URL of the page you want to crawl and the XPath queries that identify the elements to be scanned.

Once executed, the function imports in the Google file the data of the page you are crawling.

  • Pros: allows the combination of imported data to any other information thanks to the native functions of the spreadsheets

  • Cons: The processing of imported data has a limit that is not very clear (once it was 50 formulas, then 500.) which can still create inconvenience when importing large volumes of data.

6 .ScraperApi

ScraperApi is a service designed for those who engage in voluminous scraping activities. It offers an API that allows you to manage operations of proxy rotation, resolution of CAPTCHAs, setting headless browsers, basically, and everything you need to avoid blockage during crawling/scraping activity.

ScraperApi offers its customers over 20 million IPs in 12 different countries providing unlimited bandwidth and a guaranteed uptime of 99.99% with subscription plans ranging from $ 29 to $ 249.

  • Pros: with ScraperApi, you can manage unlimited scraping activities without running into blocks of any kind.

  • Cons: To use it, you need specific expertise in the use of APIs and programming oriented scraping.


Web crawler is a program (or bot) that visits websites to scan/read their pages or specified information which is then indexed for accessibility. By implementing any of the enumerated web scraping tools above, you can automate your web crawlers to extract specified information based on your preference.

On the other hand, you can make your web crawler anonymous by making use of ProxyRack’s proxies. This will ensure that your web crawlers remains anonymous without fear of being blocked while crawling.

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