Web crawler – ScienceDaily
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What is a web crawler? | How web spiders work | Cloudflare
What is a web crawler bot?
A web crawler, spider, or search engine bot downloads and indexes content from all over the Internet. The goal of such a bot is to learn what (almost) every webpage on the web is about, so that the information can be retrieved when it’s needed. They’re called “web crawlers” because crawling is the technical term for automatically accessing a website and obtaining data via a software program.
These bots are almost always operated by search engines. By applying a search algorithm to the data collected by web crawlers, search engines can provide relevant links in response to user search queries, generating the list of webpages that show up after a user types a search into Google or Bing (or another search engine).
A web crawler bot is like someone who goes through all the books in a disorganized library and puts together a card catalog so that anyone who visits the library can quickly and easily find the information they need. To help categorize and sort the library’s books by topic, the organizer will read the title, summary, and some of the internal text of each book to figure out what it’s about.
However, unlike a library, the Internet is not composed of physical piles of books, and that makes it hard to tell if all the necessary information has been indexed properly, or if vast quantities of it are being overlooked. To try to find all the relevant information the Internet has to offer, a web crawler bot will start with a certain set of known webpages and then follow hyperlinks from those pages to other pages, follow hyperlinks from those other pages to additional pages, and so on.
It is unknown how much of the publicly available Internet is actually crawled by search engine bots. Some sources estimate that only 40-70% of the Internet is indexed for search – and that’s billions of webpages.
What is search indexing?
Search indexing is like creating a library card catalog for the Internet so that a search engine knows where on the Internet to retrieve information when a person searches for it. It can also be compared to the index in the back of a book, which lists all the places in the book where a certain topic or phrase is mentioned.
Indexing focuses mostly on the text that appears on the page, and on the metadata* about the page that users don’t see. When most search engines index a page, they add all the words on the page to the index – except for words like “a, ” “an, ” and “the” in Google’s case. When users search for those words, the search engine goes through its index of all the pages where those words appear and selects the most relevant ones.
*In the context of search indexing, metadata is data that tells search engines what a webpage is about. Often the meta title and meta description are what will appear on search engine results pages, as opposed to content from the webpage that’s visible to users.
How do web crawlers work?
The Internet is constantly changing and expanding. Because it is not possible to know how many total webpages there are on the Internet, web crawler bots start from a seed, or a list of known URLs. They crawl the webpages at those URLs first. As they crawl those webpages, they will find hyperlinks to other URLs, and they add those to the list of pages to crawl next.
Given the vast number of webpages on the Internet that could be indexed for search, this process could go on almost indefinitely. However, a web crawler will follow certain policies that make it more selective about which pages to crawl, in what order to crawl them, and how often they should crawl them again to check for content updates.
The relative importance of each webpage: Most web crawlers don’t crawl the entire publicly available Internet and aren’t intended to; instead they decide which pages to crawl first based on the number of other pages that link to that page, the amount of visitors that page gets, and other factors that signify the page’s likelihood of containing important information.
The idea is that a webpage that is cited by a lot of other webpages and gets a lot of visitors is likely to contain high-quality, authoritative information, so it’s especially important that a search engine has it indexed – just as a library might make sure to keep plenty of copies of a book that gets checked out by lots of people.
Revisiting webpages: Content on the Web is continually being updated, removed, or moved to new locations. Web crawlers will periodically need to revisit pages to make sure the latest version of the content is indexed.
requirements: Web crawlers also decide which pages to crawl based on the protocol (also known as the robots exclusion protocol). Before crawling a webpage, they will check the file hosted by that page’s web server. A file is a text file that specifies the rules for any bots accessing the hosted website or application. These rules define which pages the bots can crawl, and which links they can follow. As an example, check out the file.
All these factors are weighted differently within the proprietary algorithms that each search engine builds into their spider bots. Web crawlers from different search engines will behave slightly differently, although the end goal is the same: to download and index content from webpages.
Why are web crawlers called ‘spiders’?
The Internet, or at least the part that most users access, is also known as the World Wide Web – in fact that’s where the “www” part of most website URLs comes from. It was only natural to call search engine bots “spiders, ” because they crawl all over the Web, just as real spiders crawl on spiderwebs.
Should web crawler bots always be allowed to access web properties?
That’s up to the web property, and it depends on a number of factors. Web crawlers require server resources in order to index content – they make requests that the server needs to respond to, just like a user visiting a website or other bots accessing a website. Depending on the amount of content on each page or the number of pages on the site, it could be in the website operator’s best interests not to allow search indexing too often, since too much indexing could overtax the server, drive up bandwidth costs, or both.
Also, developers or companies may not want some webpages to be discoverable unless a user already has been given a link to the page (without putting the page behind a paywall or a login). One example of such a case for enterprises is when they create a dedicated landing page for a marketing campaign, but they don’t want anyone not targeted by the campaign to access the page. In this way they can tailor the messaging or precisely measure the page’s performance. In such cases the enterprise can add a “no index” tag to the landing page, and it won’t show up in search engine results. They can also add a “disallow” tag in the page or in the file, and search engine spiders won’t crawl it at all.
Website owners may not want web crawler bots to crawl part or all of their sites for a variety of other reasons as well. For instance, a website that offers users the ability to search within the site may want to block the search results pages, as these are not useful for most users. Other auto-generated pages that are only helpful for one user or a few specific users should also be blocked.
What is the difference between web crawling and web scraping?
Web scraping, data scraping, or content scraping is when a bot downloads the content on a website without permission, often with the intention of using that content for a malicious purpose.
Web scraping is usually much more targeted than web crawling. Web scrapers may be after specific pages or specific websites only, while web crawlers will keep following links and crawling pages continuously.
Also, web scraper bots may disregard the strain they put on web servers, while web crawlers, especially those from major search engines, will obey the file and limit their requests so as not to overtax the web server.
How do web crawlers affect SEO?
SEO stands for search engine optimization, and it is the discipline of readying content for search indexing so that a website shows up higher in search engine results.
If spider bots don’t crawl a website, then it can’t be indexed, and it won’t show up in search results. For this reason, if a website owner wants to get organic traffic from search results, it is very important that they don’t block web crawler bots.
What web crawler bots are active on the Internet?
The bots from the major search engines are called:
Google: Googlebot (actually two crawlers, Googlebot Desktop and Googlebot Mobile, for desktop and mobile searches)
Yandex (Russian search engine): Yandex Bot
Baidu (Chinese search engine): Baidu Spider
There are also many less common web crawler bots, some of which aren’t associated with any search engine.
Why is it important for bot management to take web crawling into account?
Bad bots can cause a lot of damage, from poor user experiences to server crashes to data theft. However, in blocking bad bots, it’s important to still allow good bots, such as web crawlers, to access web properties. Cloudflare Bot Management allows good bots to keep accessing websites while still mitigating malicious bot traffic. The product maintains an automatically updated allowlist of good bots, like web crawlers, to ensure they aren’t blocked. Smaller organizations can gain a similar level of visibility and control over their bot traffic with Super Bot Fight Mode, available on Cloudflare Pro and Business plans.
Best 3 Ways to Crawl Data from a Website | Octoparse
The need for crawling web data has become larger in the past few years. The data crawled can be used for evaluation or prediction in different fields. Here, I’d like to talk about 3 methods we can adopt to crawl data from a website.
1. Use Website APIs
Many large social media websites, like Facebook, Twitter, Instagram, StackOverflow provide APIs for users to access their data. Sometimes, you can choose the official APIs to get structured data. As the Facebook Graph API shows below, you need to choose fields you make the query, then order data, do the URL Lookup, make requests and etc. To learn more, you can refer to
2. Build your own crawler
However, not all websites provide users with APIs. Certain websites refuse to provide any public APIs because of technical limit or other reasons. Someone may propose RSS feeds, but because they put a limit on their use, I will not suggest or make further comments on it. In this case, what I want to discuss is that we can build a crawler on our own to deal with this situation.
How does a crawler work? A crawler, put it another way, is a method to generate a list of URLs that you can feed through your extractor. The crawlers can be defined as tools to find the URLs. You first give the crawler a webpage to start, and they will follow all these links on that page. Then this process will keep going on in a loop.
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Then, we can proceed with building our own crawler. It’s known that Python is an open-source programming language, and you can find many useful functional libraries. Here, I suggest the BeautifulSoup (Python Library) for the reason that it is easier to work with and possesses many intuitive characters. More exactly, I will utilize two Python modules to crawl the data.
BeautifulSoup does not fetch the web page for us. That’s why I use urllib2 to combine with the BeautifulSoup library. Then, we need to deal with HTML tags to find all the links within page’s tags and the right table. After that, iterate through each row (tr) and then assign each element of tr (td) to a variable and append it to a list. Let’s first look at the HTML structure of the table (I am not going to extract information for table heading
By taking this approach, your crawler is customized. It can deal with certain difficulties met in the API extraction. You can use the proxy to prevent it from being blocked by some websites and etc. The whole process is within your control. This method should make sense for people with coding skills. The data frame you crawled should be like the figure below.
3. Take advantage of ready-to-use crawler tools
However, to crawl a website on your own by programming may be time-consuming. For people without any coding skills, this would be a hard task. Therefore, I’d like to introduce some crawler tools.
Octoparse is a powerful visual windows-based web data crawler. It is really easy for users to grasp this tool with its simple and friendly user interface. To use it, you need to download this application on your local desktop.
As the figure shown below, you can click-and-drag the blocks in the Workflow Designer pane to customize your own task. Octoparse provides two editions of crawling service subscription plans – the Free Edition and Paid Edition. Both can satisfy the basic scraping or crawling needs of users. With the Free Edition, you can run your tasks on the local side.
If you switch your free edition to a Paid Edition, you can use the Cloud-based service by uploading your tasks to the Cloud Platform. 6 to 14 cloud servers will run your tasks simultaneously with a higher speed and crawl in a larger scale. Plus, you can automate your data extraction leaving without a trace using Octoparse’s anonymous proxy feature that could rotate tons of IPs, which will prevent you from being blocked by certain websites. Here’s a video introducing Octoparse Cloud Extraction.
Octoparse also provides API to connect your system to your scraped data in real-time. You can either import the Octoparse data into your own database or use the API to require access to your account’s data. After you finish the configuration of the task, you can export data into various formats, like CSV, Excel, HTML, TXT, and database (MySQL, SQL Server, and Oracle).
is also known as a web crawler covering all different levels of crawling needs. It offers a Magic tool which can convert a site into a table without any training sessions. It suggests users to download its desktop app if more complicated websites need to be crawled. Once you’ve built your API, they offer a number of simple integration options such as Google Sheets,, Excel as well as GET and POST requests. When you consider that all this comes with a free-for-life price tag and an awesome support team, is a clear first port of call for those on the hunt for structured data. They also offer a paid enterprise-level option for companies looking for more large scale or complex data extraction.
Mozenda is another user-friendly web data extractor. It has a point-and-click UI for users without any coding skills to use. Mozenda also takes the hassle out of automating and publishing extracted data. Tell Mozenda what data you want once, and then get it however frequently you need it. Plus, it allows advanced programming using REST API the user can connect directly with Mozenda account. It provides the Cloud-based service and rotation of IPs as well.
SEO experts, online marketers and even spammers should be very familiar with ScrapeBox with its very user-friendly UI. Users can easily harvest data from a website to grab emails, check page rank, verify working proxies and RSS submission. By using thousands of rotating proxies, you will be able to sneak on the competitor’s site keywords, do research on sites, harvesting data, and commenting without getting blocked or detected.
Google Web Scraper Plugin
If people just want to scrape data in a simple way, I suggest you choose the Google Web Scraper Plugin. It is a browser-based web scraper that works like Firefox’s Outwit Hub. You can download it as an extension and have it installed in your browser. You need to highlight the data fields you’d like to crawl, right-click and choose “Scrape similar…”. Anything that’s similar to what you highlighted will be rendered in a table ready for export, compatible with Google Docs. The latest version still had some bugs on spreadsheets. Even though it is easy to handle, notice to all users, it can’t scrape images and crawl data in a large amount.
Artículo en español: 3 Mejores Formas de Crawl Datos desde WebsiteTambién puede leer artículos de web scraping en el Website Oficial
Artikel auf Deutsch: Die 3 besten Methoden zum Crawlen von Daten aus einer WebsiteSie können unsere deutsche Website besuchen.
Author: The Octoparse Team
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Frequently Asked Questions about what does crawling a website mean
What is a Web crawler and how does it work?
A web crawler, or spider, is a type of bot that is typically operated by search engines like Google and Bing. Their purpose is to index the content of websites all across the Internet so that those websites can appear in search engine results.
How do I crawl a website?
Best 3 Ways to Crawl Data from a WebsiteUse Website APIs. Many large social media websites, like Facebook, Twitter, Instagram, StackOverflow provide APIs for users to access their data. … Build your own crawler. However, not all websites provide users with APIs. … Take advantage of ready-to-use crawler tools.Sep 8, 2021
Is crawling a website legal?
If you’re doing web crawling for your own purposes, it is legal as it falls under fair use doctrine. The complications start if you want to use scraped data for others, especially commercial purposes. … As long as you are not crawling at a disruptive rate and the source is public you should be fine.Jul 17, 2019