See also Web Bot, Web Spider, Web Scraping, Web Parsing
Web Crawlers are computer programs or “bots” that systematically navigate the web for the purpose of indexing pages and data. Web crawlers usually reference a list of starting URLs and then “crawl” through the HTML of the page identifying elements of the page and all hyperlinks which lead to other pages. A web crawler is not to be confused with web scraping, data extraction, or screen scraping.
Learn more about Mozenda – Watch the 1 minute video below!
Some companies use web crawlers to perform indexing functions on a variety of sites for a variety of reasons. Some of those functions include indexing how many and what types of pages exist, how many times certain terms are referenced, broken links or elements on a page, how many hyperlinks and backlinks are found, changes to certain web pages, or to collect page ranking information to name a few. The most popular crawlers are search engine crawlers. These crawlers have the specific task of attempt to index the web to make content more searchable and available to users.
- Indexing how many and what types of pages exist
- Counting how many times certain terms are referenced
- Noting broken links or elements on a page
- Tracking changes to certain web pages
- Collecting pagerank information
The most popular crawlers are search engine crawlers. These crawlers have the specific task of attempting to index the entire web to make content more searchable and available to users.
Web crawlers use sets of instructions or policies to determine the crawling behavior. Many crawlers do not hit “every” page, but hit enough pages to determine what is important and what
is not. The instructions that crawlers use are called policies:
- Restriction policy– exclude MIME type pages
- Normalization policy– avoid crawling the same resource more than once
- Selection policy– which pages to download and in what order
- Revisit policy– checks for changes on the pages being crawled
- Politeness policy– when and how frequently requests can be made to the website
Crawling vs. Scraping
Web crawling should not be confused with web scraping. The purpose of crawling is to find and index. The purpose of scraping is to extract textual data fields into usable file formats. Crawlers that employ regular expression algorithms can achieve some of the same results as a scraper, but these types of crawlers are usually only used for extremely specific tasks such as finding and extracting emails or lists of hyperlinks, etc. Web scrapers are designed to assist individuals who know what they want and where to find it, but don’t want to copy and paste the data from the web pages. Both crawlers and scrapers use automated processes to replicate similar actions across many pages. Certain crawlers and scrapers have the ability to access pdf and word documents and determine the contents within the file, and both can download files or images.
If you want to collect information from the web, chances are you are looking for a web scraper, not a web crawler. The Mozenda application makes it easy to capture any text or image you see on a web page. With Mozenda, all you have to do is click on the values of the fields you want to capture, and the underlying text is added to your database. You can navigate through categories and sub categories and even go as many pages deep as you need. The best thing about Mozenda is that it is all automated. After you teach Mozenda what to do once, it will copy the process and apply it to as many pages, products, locations, or items needed.
See how Mozenda works
|Refine Captured Text||4:44|
|Click the “Next” Button to Load the Next Page of Results||1:58|
|Schedule an Agent to Run Regularly||1:08|
|Combine the Contents of Two Fields||1:16|