What is a Web Parser?
Definition of a Web Parser
A web parser is a software robot that scans the Internet and downloads data it finds. Most parsers are operated by search engines such as Google, Bing, Baidu, and DuckDuckGo. Search engines apply their search algorithms to the collected data, building a search engine index. Indexes enable search engines to provide users with relevant links based on their search queries.
There are web parsers that serve not only search engines but also other purposes, such as The Wayback Machine from the Internet Archive, which provides snapshots of websites at a specific point in time in the past.
How Do Web Parsers Work?
Web parsers, such as Google's Googlebot, start each day with a list of websites they want to visit. This is called the crawl budget. The budget reflects the need for indexing pages. The crawl budget is influenced by two main factors: popularity and staleness. URLs that are more popular on the web are usually crawled more frequently to keep them fresh in the index. Web parsers also try to prevent URLs from becoming outdated in the index.
When a web parser connects to a site, it starts by downloading and reading the robots.txt file. The robots.txt file is part of the Robots Exclusion Protocol (REP) – a set of web standards that govern how robots operate on the web, access content, index it, and serve it to users. Site owners can specify which user agents may or may not access the site. The robots.txt file may also define a crawl-delay directive to slow down the rate of requests to the site. Robots.txt also lists the site's sitemaps, so the parser can find every page and its last updated time. If a page has not changed since the parser's last visit, it will be skipped this time.
When a web parser reaches a page that needs to be crawled, it renders it in a browser, loading all HTML, third-party code, JavaScript, and CSS. This information is stored in the search engine's database and later used for indexing and ranking the page. All links on the page are also loaded. Links that are not yet in the search engine's index are added to a list for future crawling.
Compliance with the directives in the robots.txt file is voluntary. Most major search engines follow robots.txt directives, but some do not. Bad actors, such as spammers and botnets, ignore robots.txt directives. Even some legitimate search engines, like the Internet Archive, ignore robots.txt.
Examples of Web Parsers
Search engines have several types of web parsers. For example, Google has 17 types of bots:
- APIs-Google
- AdSense
- AdsBot Mobile Web Android
- AdsBot Mobile Web
- Googlebot Image
- Googlebot News
- Googlebot Video
- Googlebot Desktop
- Googlebot Smartphone
- Mobile Apps Android
- Mobile AdSense
- Feedfetcher
- Google Read Aloud
- Duplex on the web
- Google Favicon
- Web Light
- Google StoreBot
Why Web Parsers Are Important for SEO
The goal of SEO is to make your content easily discoverable when a user searches for a relevant query. Google cannot determine where to rank your content if it has not been crawled and indexed.
Web parsers can also be useful in other areas: e-commerce sites often crawl competitor sites to analyze product selection and pricing.
Challenges Faced by Web Parsers
There are several challenges that web parsers may encounter.
| Problem | Description |
| Robots.txt Restrictions | If a web parser complies with robots.txt restrictions, it may not be able to access certain web pages or may send requests exceeding an arbitrary limit. |
| IP Address Bans | Since some web parsers do not comply with robots.txt restrictions, sites may use a range of other tools to limit web parsing. Websites may ban IP addresses known to be malicious, such as free proxy servers used by fraudsters, or certain data center IP addresses. |
| Geolocation Restrictions | Some websites require the visitor to be in a specific geographic region to access content. An example is trying to access Netflix USA content from another country. Most geographic restrictions can be bypassed using residential proxy networks. |
| CAPTCHAs | When a site detects high activity from suspicious sources, it may display a CAPTCHA to verify that a human is behind the request. CAPTCHAs can disrupt web parsers. Many web parsing solutions have tools and technologies to overcome such blocks. These tools often use a CAPTCHA solving service. |
Conclusion
Web parsers are a crucial part of the Internet infrastructure. They allow search engines to collect the data needed to build search indexes, enabling them to deliver search results for user queries. Many companies find that web parsers help them in research. In reality, they are often targeted at just one or two sites, such as Amazon, Adidas, or Airbnb listings.