Crawler

A crawler is an automated program that systematically visits web pages, follows links, and collects information from websites.

Crawlers are widely used by search engines to discover new or updated pages and keep their indexes current. They can also support web data collection, site auditing, monitoring, and other automated workflows.

Also known as: Web crawler, spider, web spider, web bot.

How Does a Crawler Work?

A crawler usually begins with one or more starting URLs, often called seed URLs.

It then:

1. Visits the starting page.
2. Reads the page content and identifies links.
3. Adds new links to a queue.
4. Visits those pages according to its crawling rules.
5. Repeats the process until the crawl is complete.

Crawlers may also apply rules for duplicate detection, crawl depth, page priority, robots directives, and request rate.

Crawler vs. Scraper

A crawler focuses on discovering and navigating web pages by following links.

A scraper focuses on extracting specific information from those pages.

In simple terms:

• Crawler: Finds and explores pages.
• Scraper: Extracts targeted data from pages.

The two are often used together in web data collection systems.

Crawler vs. Indexer

A crawler discovers and retrieves webpage content.

An indexer organizes the collected information into a searchable structure so it can be retrieved efficiently later.

For example, a search engine crawler finds pages across the web, while the indexer processes and stores their content for search results.

Advantages of Crawlers

• Broad coverage: Can discover large numbers of interconnected pages.
• Automation: Operates without manual page-by-page navigation.
• Scalability: Can process large websites or collections of websites.
• Continuous discovery: Can revisit pages to detect new or updated content.

Limitations of Crawlers

• Resource usage: Large crawls can require significant bandwidth, computing power, and storage.
• Server load: Excessive request rates can place unnecessary pressure on target websites.
• Complex websites: JavaScript-heavy pages may require browser rendering or additional logic.
• Duplicate content: Poor crawl management can lead to repeated requests for the same or similar pages.
• Compliance considerations: Crawlers should respect applicable access rules, privacy requirements, and website policies.

Crawlers and Web Scraping

In many scraping systems, the crawler is responsible for discovering which pages should be visited, while the scraper extracts the required information from each page.

For example, an e-commerce workflow may use a crawler to discover category and product URLs, then use a scraper to collect:

• Product names
• Prices
• Availability
• Ratings
• Product descriptions

This separation helps make large-scale data collection easier to organize and maintain.

Crawlers and Proxies

Large crawling jobs may generate many requests across websites or geographic regions.

Proxy infrastructure can help distribute authorized requests across different IP addresses and locations, while the crawler manages page discovery and scheduling.

A scalable crawling workflow may combine:

• URL queues
• Crawl-depth rules
• Rate limiting
• Proxy rotation
• Retry logic
• Duplicate detection
• Data extraction

Proxies support the network layer, while the crawler controls which pages are visited and in what order.

Example

A search engine crawler starts from a set of known webpages and follows the links it discovers.

When it finds a new page, it retrieves the content and sends relevant information to an indexing system.

The crawler later revisits pages to check whether they have changed, helping the search engine maintain a more current index.

Key Takeaway

A crawler is an automated program that discovers and navigates webpages by following links.

It differs from a scraper, which extracts specific data, and from an indexer, which organizes collected information for retrieval. Crawlers are fundamental to search engines, website auditing, monitoring, and large-scale web data collection.