14 Questions
What is the initial step in the basic crawler operation?
Begin with known seed URLs
What is a potential issue with non-malicious pages?
They vary in latency and bandwidth
What is a spider trap?
A set of web pages that may intentionally or unintentionally cause a crawler to make an infinite number of requests
What is an aspect of being a polite crawler?
Respecting both implicit and explicit politeness considerations
What is an example of explicit politeness?
Specifications from webmasters on what portions of a site can be crawled
What should a crawler be immune to?
Spider traps and other malicious behavior from web servers
What is one of the key capabilities that a crawler should have?
Be capable of distributed operation
What is one of the advantages of a crawler being scalable?
It can increase the crawl rate by adding more machines
What should a crawler prioritize when fetching pages?
Fetch pages of higher quality first
What is a key aspect of continuous operation in a crawler?
Continuing to fetch fresh copies of previously fetched pages
What is an advantage of a crawler being extensible?
It can adapt to new data formats and protocols
What is a consideration when crawling multiple pages from the same host?
Avoid trying to fetch them all at the same time
What is a benefit of designing a crawler for distributed operation?
It can make full use of available processing and network resources
What is a key consideration when designing a crawler?
Permitting full use of available processing and network resources
Study Notes
Basic Crawler Operation
- Crawler operation starts with known seed URLs
- Fetch and parse these URLs, extracting new URLs they point to
- Place extracted URLs on a queue for further processing
Complications
- Malicious pages can be encountered, including spam pages and spider traps
- Spider traps can cause a crawler to make an infinite number of requests or crash
- Non-malicious pages can also pose challenges, such as varying latency and bandwidth
- Webmasters may have stipulations that need to be considered
Spider Traps
- A spider trap is a set of web pages that intentionally or unintentionally cause a crawler to make an infinite number of requests or crash
- Spider traps can be dynamically generated
Robustness and Politeness
- A crawler must be robust to handle spider traps and malicious behavior
- A crawler must be polite and respect implicit and explicit politeness considerations
Politeness Considerations
- Explicit politeness: robots.txt specifies what portions of a site can be crawled
- Implicit politeness: avoid hitting a site too often, even with no explicit specification
Additional Crawler Requirements
- Be capable of distributed operation to run on multiple machines
- Be scalable to increase the crawl rate by adding more machines
- Optimize performance and efficiency to use available processing and network resources
- Fetch pages of higher quality first
- Continuously operate and fetch fresh copies of previously fetched pages
- Be extensible to adapt to new data formats and protocols
URL Frontier
- The URL frontier can include multiple pages from the same host
- Avoid trying to fetch all pages from the same host at the same time
- Keep all crawling threads busy to optimize performance
Learn about the basics of web crawling, including how crawlers operate and common challenges they face, such as malicious pages and varying latency.
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