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Questions and Answers
What is the initial step in the basic crawler operation?
What is a potential issue with non-malicious pages?
What is a spider trap?
What is an aspect of being a polite crawler?
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What is an example of explicit politeness?
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What should a crawler be immune to?
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What is one of the key capabilities that a crawler should have?
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What is one of the advantages of a crawler being scalable?
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What should a crawler prioritize when fetching pages?
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What is a key aspect of continuous operation in a crawler?
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What is an advantage of a crawler being extensible?
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What is a consideration when crawling multiple pages from the same host?
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What is a benefit of designing a crawler for distributed operation?
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What is a key consideration when designing a crawler?
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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
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Description
Learn about the basics of web crawling, including how crawlers operate and common challenges they face, such as malicious pages and varying latency.
