The Inverted Index
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Web crawlers are just programs that download webpages, collect all the links, and visit all those links to repeat the process. This understates the potential complexity of the process quite a bit. There are many other additions and features to be added to a web crawler.
To figure out what features a web crawler must implement you need to produce a list of characteristics for the system. This can be difficult because many characteristics interfere with one another leading to trade-offs.
Features and Characteristics
- Scalability: the web is absolutely enormous and indexing any sizable portion requires a scalable system. Having a distributed web crawler
- is a good strategy for scalability.
- Niceness: It is considered rude for a crawler to send very frequent requests to a given server, some might even block the crawler’s IP if it is running too fast
- Freshness: The web is always changing so keeping the index up to date is ideal
- Robustness: The web can have spider traps either put there maliciously to stop web crawlers or created on accident. A crawler should be resilient when finding these traps.
- Extensibility: The web is always changing and a web crawler should be able to adapt easily 1.