题目内容
The Web has a very dynamic nature, meaning it can be created, updated and deleted at any moment.
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It's usual and doesn't matter that a crawler does not detect an event.
There are two measures, namely freshness and age, indicating the local copy is accurate or outdated.
It's difficult for a server to satisfy multiple requests per second and/or downloading large files from a single crawler or the requests from multiple crawlers.
To use Web crawlers will cost a lot of network resources.
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