Ask HN: How would you build a Google Alerts substitute?

Assuming you can't leverage any 3rd party API, but have access to a hose of URLs, how would you create a Google Alerts substitute that would scale for 10,000 + keywords? Would you: 1) Import all the documents into a search index, and have a Cron Job that iterates through all the keywords and searches for it in the index? 2) As soon as an URL is received, iterate through all 10,000 keywords and see if the contents of the URL contains that keyword? 3) Another efficient way?