How to write a crawler?

You’ll be reinventing the wheel, to be sure. But here’s the basics:

• A list of unvisited URLs – seed this with one or more starting pages
• A list of visited URLs – so you don’t go around in circles
• A set of rules for URLs you’re not interested in – so you don’t index the whole Internet

Put these in persistent storage, so you can stop and start the crawler without losing state.

Algorithm is:

while(list of unvisited URLs is not empty) {
    take URL from list
    remove it from the unvisited list and add it to the visited list
    fetch content
    record whatever it is you want to about the content
    if content is HTML {
        parse out URLs from links
        foreach URL {
           if it matches your rules
              and it's not already in either the visited or unvisited list
              add it to the unvisited list
        }
    }
}