Question

There should be a frontier object – Holding a set of visited and waiting to crawl URL’s.
There should be some thread responsible for crawling web pages.
There would be also some kind of controller object to create crawling threads.

I don’t know what architecture would be faster, easier to extend. How to divide responsibilities to make as as few synchronization as possible and also minimize number of checking if current URL has been already visited.

Should controller object be responsible of providing new URL’s to working threads – this mean working threads will need to crawl all given URL’s and then sleep for undefined time. Controller will be interpreting this threads so crawling thread should handle InterruptedException (How expensive it is in Java – it seems that exception handling is not very fast ).
Or maybe controller should only starts the threads and let crawling threads to fetch frontier themselves?

Answer 1

create a shared, thread-safe list with the URL’s to be crawled. create an Executor with the number of threads corresponding to the number of crawlers you desire to run concurrently. start your crawlers as Runnables with a reference to the shared list and submit each of them to the Executor. each crawler removes the next URL from the list and does whatever you need it to do, looping until the list is empty.