Neither ProcessPool nor ThreadPool respawn dead workers

[movermeyer]

If you are processing a stream that has a chance of failure, each workers in your ThreadPool/ProcessPool will eventually hit a failure case and die.

Once all of the workers have died, processing stops.

For example, your "Retrieving web pages concurrently" only works because your ThreadPool has more workers than failures (4 workers vs 2 failures). If you reduce the number of workers to 1, it only processes the first two URLs before dying.