In the book, iirc the example for Circuit Breaker was a timeout of Oracle db connections in a connection pool, due to a firewall in the middle. You'd have to read the book to get the story done well, but basically it was a resource contention issue because a socket was cut off in the middle without closing at the connection end. Sockets, by nature, don't time out. That's why Nygard says in the book that firewalls are a way of violating TCP/IP standards.