so when should we actually put circuit breakers in we should put them in when a service starts to fail or become

I've been thinking about system resilience lately, you know, especially with distributed systems. We've all experienced those awful cascading failures where one service hiccups and brings down everything else. So, I'm wondering when it really makes sense to add circuit breakers to the mix. Is it mostly for when services talk to each other internally, or do you put them on the edge too? What are your favorite libraries or ways to do this?