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The client can track its own error rate to the service, but it would need information from a server to get the overall health of the service, which is what the author probably means. Furthermore the load balancer can add a Retry-After header to have more control over the client's retries.

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I think I've misunderstood what circuit breakers are for years! I did indeed think they were a server-side mechanism. The original commenter's description of them is great, you can essentially create a heuristic based on the observed behaviour of the server and decide against overwhelming it further if you think it's unhealthy.

TIL! Seems like it can have tricky emergent behaviour. I bet if you implement it wrong you can end up in very weird situations. I should visualise it. :)

I mean, they can and should be both. Local decisions can be cheap, and very simple to implement. But global decisions can be smarter, and more predictable. In my experience, it's incredibly hard to make good decisions in pathological situations locally, as you often don't know you're in a pathological situation with only local data. But local data is often enough to "do less harm" :)

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