I vaguely remember some joke about how difficult networking is and how some Jeff Atwood blog post I think makes the comparison about analogy of sending actors in a taxi to somewhere being compared to a packet, anything can happen inbetween but you will get the packet or something indistinguishable from the original at the other end if all goes well.

Are occasional/intermittent 503 service unavailable or 504 gateway timeouts unavoidable for microservices calling another external microservice?

Like at that point isn't receiving a 503 or 504 from something else, somewhat outside my jurisdriction, albeit I am fucked if I am depending on them and need to fail gracefully.

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