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  • And for the extra bit of clarity; in a standard CRUD client-server relationship, both ends are both upstream and downstream to each other. The client can't get any data or updates if the server's down, and the server doesn't have any instructions to execute if there isn't a client. Commented Aug 7, 2019 at 16:53
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    @Delioth disagree. The backend can have many clients, but doesn’t depend on any single one of them. If you removed a client, backend would still work. The client can have many backends it could use. If one backend is removed without the client knowing, the client can’t work properly. Client is downstream. Backend is upstream. Commented Aug 7, 2019 at 16:56
  • I have seen nginx/apache in reverse proxy context calling the backend services as upstream services. Then I saw an API gateway called ocelot that call the backend services as downstream. That is the reason why I'm reading this thread. Commented Sep 29, 2021 at 4:52
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    I guess it is just a difference caused by some people thinking about it in terms of flow of message/control, while some others thinking in terms of flow of dependency? Commented Sep 29, 2021 at 5:27
  • This discussion shows the terms are confusing when used in context of dependencies and bidirectional data flows (like request+response). I therefore suggest to only use them when the directionality is very clear: up=before, down=afterwards (e.g. batch processing) Commented Aug 30, 2023 at 12:42