First and foremost, I don’t think there’s any right or wrong answer. There are just two answers and you need to clarify with everyone each time or go with the flow.
If we think of the upstream and downstream concepts as a river and your service is a point somewhere.
What does the WATER represent?
Choice A: Response
Service A calls service B to get some data. Service A is downstream. Service B is upstream.
This one is a popular one. Data flows from upstream to downstream. If you dig around on the world wide web, you’ll probably end up with this definition.
Choice B: Request
Service A calls to service B. The request flows from a to B. Service A is upstream. Service B is downstream.
After searching on the Internet, I find this one more obscure but nevertheless something that, anecdotally, I find my peers think of first before choice A.
Verdict?
if you are here because you’re confused like me when we switched between choices and B, just don’t use it. Don’t feel like you have to be nice and use a term that everyone keeps interchangeably using and use a term that is clear.
However, if you are hell bent to find some resemblance of consistency like me, the answer is both A and B. If service A needs data from service B to do what it’s doing, the answer is choice A. If service A does not care for the response from service B but rather kicking off a chain of workflows, the answer is choice B.