This is similar to the old debate about the meaning of "frontend" and "backend". I maintain that something cannot be a "frontend" or a "backend" in isolation, it can only be a frontend or a backend to some other system. In a stack with many layers, (nearly) every layer is a frontend to one thing and a backend to another.
That said, I think I tend to use "upstream" in a similar sense to how package maintainers talk about packages: "upstream" is the source (where the thing originally came from) and "downstream" is the sink (where I will send it to). This implies that "upstream" for a request is "downstream" for a response, and vice versa. I don't know if that's necessarily correct usage, but that's how I've done it.
That said, I think I tend to use "upstream" in a similar sense to how package maintainers talk about packages: "upstream" is the source (where the thing originally came from) and "downstream" is the sink (where I will send it to). This implies that "upstream" for a request is "downstream" for a response, and vice versa. I don't know if that's necessarily correct usage, but that's how I've done it.