🤔 Do you know the difference between an upstream and a downstream flow in Kotlin coroutines?

If you go looking through flow documentation in coroutines, you will eventually run into something like this:

𝘐𝘯 𝘱𝘢𝘳𝘵𝘪𝘤𝘶𝘭𝘢𝘳, 𝘵𝘩𝘪𝘴 𝘰𝘱𝘦𝘳𝘢𝘵𝘰𝘳 𝘦𝘯𝘴𝘶𝘳𝘦𝘴 𝘵𝘩𝘢𝘵 𝘥𝘰𝘸𝘯𝘴𝘵𝘳𝘦𝘢𝘮 𝘧𝘭𝘰𝘸 𝘥𝘰𝘦𝘴 𝘯𝘰𝘵 𝘳𝘦𝘴𝘶𝘮𝘦 𝘰𝘯 𝘤𝘢𝘯𝘤𝘦𝘭𝘭𝘢𝘵𝘪𝘰𝘯 𝘦𝘷𝘦𝘯 𝘪𝘧 𝘵𝘩𝘦 𝘦𝘭𝘦𝘮𝘦𝘯𝘵𝘸𝘢𝘴 𝘢𝘭𝘳𝘦𝘢𝘥𝘺 𝘦𝘮𝘪𝘵𝘵𝘦𝘥 𝘣𝘺 𝘵𝘩𝘦 𝘶𝘱𝘴𝘵𝘳𝘦𝘢𝘮 𝘧𝘭𝘰𝘸.

You will see a reference to an "upstream" and a "downstream" flow everywhere in the docs.

It was challenging for me to understand what those definitions mean, so I put together an animation to help you understand as well.

⬆️ Upstream flow - When you apply an intermediate operator to a flow, you are affecting the part of the flow that comes before (or "upstream of") the operator.

⬇️ Downstream flow - Side of the flow that proceeds after applying an intermediate operator.

Hope my animation helps you clear up the confusion!