General Programming Principal

Upstream vs Downstresam API
All this discussion is relative based on an API that uses another API and that API is being used by another API. 

 

 Upstream API 

 You refer to API that your API uses as upstream if it produces data that YOUR API consumes in the middle. 

 Downstream API 

 You refer to API that depends on YOUR API as downstream. They are waiting for your output in order to process the request that they have received. 

 Some pictures