The concept of upstream and downstream in microservices can be confusing, but understanding the difference is crucial for long-term success. APIs are interconnected systems that rely on other components to function, forming a flow often referred to as a stream. In this context, upstream refers to the source of raw data, while downstream refers to the system that consumes the processed output. Dependencies complicate the upstream/downstream relationship, and understanding whether a dependency is needed for input or output is essential. Inherited dependencies further increase complexity, creating a chain of dependencies extending further downstream as systems evolve. A clear grasp of upstream and downstream dynamics is vital for informed decisions, contributing to long-term success and user satisfaction.