Difference Between Upstream & Downstream in Microservice

The concept of upstream and downstream often puzzles novice developers, especially when dependencies and inheritance are involved. Mastering these ideas is crucial for long-term success in software development.

Exploring Stream Concepts

APIs are interconnected systems. Regardless of their power, APIs rely on other components to function. Think of APIs as nodes within a network that exchange information. These nodes receive input data and send output data, forming a flow often referred to as a stream.

To visualize this, imagine a factory situated along a river. Raw materials arrive from point A upstream and are processed at the factory (point B). Finished products are then sent downstream to a store at point C, where consumers purchase them. Similarly, APIs require input data from an upstream source, process it, and pass the output downstream.

Upstream vs. Downstream

In this context, understanding upstream and downstream is essential. Using the manufacturing analogy, the raw material supplier is upstream, while the store is downstream. For APIs, an upstream system provides raw data, and a downstream system consumes the processed output. 

Consider an API designed to find train ticket discounts and send this data via text. This API acts as a middle processor. It fetches data from external sources (upstream) and forwards it to users through a messaging service (downstream). Here, the user is the final downstream point, while the local provider APIs supplying ticket prices are upstream.

Each component's position in the flow determines its upstream or downstream status. For instance, the ticket data API sees the processing API and SMS service as downstream, while the SMS service views everything preceding it as upstream.

Dependencies in Stream Flow

Dependencies complicate the upstream/downstream relationship. Dependencies are essential components required for an API to function. These can include other APIs, libraries, or systems. Understanding whether a dependency is needed for input or output is crucial.

Revisiting the ticketing API example, several upstream dependencies might be involved. These could include external libraries and APIs that supply data in the required format. For instance, a GraphQL API might provide data, which is then processed and compressed by other systems. These are all upstream dependencies.

On the downstream side, processed data must be transmitted somewhere. This could involve multiple endpoints, such as SMS APIs or data presentation systems, each with its own dependencies. These downstream systems ensure the processed data reaches its final destination.

Inherited Dependencies

The complexity increases with inherited dependencies. Choosing specific technical solutions often brings additional dependencies. Every decision impacts not just the API in question but also the downstream systems interacting with it.

In the ticketing API scenario, suppose a security protocol requires two libraries. This choice creates direct dependencies for the API and indirectly affects downstream systems. Any downstream entity must accommodate this security protocol, adding it to their own dependencies. This creates a chain of dependencies extending further downstream.

As downstream systems evolve, they may introduce new dependencies, propagating the chain. Even if the initial API no longer requires a specific protocol, downstream systems might still depend on it, creating a complex web of dependencies.

Real-World Authentication Example

Consider a modern authentication flow. Basic username and password pairs are often insufficient. Additional checks, such as one-time passwords or biometric verification, may be triggered by upstream services, leading to downstream API interactions.

A Hypermedia Authentication API (HAAPI) provides hyperlinks for actions based on the current state. In an authentication flow, this helps negotiate further steps. For example, if a user logs in from a new location, the system might trigger a downstream service to send an SMS for additional verification. This interaction involves multiple upstream and downstream dependencies before granting access.

Importance of Understanding Dependencies

Dependencies are an inherent part of modern development. Developers must understand the systems, libraries, and APIs integrated into their projects. This ensures informed decisions, contributing to long-term success and user satisfaction.

Documenting and understanding dependencies is crucial for future development and customer satisfaction. A clear grasp of upstream and downstream dynamics is vital for the health of the industry and the success of any API implementation.