Upstream / Downstream

Noun · Adjective · Open Source

Definitions

Upstream and Downstream in software development describe the relationship between original source repositories and their derivatives. Upstream refers to the original project or repository from which your copy was forked or from which you receive updates. Downstream refers to projects or forks that depend on or derive from yours. When contributing to open source, you pull changes from upstream (the original repository) and push your contributions back upstream via pull requests. In dependency management, upstream packages are those your project depends on, and changes in them flow downstream to your project. In data engineering, upstream refers to data sources and earlier processing stages, while downstream refers to consumers and later stages. Understanding the upstream/downstream relationship is essential for managing software supply chains, as a vulnerability or breaking change in an upstream dependency can cascade to all downstream consumers. The terms help clarify direction of data flow, responsibility, and where changes should be proposed.
In plain English: Upstream is the original source of a project; downstream is anything built from it. Sending changes 'upstream' means contributing them back to the original.

1800s: The river metaphor is ancient: upstream is the source, downstream is where things flow to
1990s: Linux kernel development establishes the upstream/downstream convention: patches flow upstream to mainline, distributions are downstream
2000s: Git and GitHub make the concept everyday vocabulary — 'pull from upstream' means syncing with the original repository