A software stack refers to a collection of software subsystems or components needed to create a complete platform such that no additional software is needed to support applications. Applications are said to “run on” or “run in” the resulting platform.
The concept can be visualized as a stack of layers, with each layer being a piece of software or technology. For example, a common web development stack is the LAMP stack, which stands for Linux (operating system), Apache (web server), MySQL (database), and PHP (programming language).
The most “upstream” part of a software stack usually refers to the lower layers of the stack. This is often the operating system, as it forms the base on which everything else runs. Changes to these lower layers can have wide-reaching effects on the upper layers of the stack, hence the term “upstream”.
However, the notion of “upstream” can also refer to the source of a software distribution. In this context, “upstream” refers to the original developers or maintainers of the software, as opposed to those who package and distribute the software downstream. For instance, if you’re using a package from Ubuntu’s repositories, the Ubuntu maintainers are “downstream” and the original software authors are “upstream”.
In the context of the open source community, changes to the software are often contributed back “upstream,” i.e., to the original project, so that everyone in the community can benefit from the improvements.
In summary, “upstream” can either refer to the foundational layers of a software stack or the original sources in a software distribution, depending on context.
