Google’s release engineering docs describe hermetic build processes as those which are “insensitive to the libraries and other software installed on the build machine. Instead, builds depend on known versions of build tools, such as compilers, and dependencies, such as libraries. The build process is self-contained and must not rely on services that are external to the build environment.”

Basically, if your build process requires you to pull code from any repository that you do not own, it isn’t hermetic. If you have to `pip install` or `go get` a third party (or even in-house!) dependency from a source that you do not control, your build is not hermetic.

Effectively, this means that you have to have versioned copies of all of your third party dependencies, and version-specified build graphs. Very hard to do without a mono repo and a build tool.

In the context of the GP, I’d say that deterministic would have been a better word choice. A reliance on time stamps technically wouldn’t make a build process non-hermetic, but it would definitely make it non deterministic. It’s technically possible to have hermetic builds without having reproducible builds, although that would be a very bizarre org. :)

The (subtle?) distinction is around "what it is" rather than "what it is for":

A deterministic process always produces the same output given the same set of inputs.

A hermetic behavior ensures that indeed you'll always have the same inputs. It can just be a set of best practices (e.g. being very careful of not depending on external inputs that might change outside of your control) or it can involve an active barrier that sandboxes your environment in order to ensure that you indeed always have the same inputs.

A reproducible process is a process that can be repeated later in the future. There are various degrees of reproducibility you might be interested in. For example, you might want "bit for bit" reproducibility (important for security) or you just want to make sure you can rebuild something functionally equivalent (e.g. the compilation or link phase might not be fully deterministic in the order and layout of compilation units).

Reproducible processes usually rely on a deterministic system and leverage hermetic behaviours to ensure reproducibility (over time or across locations)