[Go] module version design reasoning

Reference:
https://research.swtch.com/vgo-principles


Go modules uses import path syntax called semantic import versioning, along with a new algorithm for selecting which versions to use, called minimal version selection.


For Go modules, the import compatibility rule can be written as:
If an old package and a new package have the same import path,
the new package must be backwards compatible with the old package.

Semantic import versioning:

(figures excerpted from https://research.swtch.com/vgo-principles)

For major version difference build:

e.g

For minor version difference build (consider repeatability):

The algorithm used for Go modules is very simple, despite the imposing name "minimal version selection"

It works like this:

1. Each package specifies a minimum version of each dependency. For example, suppose B 1.3 requests D 1.3 or later, and C 1.8 requests D 1.4 or later.
2. In Go modules, the go command prefers to use those exact versions, not the latest versions. If we’re building B by itself, we’ll use D 1.3. If we’re building C by itself, we’ll use D 1.4. The builds of these libraries are repeatable.

Also shown in the figure, if different parts of a build request different minimum versions, the go command uses the latest requested version.

The build of A sees requests for D 1.3 and D 1.4, and 1.4 is later than 1.3, so the build chooses D 1.4. That decision does not depend on whether D 1.5 and D 1.6 exist, so it does not change over time.