2017-08-10

Meaningful Versions with Continuous Everything

Q: How should I version my software? A: Automated!

All continuous delivery processes follow the same basic pattern:

Engineers working on source code, configuration or other content commit their work into a git repository (or another version control system, git is used here as an example). A build system is triggered with the new git commit revision and creates binary and deployment artefacts and also applies the deployments.

Although this pattern exists in many different flavors, at the core it is always the same concept. When we think about creating a version string the following requirements apply:
  • Every change in any of the involved repositories or systems must lead to a new version to ensure traceability of changes.
  • A new version must be sorted lexicographically after all previous versions to ensure reliable updates.
  • Versions must be independent of the process execution times (e.g. in the case of overlapping builds) to ensure a strict ordering of the artefact version according to the changes.
  • Versions must be machine-readable and unambiguous to support automation.
  • For every version component there must be a single source of truth to make it easy to analyse issues and track back changes to their source.
Every continuous delivery process has at least two main players: The source repository and the build tool. Looking at the complete process allows us to identify the different parts that should contribute to a unique version string, in order of their significance:

1. Version from Source Code

The first version component depends only on the source code and is independent from the build tooling. All version components must be derived only from the source code repository. This version is sometimes also called software version.

1.1 Static Source Version

The most significant part of the version is the one that is set manually in the source code. This can be for example a simple VERSION file or also a git tag. It typically denotes a compatibility promise to the users of the software. Semantic Versioning is the most common versioning scheme here.

To automate this version one could think about analysing an API definition like OpenAPI or data descriptions in order to determine breaking changes. Each braking change should increment the major version, additions should increment the minor version and everything else the patch version.

In a continuous delivery world we can often reduce this semantic version to a single number that denotes breaking changes

1.2 Source Version Counter

Every commit in the source repository can potentially produce a new result that is published. To enforce the creation of a new version with every change, a common practice is adding an automatically calculated and strictly increasing component to the version. For Subversion this is usually the revision. For git we can use the git commit count as given by git rev-list HEAD --count --no-merges

If the project uses git tags then the following git command generates the complete and unique version string: git describe --tags --always --dirty=-changed which looks like this: 2.2-22-g26587455 (22 is the commit count since the 2.2 tag). In this case the version also contains the short commit hash from git which identifies the exact state.

2. Version from Build System

The build tools and automation also influence the resulting binaries. To distinguish building the same source version with different build tooling we make sure that the build tooling also contributes to the resulting version string. To better distinguish between the version from source code and the version from the build tooling I like to use the old term release for the version from the build tooling.

2.1 Tool Version

All the tooling that builds our software can be summarized with a version number or string. The build system should be aware of its version and set this version.

If your build system doesn't have such a version then this is a sign that you don't practice continuous delivery for the build automation itself. You can leave this version out and rely only on the build counter.

2.2 Build Counter

The last component of the version is a counter that is simply incremented for each build. It ensures that repeated builds from the same source yield different versions. It is important that the build counter is determined at the very beginning of the build process.

If possible, use a build counter that is globally unique - at least for each source repository. Timestamps are not reliable and depend on the quality of the time synchronization.

Versions for Continuous Delivery

If all your systems are continuously built and deployed than there is a big chance that you don't need semantic versioning. In this case you can simplify the version schema to use only the automatic counter version and release:
<git revision counter>.<build counter>
On the other hand you might want to add more components to your version strings to reflect modularized source repos. For example, if you keep the operational configuration separate from the actual source code then you might want to have a version with three parts, again in simplified form:
The most important take away is to automate the generation of version strings as much as possible. In the world of continuous delivery it becomes possible to see versions as a technical number without any attached emotions or special meaning.

Version numbers should be cattle, not pets.