Automatic Semantic Versioning with GitHub and TeamCity

You’ve quite possibly come across the idea of semantic versioning. It's a set of rules for versioning releases of software, designed to minimise disruption to people who are relying on your APIs not to change. In a nutshell, every release of your software has a three-part version number.

Given a version number MAJOR.MINOR.PATCH, increment the:

  1. MAJOR version when you make incompatible API changes,
  2. MINOR version when you add functionality in a backwards-compatible manner, and
  3. PATCH version when you make backwards-compatible bug fixes.

Additional labels for pre-release and build metadata are available as extensions to the MAJOR.MINOR.PATCH format.

Pretty simple, huh? In other words, 2.3.4 should offer the same feature set as 2.3.1 but with extra bug fixes. 2.4.0 is backwards-compatible with 2.3.2, and with 2.2.*, and with 2.1.*, and when you go for the ‘big rewrite’ and break everything, that’s version 3.0.0 which isn’t compatible with any previous versions.

Our main development pipeline is now based on TeamCity 8 and GitHub – both of which are absolutely wonderful once you get the hang of using them properly – and I wanted to automate the versioning of our builds according to semantic versioning principles. Here’s the workflow I settled on:

  • Every build has a MAJOR, MINOR, PATCH, and BUILD version, and an optional SUFFIX
  • Each part of the version number is managed as a TeamCity build parameter.
  • MAJOR and MINOR are managed manually. TeamCity can’t tell whether your commit is a feature or a bugfix, so for now we’ve just settled for manually changing the relevant parameters when you release a major or minor release.
  • All development is done on feature branches and submitted via pull requests.
  • Any pull request is assumed to be a PATCH release unless a developer changes the major or minor numbers when they accept it.
  • The SUFFIX is used to denote a pre-release package, and should indicate which branch’s features are included in that prerelease
    • e.g. Dylan.Web- should be the code you’d get if you accepted branch7 into master 1.2.3
  • Anything built off master should be releasable, and tagged as such
  • Finally, whenever a production release is built, we’ll tag the GitHub repository with the version of that release – for tagging, we ignore the build number, so the tag will be something like v0.0.0

This lot actually distils to quite a simple guideline: The patch number is the number of pull requests accepted since the last tagged minor release.

Now that sounds like something you could automate… so with Powershell on one screen and the Github API documentation on the other, I hacked together the following script, which is now happily running as a build step on our TeamCity server.

$MajorVersion = %MajorVersion% # The major version of your current build
$MinorVersion = %MinorVersion% # The minor version of your current build
$RepoUrl = "%vcsroot.url%" # The HTTPS path to your repo –
$Token = "%GitHubAccessToken%" # Your GitHub access token, from

try {
    # Parse the supplied Git repo URL to determine the repo name and owner
    $RepoUrl -match "[^/]+)/(.+)\.git$"
    $repoOwner = $matches[1]
    $repoName = $matches[2]
    Write-Host "Reading repo $repoName owned by $repoOwner"
    # Find the tag resource matching the baseline of the current major/minor version
    # Remember, we’re not looking for X.Y.Z, we’re looking for X.Y.0 since patch numbers
    # should increment until the next minor release.

    $uri = "$repoOwner/$repoName/git/refs/tags/v$MajorVersion.$MinorVersion.0?access_token=$Token"
    Write-Output "Looking for tag v$MajorVersion.$MinorVersion.0 at $uri"
    $tag = Invoke-RestMethod -Uri "$uri"

    # $tag.object.url will now give us a more detailed tag resource, including the commit that was tagged
    $uri = $tag.object.url
    Write-Output "Getting tag info from $uri`?access_token=$Token"
    $tag = Invoke-RestMethod -Uri "$uri`?access_token=$Token"
    # $tag.object.url is now a link to the tagged commit
    $uri = $tag.object.url
    $tag = Invoke-RestMethod -Uri "$uri`?access_token=$Token"

    # now we can dig into the commit itself and find out WHEN the baseline release was tagged...
    $since = $

    # Now we can retrieve all the commits in this repo SINCE that date
    $commitsUri = "$repoOwner/$repoName/commits?since=$since&access_token=$Token"
    Write-Host "Retrieving commit log from $commitsUri"
    $commits = Invoke-RestMethod -Uri "$commitsUri"
    $merges = @($commits | Where-Object { $_.commit.message -match "^Merge pull request" })

    # Reversing the merges just means they show up in TeamCity’s build log in chronological order.
    # Which is nice.

    $mergeCount = $merges.Count
    Write-Host "Found $mergeCount merges since last release tag"
    for($i = 0; $i -lt $merges.Count; $i++) {
        $merge_number = $i+1
        $merge_message = $merges[$i].commit.message.split("`r`n",[StringSplitOptions]"RemoveEmptyEntries") -join "`r`n    "
        $merge_sha = $merges[$i].sha
        $merge_web_url = $RepoUrl.Replace(".git", "/commit/$merge_sha")
        "`r`n  Merge #$merge_number`: $merge_web_url"
        "    $merge_message"
    Write-Host "`r`n"
    Write-Output "##teamcity[setParameter name='PatchVersion' value='$mergeCount']"
} catch [System.Exception] {
    Write-Host "Exception trying to determine patch number from Github API"
    Write-Host $_
    Write-Host "Using default patch number 0"
    Write-Output "##teamcity[setParameter name='PatchVersion' value='0']"

A couple of fun details to look out for:

  • If we’re doing the first build of a new minor version, there won’t be any ‘baseline’ tagged version yet – which is fine; we just set the patch number to zero and off we go.
  • You can control TeamCity build parameters by outputting specially-formatted messages in your build log – that’s what those lines

    Write-Output "##teamcity[setParameters name='SomeParameter' value='NewValue']"

    are doing. See Build Script Interaction with TeamCity for more info.

Finally, if you'd like the commit/merge hyperlinks in your TeamCity build log to be clickable, check out this awesome tip from Christian Rodemeyer about how to embed clickable URL links in TeamCity build logs.

Happy versioning!