What is the Maven way for automatic project versions when doing continuous delivery?

Question

I have a web application where we deploy to production whenever a feature is ready, sometimes that can be a couple of times a day, sometimes it can be a couple of weeks between releases.

Currently, we don't increment our version numbers for our project, and everything has been sitting at version 0.0.1-SNAPSHOT for well over a year. I am wondering what is the Maven way for doing continuous delivery for a web apps. It seems overkill to bump up the version number on every commit, and never bumping the version number like we are doing now, also seems wrong.

What is the recommend best practice for this type of Maven usage?

The problem is actually a two-fold one:

• Advancing project version number in individual pom.xml file (and there can be many).
• Updating version number in all dependent components to use latest ones of each other.

Answer 1

I recommend the following presentation that discusses the practical realities of doing continuous delivery with Maven:

• You tube presentation on CD with Maven
• Slides

The key takeaway is each build is a potential release, so don't use snapshots.

Answer 2

This is my summary based on the video linked by Mark O'Connor's answer.

• The solution requires a DVCS like git and a CI server like Jenkins.
• Don't use snapshot builds in the Continuous Delivery pipeline and don't use the maven release plugin.
• Snapshot versions such as 1.0-SNAPSHOT are turned into real versions such as 1.0.buildNumber where the buildNumber is the Jenkins job number.

Algorithm steps:

1. Jenkins clones the git repo with the source code, and say the source code has version 1.0-SNAPSHOT
2. Jenkins creates a git branch called 1.0.JENKINS-JOB-NUMBER so the snapshot version is turned into a real version 1.0.124
3. Jenkins invokes the maven versions plugin to change the version number in the pom.xml files from 1.0-SNAPSHOT to 1.0.JENKINS-JOB-NUMBER
4. Jenkins invokes mvn install
5. If the mvn install is a success then Jenkins will commit the branch 1.0.JENKINS-JOB-NUMBER and a real non-snapshot version is created with a proper tag in git to reproduce later. If the mvn install fails then Jenkins will just delete the newly created branch and fail the build.

I highly recommend the video linked from Mark's answer.

Answer 3

Starting from Maven 3.2.1 continuous delivery friendly versions are supported out of the box : https://issues.apache.org/jira/browse/MNG-5576 You can use 3 predefined variables in version:

${changelist}
${revision}
${sha1}

So what you basically do is :

1. Set your version to e.g. 1.0.0-${revision}. (You can use mvn versions:set to do it quickly and correctly in multi-module project.)
2. Put a property <revision>SNAPSHOT</revision> for local development.
3. In your CI environment run mvn clean install -Drevision=${BUILD_NUMBER} or something like this or even mvn clean verify -Drevision=${BUILD_NUMBER}.

You can use for example https://wiki.jenkins-ci.org/display/JENKINS/Version+Number+Plugin to generate interesting build numbers.

Once you find out that the build is stable (e.g. pass acceptance tests) you can push the version to Nexus or other repository. Any unstable builds just go to trash.

Answer 4

There are some great discussions and proposals how to deal with the maven version number and continuous delivery (CD) (I will add them after my part of the answer).

So first my opinion on SNAPSHOT versions. In maven a SNAPSHOT shows that this is currently under development to the specific version before the SNAPSHOT suffix. Because of this, tools like Nexus or the maven-release-plugin has a special treatment for SNAPSHOTS. For Nexus they are stored in a separate repository and its allowed to update multiple artefacts with the same SNAPSHOT release version. So a SNAPSHOT can change without you knowing about it (because you never increment any number in your pom). Because of this I do not recommend to use SNAPSHOT dependencies in a project especially in a CD world since the build is not reliable any more.

SNAPSHOT as project version would be a problem when your project is used by other ones, because of the above reasons.

An other problem of SNAPSHOT for me is that is not really traceable or reproducibly any more. When I see a version 0.0.1-SNAPSHOT in production I need to do some searching to find out when it was build from which revision it was build. When I find a releases of this software on a filesystem I need to have a look at the pom.properties or MANIFEST file to see if this is old garbage or maybe the latest and greatest version.

To avoid the manual change of the version number (especially when you build multiple builds a day) let the Build Server change the number for you. So for development I would go with a

<major>.<minor>-SNAPSHOT

version but when building a new release the Build Server could replace the SNAPSHOT with something more unique and traceable.

For example one of this:

<major>.<minor>-b<buildNumber>
<major>.<minor>-r<scmNumber>

So the major and minor number can be used for marketing issues or to just show that a new great milestone is reached and can be changed manually when ever you want it. And the buildNumber (number from your Continuous Integration server) or the scmNumber (Revision of SUbversion or GIT) make each release unique and traceable. When using the buildNumber or Subversion revision the project versions are even sortable (not with GIT numbers). With the buildNumber or the scmNumber is also kinda easy to see what changes are in this release.

An other example is the versioning of stackoverflow which use

<year>.<month>.<day>.<buildNumber>

And here the missing links:

• Versioning in a Pipeline
• Continuous Delivery and Maven

Answer 5

DON'T DO THIS!

<Major>.<minor>-<build>

will bite you in the backside because Maven treats anything after a hyphen as LEXICAL. This means version 1 will be lexically higher than 10.

This is bad as if you're asking for the latest version of something in maven, then the above point wins.

The solution is to use a decimal point instead of a hyphen preceding the build number.

DO THIS!

<Major>.<minor>.<build>

It's okay to have SNAPSHOT versions locally, but as part of a build, it's better to use

mvn versions:set -DnewVersion=${major}.${minor}.${build.number}

There are ways to derive the major/minor version from the pom, eg using help:evaluate and pipe to a environment variable before invoking versions:set. This is dirty, but I really scratched my head (and others in my team) to make it simpler, and (at the time) Maven wasn't mature enough to handle this. I believe Maven 2.3.1 might have something that go some way in helping this, so this info may no longer be relevant.

It's okay for a bunch of developers to release on the same major.minor version - but it's always good to be mindful that minor changes are non-breaking and major version changes have some breaking API change, or deprecation of functionality/behaviour.

From a Continuous Delivery perspective every build is potentially releasable, therefore every check-in should create a build.

Answer 6

As a starting point you may have a look at Maven: The Complete Reference. Project Versions.

Then there is a good post on versioning strategy.

Answer 7

At my work for web apps we currently use this versioning pattern:

<jenkins build num>-<git-short-hash>

Example: 247-262e37b9.

This is nice because it it gives you a version that is always unique and traceable back to the jenkins build and git revision that produced it.

In Maven 3.2.1+ they finally killed the warnings for using a ${property} as a version so that makes it really easy to build these. Simply change all your poms to use <version>${revision}</version> and build with -Drevision=whatever. The only issue with that is that in your released poms the version will stay at ${revision} in the actual pom file which can cause all sorts of weird issues. To solve this I wrote a simple maven plugin (https://github.com/jeffskj/cd-versions-maven-plugin) which does the variable replacement in the file.