Friday, 24 February 2012

Inconsistency on Maven model

Introduction

Trying to build maven from a single svn commit (actually it was a TAG commit), I found that it is impossible without a central repository or information from other svn commits. I present details about this case of "cross commit version cycle". I call this a "inconsistency" on Maven Model because it allows to create commit versions of a project that are not self-sufficient.
Any inconsistency about this inconsistency is welcome.

Maven project model (fragment)

In maven there are two different project relations Parent Relation and Module Relation.
Parent Relation inherent project properties and Module Relation manage the build process: "When building this project, build also this submodules".
Look at the maven reference documentation for full reference: http://www.sonatype.com/books/mvnref-book/reference/pom-relationships-sect-project-relationships.html

Snapshot versions and release versions

Most Maven projects have a main parent and many submodules as on figure 1.
Figure 1. Project structure and relations

Snapshot versions are used during development, to get the last version of the source code. Maven look on the central (or project if configured) repository to look for the last build.
When a release is ready, all the projects may be commited with version 1.0. You get then a concrete release and relations as on figure 2.
Figure 2. Release commit with version 1.0

Problem on partial version upgrade

The problem with Parent/Module relations appears on partial increase of versions. Assume that only some submodules versions are increased on release 1.1 (fig 3). As parent project has old version 1.0 (it was not changed) then submodules 1.1 refere to version 1.0. Lets call this commit 1.1_partial.
Figure 3. 1.1_partial commit

Now, increase on a new commit 1.1_rest the version of remaining projects to 1.1. You get a commit where each pom has version 1.1, BUT!! submodules changed on commit 1.1_partial have as parent pom with version 1.0 (fig. 4)
Figure4. Inconsistency relations cross commits

It may look as a bad use of maven versioning, but the model allows this situations so: how to interpret such a 1.1_rest commit??

Some simple arguments

Talking about this "inconsistency" on maven model, I get the follow counterarguments:

This don't happen!

I found this trying to build maven from source from a tag commit, so here is the example:
  • PARENT: org.apache.maven.plugins:maven-plugins:pom
  • SUBMODULE: maven-clean-plugin:maven-plugin
Refer to this TAG commit on maven:
Project relations are (version in brackets)
PARENT(18) --submodule--> SUBMODULE(2.5-SNAPSHOT) --parent--> PARENT(17)
Look here to see this relations:
You may not have any problem to build maven with this commit IF you have pom of PARENT(17) on you maven repository. This means that building from scratch is possible only if you use 2 different source commits, at lease 2 if you don't get more old version references.

I will take care of my pom versions, so it will not happen on my project

You control pom versions only on you projects, so if some external dependency have such a cross commit version cycle you will inherent it

Old versions are all on central repo, so no problem

I would like to have maven projects on gentoo, so is a problem for me.

Is this a real problem??

It is not a problem, until You have a central maven repository with old release versions. It seems to be sad, that actually We are forced to have such a historical repo.

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