Maven multi module project setup with various, different dependencies - maven

Question

Let me just say this first: I am brand new to Maven. That said I searched around but have not found answers to the following question. I found answers to similar questions, just not this scenario. Or maybe I just misunderstood the answers and this can be solved simply wiht a multi module setup.
I'd have the following dependency hierarchy:
database
| |
| +---core
| | |
| | +---business
| | |
| | +------App1
| | |
| | +------App2
| |
| +---------------App3
|
+----------------------App4
I'd like to make it work so that changes only result in new releases of any "upstream" modules/Apps. Is this indeed a simple case of multi-module maven setup or do I need to do something else?

Answer 1

If you want that releasing one component produce a new release of each project, just use maven-release-plugin: http://maven.apache.org/maven-release/maven-release-plugin/.
Documentation
As per doc, this would :
Check that there are no uncommitted changes in the sources
Check that there are no SNAPSHOT dependencies
Change the version in the POMs from x-SNAPSHOT to a new version (you will be prompted for the versions to use)
Transform the SCM information in the POM to include the final destination of the tag
Run the project tests against the modified POMs to confirm everything is in working order
Commit the modified POMs
Tag the code in the SCM with a version name (this will be prompted for)
Bump the version in the POMs to a new value y-SNAPSHOT (these values will also be prompted for)
Commit the modified POMs
Because of the maven multi module structure, they are linked together, and each project would be bumped into a new release.
In a few words, this will :
move version 1.0-SNAPSHOT --> 1.1-SNAPSHOT
tag 1.0
generate 1.0.jar (ou war or anything else)
Plugin usage
Assuming that SCM is correctly defined, and repository and distribution management configured, just add these lines
<project>
[...]
<build>
[...]
<plugins>
[...]
<plugin>
<groupId>org.apache.maven.plugins</groupId>
<artifactId>maven-release-plugin</artifactId>
<version>2.4.2</version>
<!-- optional, basic format is ${project.artifactId}-${project.version}.${project.type}-->
<configuration>
<tagNameFormat>v#{project.version}</tagNameFormat>
</configuration>
</plugin>
[...]
</plugins>
[...]
</build>
[...]
</project>
And call
mvn release:prepare
mvn release:perform
Inheritance vs Dependency
You may consider the two differents Maven approches :
inheritance, that means parent and multi/sub modules
aggregation, in other words : use of dependencies
In a multi-maven project, all your modules, including parent, share the same lifecycle. Releasing one imply releasing all, and so, releasing just one is a non sense.
In your case, you can't modify app 1 to 3 whithout impacting app 4.
If App 4 depends App 1, obviously App 1 can't depends on App 4 (circular references are not allowed).
So, you want to isolate App4 and App1 to 3 lifecycles, you should not use multi-modules, but just share a parent project, or a hierachy of pom like corporate > main project > sub project (not submodule).
After that, just declare a dependency between App 4 and App 1. (... into app4 pom.xml)
Just another thought : the name of your projects and submodules sounds strange. "Classical" hierarchy is often (considering multi business object domain for a large project):
Main Project (sometimes EAR) --> POM
|-- Business Object / DAO --> POM
| |-- Domain 1 --> JAR
| `-- Domain 2 --> JAR
|-- Core (depends on BO) --> JAR
`-- IHM / Web App (depends on core) --> WAR
So, database is rarely at the top of hierarchy.