Joseph Leonard created MNG-8096:
-----------------------------------
Summary: Inconsistent dependency resolution behaviour for
concurrent multi-module build can cause failures
Key: MNG-8096
URL: https://issues.apache.org/jira/browse/MNG-8096
Project: Maven
Issue Type: Bug
Components: Core
Affects Versions: 3.9.6, 3.8.2
Reporter: Joseph Leonard
h2. tl;dr
Maven can resolve dependencies either from:
* an external repo
* a class directory of a module being built within the reactor
* a packaged jar of a module being built within the reactor
If you run a concurrent multi-module build it is possible to get a race
condition whereby the build of module _Foo_ may resolve module _Bar_ from
either of the three resolution {_}channels{_}. This inconsistency can result in
the Maven _war_ plugin sometimes failing to build a functional war file. I
would expect a consistent resolution would always take place.
h2. Full details
h4. Scenario
Consider you have a repo with the following structure:
{noformat}
App
/ \
/ \
(compile scope) (test scope)
/ \
\/_ _\/
ModuleA TestSupportModule1
/
/
(compile scope)
/
\/_
ModuleB
/
/
(test scope)
/
\/_
TestSupportModule2
{noformat}
If you were to make a src code change to the following test support modules:
* TestSupportModule1
* TestSupportModule2
Then the minimum number of modules we need to build to verify the change set is
OK is:
* TestSupportModule1
* TestSupportModule2
* ModuleB
* App
i.e. there is no requirement to build ModuleA because we know that none of the
src code changes could impact the classpaths used in its maven build.
We know that despite 'App' depending (transitively) on ModuleB there is no need
for the 'App' build to wait for ModuleB to complete its build because the src
code change to TestSupportModule2 will not impact any of the classpaths used in
the App maven build. Therefore to get the most efficient build possible we
ideally would invoke Maven to run with 2 threads and with instruction to build
*two distinct* 'dependency graphs':
* TestSupportModule1 followed by ModuleB
* TestSupportModule1 followed by App
The following Maven command achieves exactly what we want because the reactor
build order is based only on the *direct* (i.e. non-transitive) dependencies of
the modules provided to the reactor in the build command. Therefore the absence
of ModuleA results in two distinct 'dependency graphs':
{noformat}
mvn clean verify -pl TestSupportModule1,TestSupportModule2,ModuleB,App -T 2
{noformat}
Note: In reality the code base I maintain has a very large monobuild with 100s
of modules and this type of build optimisation makes a significant difference
to the speed of our monobuild (we use
[https://github.com/gitflow-incremental-builder/gitflow-incremental-builder] to
automate the logic of determining which modules to include in the reactor based
on our change set).
h4. Issue
We have encountered an issue in the above scenario because the 'App' build has
a race condition with the ModuleB build which will result in one of the
following three outcomes:
* If the 'App' build starts before the ModuleB build has compiled its src
classes then the 'App' build will resolve ModuleB from the external repo (i.e.
equivalent to ModuleB not being in the reactor at all)
* If the 'App' build starts after ModuleB has compiled its src classes but
before it has packaged these classes into a jar then the 'App' build will
resolve ModuleB's {{target/classes}} directory
* If the 'App' build starts after ModuleB has packaged its jar file then the
'App' build will resolve ModuleB's {{target/ModuleB.jar}} file.
In many scenarios this dependency resolution inconsistency doesn't represent a
challenge. However, it does cause an issue in our case because the 'App' POM
has its Maven {{packaging}} stanza configured to {{war}} and in the scenario
where ModuleB's {{target/classes}} directory is resolved by the 'App' then this
results in the resultant 'App' war file being packaged with a completely empty
ModuleB.jar file.
h4. Proposed solution
Ideally we would like the Maven reactor to retain isolation between the *two
distinct* 'dependency graphs' it constructs at _instantiation_ throughout the
entire Maven build. This would mean, in the simple example above, that the
'App' would *always* resolves ModuleB from the external repo (regardless of
whether the reactor has built ModuleB or not in a _separate_ 'dependency graph'
in the reactor).
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