The purpose of this module is isolating all the part of drools requiring mvel and/or asm. The main reason for this refactor is that the excutable model (and then also Kogito which relies on it) doesn't need them but it is still indirectly depending on them through drools-core and drools-compiler.

Note that this is unrelated with the dialect in use. Going through "traditional" drl compilation, both java and mvel dialect heavily relies on mvel for parsing and asm for bytecode generation and in both cases you'll need to have drools-mvel in your classpath.

Regarding the executable model it is supposed to work fine (with a few minor exceptions) without this module. In the very edge cases when this is not true, like for instance when using the legacy accumulate with inline functions, which is deprecated anyway, a RuntimeException clearly reporting the problem is thrown. It's planned to also cover without mvel these missing edge cases in future, but for now readding drools-mvel to the classpath when necessary solves this problem.

All classes in drools-core and drools-compiler having a dependency on mvel and/or asm have been moved into this module.

The points where core or compiler needs to interact with those mvel dependending classes (this never happens when using the executable model) have been gathered into specific interfaces. This interfaces are only implemented in drools-mvel and, when this module is present on the classpath, their implementations are loaded through the usual drools service discovery mechanism. All these interfaces and their implementation are then defined in the META-INF/kie.conf of this module.

When the execution requires one of these implementations but drools-mvel is not present on the classpath (for instance because an user miseed to add it even if using the "traditional" drl compilation) a RuntimeException clearly explaining the problem is thrown. This mechanism, together with the exception to be thrown, is defined in org.drools.core.base.CoreComponentsBuilder (one of the interfaces implemented in drools-mvel) and reused in all other places where this check is necessary.

In some specific cases it is needed to perform different actions depending on whether drools-mvel is present or not on the classpath or not. This test is performed by the org.drools.core.util.Drools.hasMvel() method which also check if the implementation of org.drools.core.base.CoreComponentsBuilder is available. This is used for instance in core, to skip initializations that are not necessary when not using mvel, and in the executable model, to confine and manage the features mentioned above that are still not available without mvel.