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To be Reviewed By: August 17th 19th 2020

Authors: Patrick Johnson

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Geode is comprised of multiple sub-projects, many of which depend on other sub-projects at runtime to work correctly; these dependencies are defined in each project's build.gradle file. Normally, this is not a problem because all of Geode's projects are present on the Java classpath at runtime, allowing sub-projects to access classes from other sub-projects as required. However, the changes proposed by the ClassLoader Isolation RFC will result in all sub-projects being loaded as separate modules with different ClassLoaders and no longer being on the system classpath, as they were before.

Once these modularizing changes go into effect, modules will no longer be able to access classes from other modules unless they explicitly create a dependency on them. In order to create dependencies between related modules, we need a way to determine at runtime which modules depended on which other modules, which, currently, we do not have.   In addition to creating dependencies between modules, we may also need to load modules that are not already loaded, when loading a module that depends on them. For example, imagine the scenario where module A depends on modules B and C which both have dependencies of their own represented by the below graphic.

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This proposal is intended only to solve the above problem of determining at runtime, which sub-projects/modules depend on which other sub-projects/modules and is not concerned with...

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 The proposed solution is to add a manifest file with a "Dependent-Modules" attribute (or add the a "Dependent-Modules" attribute to an existing manifest file) to the jar files of all Geode sub-projects. The "Dependent-Modules" attribute will consist of a space-delimited list of sub-projects required at runtime by the current sub-project, in the form {name}-{version}, for example:

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The sample hierarchy shown in the graphic above is much cleaner than many of the actual relationships between Geode sub-projects. Below shows a more realistic hierarchy structure.

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Notice that the above hierarchy is not a tree, but a graph and that there are multiple paths between some modules, for example, A could get to C directly, or by going through D. While this may accurately represent the current dependencies between projects as they are defined in Gradle, it is unnecessarily complex given that modules can find each other via other modules; there only needs to be a single path between a module and the other modules it depends on.   The above hierarchy can be simplified at build-time by removing dependencies that are reachable via other dependencies from the dependencies dependency list of each sub-project, dependencies that are reachable via any other dependencies. The remaining dependencies in the list become the value of the "Dependent-Modules" attribute in the sub-project's manifest file. By doing this, the above hierarchy can be simplified to something more like the diagram below.

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This simplified hierarchy has significantly fewer paths, but still satisfies all of the module's dependencies., making it easier to understand and construct, while still allowing modules access to all of their dependencies. 

Changes and Additions to Public Interfaces

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