ClassLoader Isolation

To be Reviewed By: 13 Mar 2020

Authors: Udo Kohlmeyer

Status: Draft | Discussion | Active | Dropped | Superseded

Superseded by: N/A

Problem

Apache Geode uses many libraries and frameworks to complete features. These libraries will be on the classpath for global access within the JVM using a global Java ClassLoader (default behavior). Apache Geode has the ability to run custom code deployed onto the server side. This means that logic that requires data can be run where the data is located, which results into potentially faster response times, as there are less network hop and serialization steps required to complete a single operation. Writing custom code allows users to tailor make their logic to solve their problem. This code can use either the libraries that Apache Geode has available on its classpath OR deploy the libraries that the code requires to complete the functionality. In the latter scenario, it is possible that the custom deployed code requires/uses libraries that are conflicting in version or implementation with the libraries that Apache Geode uses. A prime example of this is Apache Geode Management use of the Spring Framework.

In older versions of Apache Geode, the bundled Spring Framework version was on v4.3.x. Which meant that any user that wanted to use Spring in their custom code would be restricted to using that version. Manually upgrading the a Spring Framework version higher, like v5.x, could result in version conflicts in either the custom deployed code, Apache Geode or in both. In all cases this is behavior that is unwarranted.

Without a solution that does not restrict users on framework and versions, it will be impossible for users to most effectively use Apache Geode to their advantage. It also adds an extra burden on the Apache Geode system to always be up-to-date with all the latest versions of libraries, but also have tested with many different combinations of library versions. This of course is an impossible feat, which would result in always being on the wrong version than what the custom code required. i.e Apache Geode has upgraded to the latest version of libraryA (v2.0) but the custom deployed code is reliant on (v1.0) OR vice versa. One cannot keep everybody happy!!!

What is required is the ability to deploy custom code, that functions as expected (and tested externally) and that does not affect the Apache Geode core system or breaks in an unexpected manner due to version conflicts. The ability to deploy custom code with the freedom to use whatever libraries/frameworks and versions of those without any restriction. The expected behavior should be similar to a Web Container or Application Container. Where the web/application does not affect the container system that is runs or is affected by the libraries that the container system uses.

Anti-Goals

This proposal is aimed at ONLY solving/resolving problems that one is faced when deploying custom code, into Apache Geode, and use any library/framework or version that one chooses without affecting the core Apache Geode system or without being affected by the libraries/framework that Apache Geode uses.

This proposal is not there to help define how to modularize one's code (as per the related reading) or any other approaches (like API/SPI) to simplifying code and implementation.

Solution

As per the "related" content How to stop dependency leakage in 10 steps.pdf, the proposal hinges on the JBoss Modules library. This library will enable the capability to deploy modular code into Apache Geode and not affect or be affected by the libraries used by Apache Geode.

With JBoss Modules the following features are possible:

ClassLoader Isolation
Module Code visibility
Library and Framework usage freedom
Usage of different versions of code and libraries in the JVM at the same time
Simplified modular experience w.r.t modular code deployment maintenance

In order to have Apache Geode bootstrap correctly, Apache Geode's current bootstrapping cycle needs to be altered to accommodate for this modular loading system. The new bootstrapping mechanism will bootstrap Apache Geode components using a ModularService, which will load the Apache Geode system into its own ClassLoader. Determining what strategy is used, will be determined by the implementation of the ModuleService. Initially the JBossModuleService will be opt-in only. This way users can migrate their implementation to match the requirements of the new ModuleService.

The initial bootstrapping implementation is "simple". The implementation will bootstrap the Apache Geode system using the current bootstrapping mechanism of `LocatorLauncher` or `ServerLauncher`.

There will be two bootstrapping options:

Using the current class loading mechanism
Using the new JBoss module loading system to achieve class loading isolation

The Bootstrap module will use the ModuleService to load modules/components or load service implementations using the standard ServiceLoader functionality. Other than the bootstrapping change the "normal" behavior of the system should not change.

The biggest change that will be visible will be when opting into using the JBoss Modules class loading. The initial targeted implementation would be the `gfsh deploy jar' .

The deploy jar function initiated by running `deploy jar` from gfsh will trigger the Geode-Management register the module with the ModuleService using the uploaded JAR-file.

The ModuleService registration service will create a JBoss Module module construct, with the jar as a resource and a dependency on the Apache Geode system.

The deployment of custom code, like gfsh deploy jar, will be handled by the ModuleService. The ModuleService will load the deployed jar into its own ClassLoader with a dependency on the Apache Geode system. That way the deployed custom code will still have access to the Apache Geode's public API's, but without being exposed to its libraries.

Changes and Additions to Public Interfaces

registerModule(moduleMeta:ModuleMeta):boolean – Registers a module given the ModuleMeta data object.

loadService(clazz: Class<T>): T – Load all services of type Class<T> . This functionality is equivalent to the ServiceLoader semantics currently present in Java

unloadModule(moduleName:String): boolean – Unloads the registered module

Performance Impact

No anticipated performance impact.

Backwards Compatibility and Upgrade Path

No backward compatibility impact.

No public API's being changed

Deploying of jars might have a small change in behavior. The initial implementation might require the user to deploy a fully build jar, not part-jars where the sum of the jars make up a whole. Upgrading of libraries will result in the old jar being undeployed and the new one deployed in its place.

Prior Art

An alternative to this solution is to either leave the ClassLoader model as it is currently working in Geode. Whilst the current approach has a notion of ClassLoader isolation, but the user will still be limited to the versions of the libraries that Apache Geode chooses to use.

FAQ

Answers to questions you’ve commonly been asked after requesting comments for this proposal.

Errata

What are minor adjustments that had to be made to the proposal since it was approved?

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