Working Group Proposal: AOT Support

[cescoffier]

Objective

The main objective of this working group is to consolidate, enhance, and modernize Quarkus's Ahead-of-Time (AOT) and runtime efficiency (in JVM mode) capabilities. We aim to leverage OpenJDK's Project Leyden, IBM Semeru's advanced features, and JLink to provide a unified, "native-like" startup experience on the JVM.

The Problem

Currently, AOT file handling and runtime optimization in Quarkus are fragmented. While we support features like AppCDS and AWS Snapstart, the strategy lacks cohesion:

• Usability: Configuration is obscure (legacy AppCDS naming) and hard to discover.
• Integration: There is no seamless way to generate and embed AOT caches or custom runtimes (JLink) within our container image generation pipeline.
• Missed Opportunities:
  • Project Leyden: We are not fully-prepared to exploit the standardized AOT capabilities arriving in Java 25.
  • IBM Semeru: We are underutilizing Semeru's "Shared Classes Cache" (a Leyden-like mechanism available today) and "InstantOn" (CRIU/snapshotting), which offer immediate gains.
  • JLink: We heavily rely on full JDK/JRE images (UBI Java runtime), missing the chance to build minimal, modular runtimes that complement AOT optimizations for faster startup and lower storage costs.

The Proposed Solution

This working group is structuring a dedicated effort to focus on JVM AOT support in Quarkus. The working group will focus on:

1. Unified AOT Configuration: Redesigning configuration to be intuitive and forward-looking (aligned with Project Leyden), abstracting the underlying mechanism (CDS, Leyden AOT Cache, Semeru SCC).
2. Dedicated Packaging (aot-jar (proposal)): Defining and implementing a specific packaging type (e.g., quarkus.package.type=aot-jar) that automatically orchestrates the transparent training phase to generate AOT artifacts (CDS, SCC), bundles them with the application, and configures the startup scripts, removing the need for manual cache management.
3. Container & Runtime Integration:
   • AOT Embedding: Build-time support to generate and embed AOT caches (CDS/SCC) directly into container images.
   • JLink Support: Introduce a mechanism to generate custom, stripped-down JREs using jlink. Combining AOT caches with a minimal JLink runtime will yield the smallest, fastest non-native container possible.
4. Vendor-Specific Optimizations:
   • IBM Semeru: Explicit support for Semeru InstantOn (CRaC-like snapshotting) and their Shared Classes Cache (SCC), which effectively provides "Leyden-like" benefits (cached classes + AOT code) today.
5. Measurement & Validation: Establishing benchmarks to measure RSS usage, startup time, and image size specifically attributed to these enhancements.

Definition of Done

The working group will be considered successfully closed when:

• The AOT configuration namespace has been refactored.
• aot-jar Packaging: The new packaging type is implemented and fully supported in Maven/Gradle plugins.
• Container builders (Docker/Podman) support automatic generation/embedding of AOT caches.
• JLink integration is available to build minimal runtime images with Quarkus applications.
• Semeru InstantOn and SCC are first-class, configurable options in the Quarkus build.
• A comprehensive guide on "JVM Efficiency: AOT, JLink, and Caching" is published.
• Blog Post: A comparative blog post is published analyzing Plain JVM, AOT (using the new aot-jar), and Native Image. The analysis should use the Quarkus Super-Heroes microservices suite to cover diverse use cases and provide concrete, reproducible data on Startup Time and RSS usage gains.
• All sub-tasks in Issue #51976 are resolved.

Scope of Work

In Scope:

• Refactoring quarkus.package.jar.appcds.* and related configurations.
• Implementation of the aot-jar packaging type.
• Integration with OpenJDK Project Leyden (Java 25 AOT cache).
• JLink Integration: Tooling to create custom runtime images during the build.
• IBM Semeru Integration:
  • Configuration for Shared Classes Cache (SCC).
  • Support/Guide for "InstantOn" (CRIU snapshotting).
• Modifying container image generation (Jib, Docker) to support these artifacts.
• Benchmarking: executing performance tests using the Quarkus Super-Heroes suite to validate the aot-jar benefits.
• Documentation and Blog Post creation.

⠀Out of Scope:

• General runtime performance optimizations unrelated to Startup/Warmup/Footprint.

Organizing the Work

• Point of Contact: Guillaume Smet (@gmset) / Georgios Andrianakis (@geoand)
• Timeline: Features will be delivered step by step between now and Quarkus 4.

Existing Issues

• #51976 - AOT file handling (Epic)
• #51900 - Add support for AOT cache creation in container images
• #51977 - Use better name for the AOT file config generation
• #51978 - Add Semeru's AOT features to our AOT configuration