Implement per-session compaction control and plugin hooks

[jayy-77]

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• Related: Configure context compression per session #4132

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[gemini-code-assist]

Summary of Changes

Hello @jayy-77, I'm Gemini Code Assist¹! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!

This pull request enhances the event compaction mechanism by introducing more flexible and granular control. It allows applications to define custom logic for when compaction should occur on a per-session basis and provides a new plugin hook for dynamic session configuration. This improves the adaptability of the system, enabling tailored compaction strategies based on specific session states or user requirements.

Highlights

• Per-session Compaction Predicate: Introduced a compaction_predicate in EventsCompactionConfig to allow dynamic, per-session control over whether compaction should run, based on custom logic provided by a callable.
• before_session_start Plugin Hook: Added a new plugin lifecycle hook, before_session_start, enabling plugins to inspect and modify session settings (like compaction status) before an invocation begins.
• Session-level Compaction Control: Implemented set_compaction_enabled and is_compaction_enabled methods on the Session object, providing direct control to enable or disable compaction for individual sessions.
• Centralized Compaction Logic: A new should_compact_session utility function was added to consolidate the logic for determining if compaction should occur, prioritizing session-level overrides and then app-level predicates.

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Changelog

• src/google/adk/apps/app.py
  • Added Callable and TYPE_CHECKING imports.
  • Introduced compaction_predicate to EventsCompactionConfig for dynamic compaction control.
• src/google/adk/apps/compaction.py
  • Added should_compact_session function to determine if compaction should run, considering session overrides and app predicates.
• src/google/adk/plugins/base_plugin.py
  • Imported Session for type checking.
  • Added before_session_start async callback for plugins to modify session settings.
• src/google/adk/runners.py
  • Imported should_compact_session.
  • Integrated before_session_start plugin hook into _run_with_trace, rewind_async, and run_live.
  • Updated compaction check to use should_compact_session.
• src/google/adk/sessions/session.py
  • Added set_compaction_enabled and is_compaction_enabled methods for per-session compaction control.

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[adk-bot]

Response from ADK Triaging Agent

Hello @jayy-77, thank you for creating this PR!

Could you please describe the change, including the problem and the solution you are proposing?

In addition, could you please provide a testing plan for your changes? This should include details of any unit tests you have added or updated, as well as instructions for manual end-to-end testing, with logs or screenshots to help reviewers understand the fix.

This information will help reviewers to review your PR more efficiently. Thanks!

[gemini-code-assist]

Code Review

This pull request introduces significant enhancements to the application's event compaction mechanism and plugin system. It adds per-session control over event compaction through a new compaction_predicate in EventsCompactionConfig and dedicated methods (set_compaction_enabled, is_compaction_enabled) in the Session class. Furthermore, a new before_session_start plugin hook has been integrated, allowing plugins to dynamically adjust session settings, such as compaction, before an invocation begins. The refactoring to use should_compact_session centralizes the compaction decision logic, improving maintainability. Overall, the changes are well-implemented, enhance flexibility, and maintain code clarity.