Implement Database-Backed Session Storage (DatabaseSessionService) for ADK Java

[prasadskarmarkar]

Is your feature request related to a problem? Please describe.
ADK Java currently provides only in-memory session storage, which is not suitable for production environments that require durable, consistent, and queryable session/state/event storage.

Projects building on ADK often need:

• Persistent session storage across restarts
• Reliable event logging and retrieval
• Concurrency-safe updates
• Support for standard relational databases
• Schema versioning for continuous delivery

Describe the solution you'd like
Introduce a production-ready database-backed session service (DatabaseSessionService) that provides durable storage for sessions, events, and state.

Key characteristics:

• Hibernate 6.6 ORM integration
• HikariCP connection pooling
• Flyway migrations for schema versioning and zero-downtime deployments
• Support for PostgreSQL, MySQL, H2, and other RDBMS
• Pessimistic locking for safe concurrent updates
• Dialect-aware JSON handling (JSONB for PostgreSQL, CLOB for others)
• Event filtering and pagination
• Comprehensive test coverage using H2 and Integration tests to test all the features on actual database running in docker.

This service would offer a robust, scalable alternative to the current in-memory session implementation.

Describe alternatives you've considered

1. Providing this as an external extension/library

   • Persistent sessions are commonly required; shipping this in-core provides a better out-of-the-box experience.

2. Continuing with in-memory storage only

   • Not viable for production deployments requiring durability or concurrency guarantees.

Additional context
The Python ADK includes a production-grade DatabaseSessionService that is widely used in real-world workloads (including GKE deployments). Adding equivalent persistent session storage to ADK Java would provide consistency across ADK environments and reduce the need for custom user-built solutions.

An implementation is available in PR : #654