feat: Add DatabaseSessionService with Hibernate and Flyway support by prasadskarmarkar · Pull Request #654 · google/adk-java

prasadskarmarkar · Dec 8, 2025

Summary

Implements a production-ready database-backed session service that provides
persistent storage for ADK sessions, events, and state using JPA/Hibernate.

Key Features

Hibernate 6.6 + HikariCP connection pooling for optimal performance
Flyway migrations for schema versioning and zero-downtime deployments
Multi-database support: PostgreSQL, MySQL, H2, and other RDBMS
Thread-safe operations with pessimistic locking for concurrent updates
Comprehensive test coverage with H2 in-memory database
Dialect-aware JSON storage (JSONB for PostgreSQL, CLOB for others)
Event filtering and pagination for efficient data retrieval

Architecture

Database dependencies are non-optional in the core module for ease of use,
consistent with Python ADK's approach where `DatabaseSessionService` is in the
main package.

Note: Database drivers (PostgreSQL, MySQL) are marked as
`true`, so users must add their specific driver
dependency to their project.

Files Changed

28 files, 3,874 insertions
New `DatabaseSessionService` implementation
JPA entities and converters for session/event storage
Flyway migration scripts for PostgreSQL, MySQL, and H2
Comprehensive unit tests

Test Plan

Run `DatabaseSessionServiceTest` - all 10 tests pass
Verify Flyway migrations apply correctly
Test multi-pod deployment scenarios with migration locks
Manual testing with PostgreSQL
Manual testing with MySQL"

google-cla · Dec 8, 2025

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gemini-code-assist · Dec 8, 2025

Summary of Changes

Hello @prasadskarmarkar, 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 introduces a new, production-ready DatabaseSessionService designed to provide persistent storage for ADK sessions, events, and application/user state. It leverages JPA/Hibernate for ORM capabilities and integrates Flyway for robust schema management, supporting multiple database types. The service is built with performance and data integrity in mind, incorporating connection pooling, pessimistic locking, and dialect-aware JSON storage, making it a foundational improvement for stateful ADK applications.

Highlights

New Database Session Service: Introduces a DatabaseSessionService for persistent storage of ADK sessions, events, and state using JPA/Hibernate, replacing in-memory or other non-persistent solutions.
Database Persistence Layer: Implements a robust persistence layer utilizing Hibernate 6.6 with HikariCP for efficient connection pooling, ensuring optimal performance and scalability for session management.
Automated Schema Management: Integrates Flyway for database schema versioning and migrations, enabling zero-downtime deployments and consistent schema evolution across different environments.
Multi-Database Support: Provides out-of-the-box support for multiple relational databases including PostgreSQL, MySQL, and H2, with a flexible architecture to extend to other RDBMS.
Advanced Data Handling: Features dialect-aware JSON storage (JSONB for PostgreSQL, CLOB for others) and thread-safe operations with pessimistic locking for concurrent updates, ensuring data integrity.
Session and Event Management Features: Includes comprehensive functionalities for creating, retrieving, listing, appending events to, and deleting sessions, along with event filtering and pagination for efficient data retrieval.
Comprehensive Testing: Adds extensive unit tests for the DatabaseSessionService, Flyway migrations, and state management, primarily using an H2 in-memory database for reliability.

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gemini-code-assist

Code Review

This pull request introduces a significant and well-structured feature: a database-backed session service using Hibernate and Flyway. The implementation demonstrates strong practices, including multi-database support, Flyway for schema migrations, and pessimistic locking for concurrency control. The code is generally of high quality. However, I've identified a few critical and high-severity issues that should be addressed. The most critical concern is the use of Java serialization for persistence, which is fragile and insecure. There are also bugs causing data loss during the serialization/deserialization of certain event fields. Additionally, I've noted some misleading tests and opportunities for code cleanup and dependency improvements. My detailed comments are below.

core/src/main/java/com/google/adk/sessions/db/entity/StorageEvent.java

core/src/main/java/com/google/adk/sessions/DatabaseSessionService.java

core/src/main/java/com/google/adk/sessions/db/entity/StorageEvent.java

core/pom.xml

core/src/main/java/com/google/adk/sessions/DatabaseSessionService.java

core/src/main/java/com/google/adk/sessions/db/converter/DialectAwareJsonConverter.java

core/src/test/java/com/google/adk/sessions/StateManagementTest.java

prasadskarmarkar · Dec 10, 2025

/gemini review

gemini-code-assist

Code Review

This pull request introduces a robust, production-ready DatabaseSessionService using Hibernate and Flyway, which is a significant and well-executed feature. The implementation correctly uses pessimistic locking for concurrency control and Flyway for schema migrations, supporting multiple database backends. The test coverage is comprehensive and impressive.

My review focuses on improving maintainability, robustness, and consistency. Key suggestions include:

Refactoring duplicated transaction management logic in DatabaseSessionService.
Removing potential write side-effects from the getSession method to adhere to command-query separation.
Enhancing error logging by replacing System.err with a proper logger and logging exceptions that are currently swallowed.
Improving schema consistency in the MySQL migration script.
Cleaning up potentially unused code and over-broad .gitignore rules.

core/src/main/java/com/google/adk/sessions/DatabaseSessionService.java

.gitignore

core/src/main/java/com/google/adk/runner/Runner.java

core/src/main/java/com/google/adk/sessions/DatabaseSessionService.java

core/src/main/java/com/google/adk/sessions/db/converter/DialectAwareJsonConverter.java

core/src/main/java/com/google/adk/sessions/db/entity/StorageEvent.java

core/src/main/resources/db/migration/mysql/V1__Initial_schema.sql

prasadskarmarkar · Dec 10, 2025

/gemini review

gemini-code-assist

Code Review

This pull request introduces a robust, production-ready DatabaseSessionService using Hibernate for persistence and Flyway for database migrations. The implementation is impressive, with strong support for multiple database dialects, thread-safe operations using pessimistic locking, and comprehensive test coverage. My review focuses on a few areas for improvement, including strengthening the database schema for timestamp handling, simplifying state management logic, and removing potentially obsolete code. Overall, this is a high-quality contribution that significantly enhances the ADK's session management capabilities.

core/src/main/resources/db/migration/postgresql/V1__Initial_schema.sql

core/src/main/java/com/google/adk/sessions/DatabaseSessionService.java

core/src/main/java/com/google/adk/sessions/db/util/SchemaInitializer.java

prasadskarmarkar · Dec 10, 2025

/gemini review

gemini-code-assist

Code Review

This pull request introduces a robust, production-ready DatabaseSessionService using Hibernate for persistence and Flyway for database migrations. The implementation is comprehensive, covering multi-database support, thread-safe operations with pessimistic locking, and extensive test coverage. The architecture is well-thought-out, particularly the handling of migrations in multi-pod environments.

My review focuses on improving concurrency handling and ensuring data type consistency. I've identified a race condition during the creation of new application/user states and an issue with the deep copying of state maps, both of which could affect correctness under load. I've also suggested an improvement to the MySQL schema for better performance and consistency. Overall, this is an excellent and thorough implementation.

gemini-code-assist · Dec 10, 2025

core/src/main/java/com/google/adk/sessions/DatabaseSessionService.java

+  private StorageAppState getOrCreateAppState(EntityManager em, String appName) {
+    StorageAppState appState = em.find(StorageAppState.class, appName);
+    if (appState == null) {
+      appState = new StorageAppState();
+      appState.setAppName(appName);
+      appState.setState(new HashMap<>());
+      appState.setUpdateTime(Instant.now());
+      em.persist(appState);
+    }
+    return appState;
+  }


There is a race condition in the getOrCreateAppState method (and similarly in getOrCreateUserState). The current "check-then-act" logic (em.find() followed by em.persist()) is not atomic across different transactions. If two threads concurrently call createSession for the same new application, both could find that the StorageAppState does not exist and then both attempt to persist a new one. This will cause the second transaction to fail with a PersistenceException (due to a primary key constraint violation) when it tries to commit. This can lead to failed session creations under concurrent load.

To fix this, you should handle the potential concurrency more robustly. One common pattern is to attempt the persist operation and catch the PersistenceException. If the exception is a constraint violation, it means another transaction created the entity in the meantime, so you can then safely find the existing entity.

A simplified version of this pattern would look like this:

// Inside a try-catch block for em.persist() and em.flush() } catch (PersistenceException e) { // A more robust check would inspect the exception cause for a constraint violation. em.clear(); // Detach the failed entity from the persistence context. StorageAppState existingState = em.find(StorageAppState.class, appName); if (existingState != null) { return existingState; } // If it's still null, something else went wrong. throw e; }

This change will make the session creation process more resilient in a multi-threaded or multi-pod environment.

core/src/main/java/com/google/adk/sessions/db/converter/JsonUserType.java

core/src/main/resources/db/migration/mysql/V1__Initial_schema.sql

prasadskarmarkar · Dec 11, 2025

/gemini review

gemini-code-assist

Code Review

This pull request introduces a comprehensive and well-designed DatabaseSessionService, adding persistent storage for sessions using JPA/Hibernate and Flyway for migrations. The implementation is robust, with good handling of concurrency through pessimistic locking, transaction management, and support for multiple database dialects. The test coverage is excellent, covering unit, integration, and concurrency scenarios.

My review focuses on a few key areas to further improve correctness and maintainability:

Data Serialization: There are opportunities to make the serialization of Content objects more robust by fully leveraging Jackson's capabilities, preventing potential data loss.
Schema Definition: A minor adjustment in the H2 database schema can prevent potential data truncation issues.
Code Consistency: A small refactoring in one of the methods can improve consistency with the rest of the class.

Overall, this is a high-quality contribution that significantly enhances the ADK's capabilities.

core/src/main/java/com/google/adk/sessions/db/entity/StorageEvent.java

core/src/main/resources/db/migration/h2/V1__Initial_schema.sql

core/src/main/java/com/google/adk/sessions/DatabaseSessionService.java

prasadskarmarkar · Dec 13, 2025

/gemini review

gemini-code-assist

Code Review

This pull request introduces a comprehensive and production-ready DatabaseSessionService using Hibernate for persistence and Flyway for database migrations. The implementation is robust, featuring support for multiple database backends (PostgreSQL, MySQL, H2), pessimistic locking for concurrent operations, and a well-structured approach to schema management. The addition of extensive unit and integration tests covering concurrency, state management, and migrations is commendable. My review focuses on a few minor areas for improvement in error handling and code clarity within the new service.

core/src/main/java/com/google/adk/sessions/DatabaseSessionService.java

prasadskarmarkar · Dec 14, 2025

/gemini review

gemini-code-assist

Code Review

This pull request introduces a comprehensive and production-ready DatabaseSessionService. The implementation is robust, featuring support for multiple databases (PostgreSQL, MySQL, H2) via Hibernate, schema management with Flyway, and excellent concurrency control with pessimistic locking. The separation of state into app, user, and session scopes is well-designed. The test coverage is extensive and covers critical aspects like concurrency, migrations, and state management. My review includes a few suggestions for improving dependency management for MySQL, minor code style enhancements, and ensuring consistency in the database schema scripts. Overall, this is an excellent contribution.

gemini-code-assist · Dec 14, 2025

core/src/main/resources/db/migration/mysql/V1__Initial_schema.sql

There's an inconsistency in the data types used for JSON storage in the MySQL schema. The events.actions column is defined as JSON, while other columns intended for JSON data (like sessions.state, events.content, etc.) are defined as LONGTEXT. The JsonUserType and EventActionsUserType implementations write a string to the database for non-PostgreSQL dialects. To maintain consistency with the UserType implementations and other columns, events.actions should also be LONGTEXT.

Suggested change

actions JSON,

actions LONGTEXT,

gemini-code-assist · Dec 14, 2025

core/pom.xml

+    <!-- Database Migration -->
+
+    <!-- Flyway Core - Database schema migration and versioning tool -->
+    <dependency>
+      <groupId>org.flywaydb</groupId>
+      <artifactId>flyway-core</artifactId>
+    </dependency>
+
+    <!-- Flyway PostgreSQL Support - PostgreSQL-specific migration support -->
+    <dependency>
+      <groupId>org.flywaydb</groupId>
+      <artifactId>flyway-database-postgresql</artifactId>
+      <scope>runtime</scope>
+    </dependency>


For full support of MySQL, especially in multi-pod environments where migration locking is crucial, it's recommended to include the flyway-mysql dependency. This is analogous to how flyway-database-postgresql is included for PostgreSQL-specific features. Please consider adding it here and to the parent pom.xml's dependencyManagement section.

gemini-code-assist · Dec 14, 2025

core/src/main/java/com/google/adk/sessions/DatabaseSessionService.java

+    return Completable.fromCallable(
+            () -> {
+              executeInTransaction(
+                  em -> {
+                    // Find session
+                    SessionId id = new SessionId(appName, userId, sessionId);
+                    StorageSession session = em.find(StorageSession.class, id);
+                    if (session == null) {
+                      throw new SessionNotFoundException(
+                          String.format(
+                              "Session not found: appName=%s, userId=%s, sessionId=%s",
+                              appName, userId, sessionId));
+                    }
+
+                    // Delete session (cascade will delete events)
+                    em.remove(session);
+
+                    return null;
+                  },
+                  "Error deleting session");
+              return null;
+            })


The use of Completable.fromCallable with a Callable that just returns null is a bit indirect for an operation that has no return value. The Completable.fromAction operator is a better semantic fit here.

Consider refactoring to use fromAction to make the code slightly cleaner and more idiomatic, like so:

return Completable.fromAction(() -> executeInTransaction(em -> { ... }, "Error deleting session")) .subscribeOn(Schedulers.io());

gemini-code-assist · Dec 14, 2025

core/src/main/java/com/google/adk/sessions/db/converter/ByteArrayConverter.java

This converter is added but doesn't appear to be used by any of the new entities or services in this pull request. If it's not intended for immediate use, it should be removed to avoid adding unused code to the codebase.

gemini-code-assist · Dec 14, 2025

core/src/main/resources/META-INF/persistence.xml

+      <property name="hibernate.hikari.idleTimeout" value="30000"/>
+    </properties>
+  </persistence-unit>
+</persistence>


This file is missing a final newline character. It's a common convention and good practice to end files with a newline.

Suggested change

</persistence>

</persistence>

gemini-code-assist · Dec 14, 2025

core/src/main/resources/db/migration/h2/V1__Initial_schema.sql

+CREATE INDEX IF NOT EXISTS idx_events_session ON events(app_name, user_id, session_id);
+
+-- Index for sorting events by timestamp
+CREATE INDEX IF NOT EXISTS idx_events_timestamp ON events(timestamp);


This file is missing a final newline character. It's a common convention and good practice to end files with a newline.

Suggested change

CREATE INDEX IF NOT EXISTS idx_events_timestamp ON events(timestamp);

CREATE INDEX IF NOT EXISTS idx_events_timestamp ON events(timestamp);

gemini-code-assist · Dec 14, 2025

core/src/main/resources/db/migration/postgresql/V1__Initial_schema.sql

+CREATE INDEX IF NOT EXISTS idx_events_session ON events(app_name, user_id, session_id);
+
+-- Index for sorting events by timestamp
+CREATE INDEX IF NOT EXISTS idx_events_timestamp ON events(timestamp);


This file is missing a final newline character. It's a common convention and good practice to end files with a newline.

Suggested change

CREATE INDEX IF NOT EXISTS idx_events_timestamp ON events(timestamp);

CREATE INDEX IF NOT EXISTS idx_events_timestamp ON events(timestamp);

Summary Implements a production-ready database-backed session service that provides persistent storage for ADK sessions, events, and state using JPA/Hibernate. Key Features Hibernate 6.6 + HikariCP connection pooling for optimal performance Flyway migrations for schema versioning and zero-downtime deployments Multi-database support: PostgreSQL, MySQL, H2, and other RDBMS Thread-safe operations with pessimistic locking for concurrent updates Comprehensive test coverage with H2 in-memory database Dialect-aware JSON storage (JSONB for PostgreSQL, CLOB for others) Event filtering and pagination for efficient data retrieval Architecture Database dependencies are non-optional in the core module for ease of use, consistent with Python ADK's approach where `DatabaseSessionService` is in the main package. Note: Database drivers (PostgreSQL, MySQL) are marked as `true`, so users must add their specific driver dependency to their project.

gemini-code-assist bot reviewed Dec 8, 2025

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prasadskarmarkar marked this pull request as draft December 13, 2025 00:59

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prasadskarmarkar marked this pull request as ready for review December 14, 2025 18:56

prasadskarmarkar mentioned this pull request Dec 15, 2025

Implement Database-Backed Session Storage (DatabaseSessionService) for ADK Java #665

Open

prasadskarmarkar force-pushed the feature/database-session-service branch from 7265a33 to 78506b1 Compare December 15, 2025 18:44

prasadskarmarkar mentioned this pull request Dec 18, 2025

feat: Implementation of MySQL-backed session service for ADK #647

Open

prasadskarmarkar force-pushed the feature/database-session-service branch from 78506b1 to c8e0960 Compare December 19, 2025 20:34

prasadskarmarkar mentioned this pull request Dec 20, 2025

deleteSession() of the InMemorySessionService does not delete the user to session mapping #687

Open

prasadskarmarkar closed this Jan 2, 2026

	CREATE INDEX IF NOT EXISTS idx_events_timestamp ON events(timestamp);
	CREATE INDEX IF NOT EXISTS idx_events_timestamp ON events(timestamp);

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Summary

Key Features

Architecture

Files Changed

Test Plan

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