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Update ADK doc according to issue #1292 - 9 - DatabaseSessionService Concurrency and Locking #1305

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Update ADK doc according to issue #1292 - 9 - DatabaseSessionService Concurrency and Locking #1305

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17 changes: 16 additions & 1 deletion docs/sessions/session/index.md
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Expand Up @@ -299,6 +299,21 @@ db_url = "sqlite+aiosqlite:///./my_agent_data.db"
session_service = DatabaseSessionService(db_url=db_url)
```

!!! note "Concurrency and Locking"

The `DatabaseSessionService` implements locking mechanisms to prevent race
conditions and ensure data consistency in concurrent environments.

* **In-Process Locking:** The service uses an internal, in-process lock to
serialize `append_event` calls for the same session. This prevents race
conditions when multiple requests try to update the same session
simultaneously within the same process.

* **Row-Level Locking:** For PostgreSQL, MySQL, and MariaDB, the service
uses row-level locking (via `SELECT ... FOR UPDATE`) to prevent race
conditions when multiple processes or replicas try to update the same
session simultaneously.

!!! warning "Async Driver Requirement"

`DatabaseSessionService` requires an async database driver. When using SQLite,
Expand Down Expand Up @@ -344,4 +359,4 @@ during a conversation turn:
it is no longer required.

This cycle highlights how the `SessionService` ensures conversational continuity
by managing the history and state associated with each `Session` object.
by managing the history and state associated with each `Session` object.
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