chore: Add migration guide for DatabaseSessionService

Co-authored-by: Shangjie Chen <deanchen@google.com>
PiperOrigin-RevId: 839376632

Author: DeanChensj

src/google/adk/sessions/migration/README.md

@@ -0,0 +1,109 @@
+# Process for a New Schema Version
+
+This document outlines the steps required to introduce a new database schema
+version for `DatabaseSessionService`. Let's assume you are introducing schema
+version `2.0`, migrating from `1.0`.
+
+## 1. Update SQLAlchemy Models
+
+Modify the SQLAlchemy model classes (`StorageSession`, `StorageEvent`,
+`StorageAppState`, `StorageUserState`, `StorageMetadata`) in
+`database_session_service.py` to reflect the new `2.0` schema. This could
+involve adding new `mapped_column` definitions, changing types, or adding new
+classes for new tables.
+
+## 2. Create a New Migration Script
+
+You need to create a script that migrates data from schema `1.0` to `2.0`.
+
+*   Create a new file, for example:
+    `google/adk/sessions/migration/migrate_1_0_to_2_0.py`.
+*   This script must contain a `migrate(source_db_url: str, dest_db_url: str)`
+    function, similar to `migrate_from_sqlalchemy_pickle.py`.
+*   Inside this function:
+    *   Connect to the `source_db_url` (which has schema 1.0) and `dest_db_url`
+        engines using SQLAlchemy.
+    *   **Important**: Create the tables in the destination database using the
+        new 2.0 schema definition by calling
+        `dss.Base.metadata.create_all(dest_engine)`.
+    *   Read data from the source tables (schema 1.0). The recommended way to do
+        this without relying on outdated models is to use `sqlalchemy.text`,
+        like:
+
+        ```python
+        from sqlalchemy import text
+        ...
+        rows = source_session.execute(text("SELECT * FROM sessions")).mappings().all()
+        ```
+
+    *   For each row read from the source, transform the data as necessary to
+        fit the `2.0` schema, and create an instance of the corresponding new
+        SQLAlchemy model (e.g., `dss.StorageSession(...)`).
+    *   Add these new `2.0` objects to the destination session, ideally using
+        `dest_session.merge()` to upsert.
+    *   After migrating data for all tables, ensure the destination database is
+        marked with the new schema version:
+
+        ```python
+        from google.adk.sessions import database_session_service as dss
+        from google.adk.sessions.migration import _schema_check
+        ...
+        dest_session.merge(
+            dss.StorageMetadata(
+                key=_schema_check.SCHEMA_VERSION_KEY,
+                value="2.0",
+            )
+        )
+        dest_session.commit()
+        ```
+
+## 3. Update Schema Version Constant
+
+You need to update `CURRENT_SCHEMA_VERSION` in
+`google/adk/sessions/migration/_schema_check.py` to reflect the new version:
+
+```python
+CURRENT_SCHEMA_VERSION = "2.0"
+```
+
+This will also update `LATEST_VERSION` in `migration_runner.py`, as it uses this
+constant.
+
+## 4. Register the New Migration in Migration Runner
+
+In `google/adk/sessions/migration/migration_runner.py`, import your new
+migration script and add it to the `MIGRATIONS` dictionary. This tells the
+runner how to get from version `1.0` to `2.0`. For example:
+
+```python
+from google.adk.sessions.migration import _schema_check
+from google.adk.sessions.migration import migrate_from_sqlalchemy_pickle
+from google.adk.sessions.migration import migrate_1_0_to_2_0
+...
+MIGRATIONS = {
+    _schema_check.SCHEMA_VERSION_0_1_PICKLE: (
+        _schema_check.SCHEMA_VERSION_1_0_JSON,
+        migrate_from_sqlalchemy_pickle.migrate,
+    ),
+    _schema_check.SCHEMA_VERSION_1_0_JSON: (
+        "2.0",
+        migrate_1_0_to_2_0.migrate,
+    ),
+}
+```
+
+## 5. Update `DatabaseSessionService` Business Logic
+
+If your schema change affects how data should be read or written during normal
+operation (e.g., you added a new column that needs to be populated on session
+creation), update the methods within `DatabaseSessionService` (`create_session`,
+`get_session`, `append_event`, etc.) in `database_session_service.py`
+accordingly.
+
+## 6. CLI Command Changes
+
+No changes are needed for the Click command definition in `cli_tools_click.py`.
+The `adk migrate session` command calls `migration_runner.upgrade()`, which will
+now automatically detect the source database version and apply the necessary
+migration steps (e.g., `0.1 -> 1.0 -> 2.0`, or `1.0 -> 2.0`) to reach
+`LATEST_VERSION`.