Update README.asyncpg.md

README.asyncpg.md

@@ -9,9 +9,81 @@ There is a customized version of the FastAPI SQL database tutorial for
 
 https://github.com/gordthompson/fastapi-tutorial-cockroachdb-async
 
-### Database support for Alembic
+### Default transaction isolation level
 
-CockroachDB version 23.1 or later is required to work with Alembic.
+Applications using asyncpg that were developed prior to CockroachDB's inclusion of
+READ COMMITTED transaction isolation may operate on the assumption that the default
+isolation level will be SERIALIZABLE. For example, 
+
+```python
+import asyncio
+
+from sqlalchemy.ext.asyncio import create_async_engine
+
+
+async def async_main():
+    engine = create_async_engine(
+        "cockroachdb+asyncpg://root@localhost:26257/defaultdb",
+    )
+    async with engine.begin() as conn:
+        result = await conn.exec_driver_sql("select version()")
+        print(result.scalar().split("(")[0])  # CockroachDB CCL v23.2.4
+
+        result = await conn.exec_driver_sql("show transaction isolation level")
+        print(result.scalar())  # serializable
+
+
+asyncio.run(async_main())
+```
+
+With current versions of CockroachDB, the default transaction isolation level
+**for asyncpg only** is now READ COMMITTED
+
+```python
+import asyncio
+
+from sqlalchemy.ext.asyncio import create_async_engine
+
+
+async def async_main():
+    engine = create_async_engine(
+        "cockroachdb+asyncpg://root@localhost:26257/defaultdb",
+    )
+    async with engine.begin() as conn:
+        result = await conn.exec_driver_sql("select version()")
+        print(result.scalar().split("(")[0])  # CockroachDB CCL v24.3.0
+
+        result = await conn.exec_driver_sql("show transaction isolation level")
+        print(result.scalar())  # read committed
+
+
+asyncio.run(async_main())
+```
+
+Applications that rely on the previous behavior will have to add `isolation_level="SERIALIZABLE"`
+to their `create_async_engine()` call
+
+```python
+import asyncio
+
+from sqlalchemy.ext.asyncio import create_async_engine
+
+
+async def async_main():
+    engine = create_async_engine(
+        "cockroachdb+asyncpg://root@localhost:26257/defaultdb",
+        isolation_level="SERIALIZABLE",
+    )
+    async with engine.begin() as conn:
+        result = await conn.exec_driver_sql("select version()")
+        print(result.scalar().split("(")[0])  # CockroachDB CCL v24.3.0
+
+        result = await conn.exec_driver_sql("show transaction isolation level")
+        print(result.scalar())  # serializable
+
+
+asyncio.run(async_main())
+```
 
 ### Testing
 

sqlalchemy_cockroachdb/provision.py

@@ -6,7 +6,8 @@
 def _cockroachdb_temp_table_keyword_args(cfg, eng):
     return {"prefixes": ["TEMPORARY"]}
 
+
 @update_db_opts.for_db("cockroachdb")
 def _update_db_opts(db_url, db_opts, options):
     """Set database options (db_opts) for a test database that we created."""
-    db_opts['isolation_level'] = 'SERIALIZABLE'
+    db_opts["isolation_level"] = "SERIALIZABLE"