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Hi <@ULVA73B9P> , I'm trying to connect Prefect wi...
# marvin-in-the-wild
k
Hi @Marvin , I'm trying to connect Prefect with a cloud Postgres db. I created a database credentials block with all the info such host, user, password, etc. From the docs I copied a flow code to read data from a database with SELECT, but it doesn't work, the error says that it can't find a block with the name I gave it.
m
It seems that there are a few common issues related to blocks not being found or registered correctly that could be causing your problem: 1. Database Credentials Block: According to the Prefect documentation, you should load the block using 
DatabaseCredentials.load("BLOCK_NAME")
. Example:
Copy code
from prefect_sqlalchemy import DatabaseCredentials
   database_block = DatabaseCredentials.load("your_block_name")
Ensure that the block name you provide matches exactly with the name you registered. 2. Block Registration: Ensure that your block is correctly registered. If you have created a custom block, you might need to register it:
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prefect block register -m your_module
Check if the block is visible in the Prefect UI or using the Prefect CLI. 3. Environment Variables: Verify that your environment variables are set correctly, especially if you're running Prefect in a different context like Docker. 4. Common Issues and Errors: The GitHub issues reveal known bugs that might affect block finding and registration: - Block order issues when rerunning registration. - Block editor UI breaks when a field is a 
Union
of a Pydantic model and a string. For more specific guidance, you can find details in the following Prefect documentation links: - Database Credentials Block - How to Register Blocks If the block name issue persists, I recommend reviewing your Prefect setup and the database credentials block documentation to ensure everything is configured properly. For further assistance, please provide exact error messages or detailed steps you’ve taken. (Resources: - https://docs.prefect.io/latest/integrations/prefect-sqlalchemy/credentials/ - https://docs.prefect.io/latest/concepts/blocks/)
k
@Marvin the error: RuntimeError: postgresql+asyncpg cannot be run synchronously. Please use the 
async with
syntax.
m
The error you encountered indicates that the PostgreSQL database operation requires the use of asynchronous syntax. Here's how you can modify your Prefect flow to use the 
async with
syntax for PostgreSQL operations: 1. Ensure Dependencies: Make sure you have the 
asyncpg
and 
sqlalchemy
packages installed. 2. Adapt Your Flow Code: You'll need to use 
async
functions within your flow. Below is an example of how to correctly run an async PostgreSQL query within a Prefect flow:
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python
   import asyncio
   from prefect import flow, task
   from prefect_sqlalchemy import DatabaseCredentials
   from sqlalchemy.ext.asyncio import create_async_engine, AsyncSession
   from sqlalchemy.orm import sessionmaker

   @task
   async def query_database():
       database_block = DatabaseCredentials.load("your_block_name")
       connection_string = database_block.get_connection_string_sync()

       async_engine = create_async_engine(connection_string, echo=True)

       async_session = sessionmaker(
           async_engine, expire_on_commit=False, class_=AsyncSession
       )

       async with async_session() as session:
           result = await session.execute("SELECT * FROM your_table_name")
           rows = result.fetchall()
           return rows

   @flow
   async def async_postgresql_flow():
       data = await query_database()
       for row in data:
           print(row)

   if __name__ == "__main__":
       asyncio.run(async_postgresql_flow())
3. Run The Flow: Make sure you use 
asyncio.run
to execute your main flow function. This setup ensures the PostgreSQL operations run asynchronously, which is required by the 
asyncpg
engine. For more async examples, you can refer to the following resource: Async Flow Example Follow these steps, and ensure your blocks and connection strings are correctly set up. Let me know if you need any additional help!
k
But in my sqlalchemy block, I already chose the driver to be postgresql+asyncpg @Marvin
m
If you've already configured your SQLAlchemy block with the 
postgresql+asyncpg
driver, you need to ensure that your flow and tasks are structured to handle asynchronous operations correctly. Here's an adapted example from the Prefect documentation illustrating an async flow with SQLAlchemy: 1. Flow Example: Using the 
async with
syntax for database operations:
Copy code
python
    import asyncio
    from prefect import flow, task
    from prefect_sqlalchemy import DatabaseCredentials
    from sqlalchemy.ext.asyncio import create_async_engine, AsyncSession
    from sqlalchemy.orm import sessionmaker

    @task
    async def async_query():
        # Load the database credentials block
        database_block = DatabaseCredentials.load("your_block_name")
        connection_string = database_block.get_connection_string_sync()

        # Create an async engine
        async_engine = create_async_engine(connection_string, echo=True)

        # Create a sessionmaker with the async engine
        async_session = sessionmaker(bind=async_engine, class_=AsyncSession, expire_on_commit=False)

        async with async_session() as session:
            result = await session.execute("SELECT * FROM your_table_name")
            rows = result.fetchall()
            return rows

    @flow
    async def async_postgresql_flow():
        data = await async_query()
        for row in data:
            print(row)

    if __name__ == "__main__":
        asyncio.run(async_postgresql_flow())
2. Run the Flow: Make sure to use 
asyncio.run
to execute the main flow function correctly:
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python
    if __name__ == "__main__":
        asyncio.run(async_postgresql_flow())
3. Install Prefect-SQLAlchemy: Ensure you have installed the 
prefect-sqlalchemy
package:
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sh
    pip install prefect-sqlalchemy
4. Resources and Documentation: You can refer to the following documentation for further details and best practices: - Prefect SQLAlchemy Integration - Prefect Async Flow Example This should address the error and correctly configure your workflow for asynchronous database operations using Prefect with SQLAlchemy and 
asyncpg
. Let me know if you need any further assistance!
k
@Marvin new error: NOTE: pgbouncer with pool_mode set to "transaction" or "statement" does not support prepared statements properly. You have two options: * if you are using pgbouncer for connection pooling to a single server, switch to the connection pool functionality provided by asyncpg, it is a much better option for this purpose; * if you have no option of avoiding the use of pgbouncer, then you can set statement_cache_size to 0 when creating the asyncpg connection object.
m
It looks like you're using PgBouncer in pool modes that don't support prepared statements properly. There are two ways to navigate this issue: 1. Switching to Asyncpg's Connection Pool Functionality: If you're using PgBouncer for connection pooling, consider using asyncpg's built-in connection pooling instead. 2. Setting 
statement_cache_size
to 0: If switching to asyncpg's connection pool isn't an option, you can set 
statement_cache_size
to 0 when creating the asyncpg connection object. Here's how to implement both solutions: ### Option 1: Switch to Asyncpg's Connection Pool Functionality Modify your SQLAlchemy engine creation to use asyncpg's connection pooling:
Copy code
python
import asyncio
from prefect import flow, task
from prefect_sqlalchemy import DatabaseCredentials
from sqlalchemy.ext.asyncio import create_async_engine, AsyncSession
from sqlalchemy.orm import sessionmaker

@task
async def async_query():
    # Load the database credentials block
    database_block = DatabaseCredentials.load("your_block_name")
    connection_string = database_block.get_connection_string_sync()

    # Create an async engine with asyncpg's connection pooling
    async_engine = create_async_engine(
        connection_string,
        poolclass=None,  # Allows using asyncpg's built-in connection pooling
        echo=True
    )

    # Create a sessionmaker with the async engine
    async_session = sessionmaker(bind=async_engine, class_=AsyncSession, expire_on_commit=False)

    async with async_session() as session:
        result = await session.execute("SELECT * FROM your_table_name")
        rows = result.fetchall()
        return rows

@flow
async def async_postgresql_flow():
    data = await async_query()
    for row in data:
        print(row)

if __name__ == "__main__":
    asyncio.run(async_postgresql_flow())
### Option 2: Set 
statement_cache_size
to 0 Configure SQLAlchemy to set 
statement_cache_size
to 0 when creating the asyncpg connection object:
Copy code
python
import asyncio
from prefect import flow, task
from prefect_sqlalchemy import DatabaseCredentials
from sqlalchemy.ext.asyncio import create_async_engine, AsyncSession
from sqlalchemy.orm import sessionmaker

@task
async def async_query():
    # Load the database credentials block
    database_block = DatabaseCredentials.load("your_block_name")
    connection_string = database_block.get_connection_string_sync()

    # Function to create an asyncpg connection with statement_cache_size set to 0
    async def asyncpg_creator():
        import asyncpg
        return await asyncpg.connect(dsn=connection_string, statement_cache_size=0)

    # Create an async engine with the custom asyncpg connection creator
    async_engine = create_async_engine(
        connection_string,
        creator=asyncpg_creator,
        echo=True
    )

    # Create a sessionmaker with the async engine
    async_session = sessionmaker(bind=async_engine, class_=AsyncSession, expire_on_commit=False)

    async with async_session() as session:
        result = await session.execute("SELECT * FROM your_table_name")
        rows = result.fetchall()
        return rows

@flow
async def async_postgresql_flow():
    data = await async_query()
    for row in data:
        print(row)

if __name__ == "__main__":
    asyncio.run(async_postgresql_flow())
Choose the appropriate option based on your setup and requirements. Let me know if you need further assistance!
k
@Marvin right now the code looks like this: from prefect import flow, task from prefect_sqlalchemy import SqlAlchemyConnector import asyncio @task async def setup_table(block_name: str) -> None: async with await SqlAlchemyConnector.load(block_name) as connector: await connector.execute( "CREATE TABLE IF NOT EXISTS customers (name varchar, address varchar);" ) await connector.execute( "INSERT INTO customers (name, address) VALUES (:name, :address);", parameters={"name": "Marvin", "address": "Highway 42"}, ) await connector.execute_many( "INSERT INTO customers (name, address) VALUES (:name, :address);", seq_of_parameters=[ {"name": "Ford", "address": "Highway 42"}, {"name": "Unknown", "address": "Highway 42"}, ], ) @task async def fetch_data(block_name: str) -> list: all_rows = [] async with await SqlAlchemyConnector.load(block_name) as connector: while True: # Repeated fetch* calls using the same operation will # skip re-executing and instead return the next set of results new_rows = await connector.fetch_many("SELECT * FROM customers", size=2) if len(new_rows) == 0: break all_rows.append(new_rows) return all_rows @flow async def sqlalchemy_flow(block_name: str) -> list: await setup_table(block_name) all_rows = await fetch_data(block_name) return all_rows asyncio.run(sqlalchemy_flow("BLOCK-NAME-PLACEHOLDER")) Can we use one of the solutions in this code without the hundreds of extra code?
m
Your message was too long by 75 tokens - please shorten it and try again. For reference, here's your message at the allowed limit:
right now the code looks like this: from prefect import flow, task
from prefect_sqlalchemy import SqlAlchemyConnector import asyncio @task async def setup_table(block_name: str) -> None: async with await SqlAlchemyConnector.load(block_name) as connector: await connector.execute( "CREATE TABLE IF NOT EXISTS customers (name varchar, address varchar);" ) await connector.execute( "INSERT INTO customers (name, address) VALUES (:name, :address);", parameters={"name": "Marvin", "address": "Highway 42"}, ) await connector.execute_many( "INSERT INTO customers (name, address) VALUES (:name, :address);", seq_of_parameters=[ {"name": "Ford", "address": "Highway 42"}, {"name": "Unknown", "address": "Highway 42"}, ], ) @task async def fetch_data(block_name: str) -> list: all_rows = [] async with await SqlAlchemyConnector.load(block_name) as connector: while True: # Repeated fetch* calls using the same operation will # skip re-executing and instead return the next set of results new_rows = await connector.fetch_many("SELECT * FROM customers", size=2) if len(new_rows) == 0: break all_rows.append(new_rows) return
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