Greg H
02/29/2024, 2:42 PMScout
02/29/2024, 2:42 PMIOManager
base class provided by Dagster. This custom IO manager would implement the handle_output
method to write data to the database and the load_input
method to read data from the database.
Here's a simplified example based on the documentation:
python
from dagster import IOManager, OutputContext, InputContext, asset
import pandas as pd
import sqlalchemy
class PostgresIOManager(IOManager):
def __init__(self, engine):
self.engine = engine
def handle_output(self, context: OutputContext, obj: pd.DataFrame):
# Write the DataFrame to a table in the PostgreSQL database
table_name = context.asset_key.path[-1] # Use the asset key as the table name
obj.to_sql(table_name, self.engine, if_exists='replace', index=False)
def load_input(self, context: InputContext):
# Read a DataFrame from a table in the PostgreSQL database
table_name = context.upstream_output.asset_key.path[-1]
return pd.read_sql_table(table_name, self.engine)
# Create an instance of the IO manager with a connection to the PostgreSQL database
engine = sqlalchemy.create_engine('<postgresql://username:password@host:port/dbname>')
postgres_io_manager = PostgresIOManager(engine)
# Define your assets and specify the IO manager to use
@asset(io_manager_key="my_postgres_io_manager")
def my_asset():
# Your asset logic here
return pd.DataFrame(...)
# Define your Dagster repository and include the IO manager in the resources
from dagster import repository, define_asset_job
@repository
def my_repository():
return [
define_asset_job("my_job", selection="*"),
my_asset,
{"my_postgres_io_manager": postgres_io_manager},
]
In this example, the PostgresIOManager
class is responsible for writing a Pandas DataFrame to a PostgreSQL table when handle_output
is called and for reading a DataFrame from a PostgreSQL table when load_input
is called. The engine
is an SQLAlchemy engine that provides connectivity to the PostgreSQL database.
When defining your assets, you can specify the IO manager to use by passing the io_manager_key
argument to the @asset
decorator. This key should match the key used to register the IO manager in the repository's resources.
Please note that this is a simplified example, and in a real-world scenario, you would need to handle additional considerations such as database credentials, connection management, error handling, and potentially more complex logic for reading and writing data.
Remember to install the necessary dependencies, such as sqlalchemy
and the appropriate database driver, and to adjust the connecScout
02/29/2024, 2:42 PMGreg H
02/29/2024, 2:44 PMins
to specify how to load an input asset and I use io_manager_key
to write the asset?Scout
02/29/2024, 2:44 PMins
parameter within the @asset
decorator to specify custom logic for loading input assets, and you can use the io_manager_key
parameter to specify which IO manager should be used for writing the output of the asset.
Here's a brief explanation of both:
1. `ins`: This parameter allows you to provide a dictionary where the keys are the names of the input parameters to your asset function, and the values are instances of AssetIn
. The AssetIn
can specify an input_manager_key
which tells Dagster which IO manager to use when loading the input for that particular asset.
2. `io_manager_key`: This parameter is used to specify the key of the IO manager that should be used for handling the output of the asset. When you define your asset, you can set the io_manager_key
to the key of a custom IO manager that you have defined and included in your repository's resources.
Here's an example that illustrates how to use both ins
and `io_manager_key`:
python
from dagster import asset, AssetIn, IOManager, InputContext, OutputContext
class MyCustomInputIOManager(IOManager):
def load_input(self, context: InputContext):
# Custom logic to load an input asset
pass
class MyCustomOutputIOManager(IOManager):
def handle_output(self, context: OutputContext, obj):
# Custom logic to handle the output of an asset
pass
@asset(
ins={"input_asset": AssetIn(input_manager_key="my_custom_input_io_manager")},
io_manager_key="my_custom_output_io_manager"
)
def my_asset(input_asset):
# Your asset logic here
pass
# Define your repository and include the custom IO managers in the resources
from dagster import repository, define_asset_job
@repository
def my_repository():
return [
define_asset_job("my_job", selection="*"),
my_asset,
{
"my_custom_input_io_manager": MyCustomInputIOManager(),
"my_custom_output_io_manager": MyCustomOutputIOManager(),
},
]
In this example, MyCustomInputIOManager
is used to load the input for my_asset
, and MyCustomOutputIOManager
is used to handle the output of my_asset
. The keys used in the ins
dictionary and the io_manager_key
parameter must match the keys used to register the corresponding IO managers in the repository's resources. Configure the io_manager_key
for asset... button I/O managers | Dagster button IOManagerDefinition.address_for_asset button Is there a way to specify/override the d... button Thank you for your feedback!Greg H
02/29/2024, 2:51 PMScout
02/29/2024, 2:51 PMpython
from dagster import IOManager, InputContext, OutputContext, asset, repository, define_asset_job
import pandas as pd
import sqlalchemy
class PostgresIOManager(IOManager):
def __init__(self, engine):
self.engine = engine
def handle_output(self, context: OutputContext, obj: pd.DataFrame):
# Use metadata to determine the schema and table name
table_name = context.metadata.get("table_name")
schema = context.metadata.get("schema", "public") # default to 'public' schema if not specified
obj.to_sql(table_name, self.engine, schema=schema, if_exists='replace', index=False)
def load_input(self, context: InputContext):
# Use metadata to determine the schema and table name
table_name = context.upstream_output.metadata.get("table_name")
schema = context.upstream_output.metadata.get("schema", "public")
return pd.read_sql_table(table_name, self.engine, schema=schema)
# Create an instance of the IO manager with a connection to the PostgreSQL database
engine = sqlalchemy.create_engine('<postgresql://username:password@host:port/dbname>')
postgres_io_manager = PostgresIOManager(engine)
# Define your assets and specify the IO manager to use
@asset(
metadata={"table_name": "my_input_table", "schema": "my_input_schema"},
io_manager_key="my_postgres_io_manager"
)
def my_input_asset():
# This asset represents a table in PostgreSQL and doesn't contain business logic
pass
@asset(
metadata={"table_name": "my_output_table", "schema": "my_output_schema"},
io_manager_key="my_postgres_io_manager"
)
def my_transformed_asset(my_input_asset: pd.DataFrame):
# Transform the input DataFrame and return the result
transformed_df = my_input_asset # Add your transformation logic here
return transformed_df
# Define your Dagster repository and include the IO manager in the resources
@repository
def my_repository():
return [
define_asset_job("my_job", selection="*"),
my_input_asset,
my_transformed_asset,
{"my_postgres_io_manager": postgres_io_manager},
]
In this example, the PostgresIOManager
class is responsible for writing a Pandas DataFrame to a PostgreSQL table and reading a DataFrame from a PostgreSQL table. The schema and table name are specified using metadata on the assets. When defining your assets,Scout
02/29/2024, 2:52 PMio_manager_key
argument to the @asset
decorator. This key should match the key used to register the IO manager in the repository's resources. Please note that this is a simplified example, and in a real-world scenario, you would need to handle additional considerations such as database credentials securely, connection management, error handling, and potentially more complex logic for reading and writing data.Greg H
02/29/2024, 2:55 PMScout
02/29/2024, 2:55 PMmy_input_asset
asset using the key "table_name"
. The metadata is a dictionary that contains various pieces of information about the asset, including the table name and schema where the data is located. Here's the relevant part of the code that defines the input table name: python @asset( metadata={"table_name": "my_input_table", "schema": "my_input_schema"}, io_manager_key="my_postgres_io_manager" ) def my_input_asset(): # This asset represents a table in PostgreSQL and doesn't contain business logic pass
In this snippet, the metadata
dictionary for my_input_asset
specifies that the table name is "my_input_table"
and the schema is "my_input_schema"
. When the `PostgresIOManager`'s load_input
method is called, it will use this metadata to construct the full table identifier and load the data from the specified table and schema in the PostgreSQL database. The my_input_asset
function itself doesn't contain any business logic because it represents a table in the database. The actual data loading is handled by the PostgresIOManager
when another asset depends on my_input_asset
. The IO Manager will use the metadata to know from which table to load the data.