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Asset checks

When building data applications, it is important to ensure the quality of all underlying entities. In this step, you will add checks to confirm that your assets are producing the expected data.

In Dagster, these quality measures are called asset checks and are defined in a similar way to assets. When an asset is materialized, asset checks also execute and verify that certain criteria are met based on the logic within the check.

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1. Define an asset check​

To create an asset check, define a function and decorate it with the @dg.asset_check decorator. Within the asset check, set the asset parameter to indicate which asset the check runs against.

The code below creates an asset check for the orders_aggregation asset to ensure that there are rows in the underlying DuckDB table:

src/dagster_tutorial/defs/assets.py
...


@dg.asset_check(asset="orders_aggregation")
def orders_aggregation_check(duckdb: DuckDBResource) -> dg.AssetCheckResult:
    table_name = "orders_aggregation"
    with duckdb.get_connection() as conn:
        row_count = conn.execute(f"select count(*) from {table_name}").fetchone()[0]

    if row_count == 0:
        return dg.AssetCheckResult(
            passed=False, metadata={"message": "Order aggregation check failed"}
        )

    return dg.AssetCheckResult(
        passed=True, metadata={"message": "Order aggregation check passed"}
    )

This asset check queries the table directly to determine if the data is valid. Based on the query result, it sets the AssetCheckResult to True or False.

info

The asset check uses the same DuckDBResource resource defined for the assets. Resources can be shared across all objects in Dagster.

2. Verify that the asset check runs​

In the Dagster UI at http://127.0.0.1:3000, you should now see an asset check associated with the orders_aggregation asset:

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Asset checks run automatically when an asset is materialized, but you can also execute them manually in the UI:

  1. Navigate to Assets.

  2. Click Reload definitions.

  3. Navigate to the Asset Details page for the orders_aggregation asset.

  4. Select the Checks tab.

  5. Click Execute for orders_aggregation.

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