Datasources are responsible for connecting data and compute infrastructure. Each Datasource provides Great Expectations DataAssets (or batches in a DataContext) connected to a specific compute environment, such as a SQL database, a Spark cluster, or a local in-memory Pandas DataFrame. Datasources know how to access data from relevant sources such as an existing object from a DAG runner, a SQL database, S3 bucket, or local filesystem.
To bridge the gap between those worlds, Datasources interact closely with generators which are aware of a source of data and can produce produce identifying information, called “batch_kwargs” that datasources can use to get individual batches of data. They add flexibility in how to obtain data such as with time-based partitioning, downsampling, or other techniques appropriate for the datasource.
For example, a generator could produce a SQL query that logically represents “rows in the Events table with a timestamp on February 7, 2012,” which a SqlAlchemyDatasource could use to materialize a SqlAlchemyDataset corresponding to that batch of data and ready for validation.
Since opinionated DAG managers such as airflow, dbt, prefect.io, dagster can also act as datasources and/or generators for a more generic datasource.
When adding custom expectations by subclassing an existing DataAsset type, use the data_asset_type parameter to configure the datasource to load and return DataAssets of the custom type.
See Batch Generators for more detail about how batch generators interact with datasources and DAG runners.
See datasource module docs Datasource Module for more detail about available datasources.