Feast uses an internal type system to provide guarantees on training and serving data. Feast currently supports eight primitive types - INT32, INT64, FLOAT32, FLOAT64, STRING, BYTES, BOOL, and UNIX_TIMESTAMP - and the corresponding array types. Null types are not supported, although the UNIX_TIMESTAMP type is nullable. The type system is controlled by Value.proto in protobuf and by types.py in Python. Type conversion logic can be found in type_map.py.
During feast apply, Feast runs schema inference on the data sources underlying feature views. For example, if the schema parameter is not specified for a feature view, Feast will examine the schema of the underlying data source to determine the event timestamp column, feature columns, and entity columns. Each of these columns must be associated with a Feast type, which requires conversion from the data source type system to the Feast type system.
The feature inference logic calls _infer_features_and_entities.
_infer_features_and_entities calls source_datatype_to_feast_value_type.
source_datatype_to_feast_value_type cals the appropriate method in type_map.py. For example, if a SnowflakeSource is being examined, snowflake_python_type_to_feast_value_type from type_map.py will be called.
Feast serves feature values as Value proto objects, which have a type corresponding to Feast types. Thus Feast must materialize feature values into the online store as Value proto objects.
The local materialization engine first pulls the latest historical features and converts it to pyarrow.
Then it calls _convert_arrow_to_proto to convert the pyarrow table to proto format.
This calls python_values_to_proto_values in type_map.py to perform the type conversion.
The Feast type system is typically not necessary when retrieving historical features. A call to get_historical_features will return a RetrievalJob object, which allows the user to export the results to one of several possible locations: a Pandas dataframe, a pyarrow table, a data lake (e.g. S3 or GCS), or the offline store (e.g. a Snowflake table). In all of these cases, the type conversion is handled natively by the offline store. For example, a BigQuery query exposes a to_dataframe method that will automatically convert the result to a dataframe, without requiring any conversions within Feast.
As mentioned above in the section on materialization, Feast persists feature values into the online store as Value proto objects. A call to get_online_features will return an OnlineResponse object, which essentially wraps a bunch of Value protos with some metadata. The OnlineResponse object can then be converted into a Python dictionary, which calls feast_value_type_to_python_type from type_map.py, a utility that converts the Feast internal types to Python native types.