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Features are the most granular entity in the feature store and are logically grouped by feature groups. Features in the same feature groups are semantically related to the feature groups primary keys.

The storage location of a single feature is determined by the feature group. Hence, enabling a feature group for online storage will make a feature available as an online feature.

Features Taxonomy#

Within a feature group there are different categories of features:

  • Primary Keys: The columns that uniquely identify an entity in a feature group (e.g., the customer, the transaction, ...). A primary key may consist of multiple columns, that is, a composite primary key.
  • Event Time: The column in a feature group that stores the time at which the event for the given record has happened (the observation time for the row). You need to provide the event time, if you want the feature group to be able to support point-in-time correct joins when used to creating training data.
  • Partition Keys: The columns that define the storage layout of the feature group data in the offline feature store. The partition keys may be defined over one or more columns, the data is organized in partitions which are stored in separate subdirectories. Partitioning can help improve query efficiency for the offline feature store, by reducing the amount of data that needs to be read from disk for a given query.
  • Features: These columns are the features that can be used to train models and perform inference on models.

Feature Data Types#

When a feature is stored in the both the online and offline feature stores, it will be stored in a data storage type native to each store.

  • Offline data type: The data type of the feature when stored on the offline feature store
  • Online data type: The data type of the feature when stored on the online feature store.

The offline data type is always required, even if the feature group is stored only online. On the other hand, if the feature group is not online_enabled, its features will not have an online data type.

Offline Data Types#

The offline feature store is based on Apache Hudi and Hive Metastore, as such, any Hive Data Type can be leveraged.

Potential offline types are:

"STRUCT < label: STRING, index: INT >", "UNIONTYPE < STRING, INT>"

Online Data Types#

The online storage is based on RonDB and hence, any MySQL Data Type can be leveraged.

Potential online types are:

"None", "INT(11)", "TINYINT(1)", "SMALLINT(5)", "MEDIUMINT(7)", "BIGINT(20)",

Complex online data types#

Additionally to the online types above, Hopsworks allows users to store complex types (e.g. ARRAY) on the online feature store. Hopsworks serializes the complex features transparently and stores them as VARBINARY in the online feature store. The serialization happens when calling the save(), insert() or insert_stream() methods. The deserialization will be executed when calling the get_serving_vector() method to retrieve data from the online feature store. If users query directly the online feature store, for instance using the fs.sql("SELECT ...", online=True) statement, it will return a binary blob.

On the feature store UI, the online feature type for complex features will be reported as VARBINARY.

Online restrictions for primary key data types:#

When a feature is being used as a primary key, certain types are not allowed. Examples of such types are Float, Double, Date, Text, Blob and Complex Types (e.g. Array<>). Additionally the size of the sum of the primary key online data types storage requirements should not exceed 3KB.

Type Mapping#

The offline and online types for each feature are identified based on the types of the columns in the Spark or Pandas DataFrame, and those types are then mapped to the online and offline data types.

In the case of a Spark DataFrame, the Spark types will be mapped to the corresponding Hive Metastore type and used as offline data type. If the feature group is online enabled, Hopsworks will then map the offline data type to the corresponding online data type. The mapping is based on the following rules:

  • If the offline data type is also supported on the online feature store (e.g. INT, FLOAT, DATE, TIMESTAMP), the online data type will be the same as the offline data type
  • If the offline data type is boolean, the online data type is going to be set as tinyint
  • If the offline data type is string, the online data type is going to be set as varchar(100)
  • If the offline data type is not supported by the online feature store and it is not one of the above exception, the online data type will be set as varbinary(100) to handle complex types.

Pandas Conversion#

When registering a Pandas DataFrame as a feature group, the following mapping rules are applied:

Pandas Type Offline Feature Type
int32 INT
int64 BIGINT
float32 FLOAT
float64 DOUBLE
datetime64[ns] TIMESTAMP
object STRING

Explicit Schema Definition#

When creating a feature group it is possible for the user to control both the offline and online data type of each column. If users explicitly define the schema for the feature group, Hopsworks is going to use that schema to create the feature group, without performing any type inference. Users can explicitly define the feature group schema as follows:

from hsfs.feature import Feature

features = [

fg = fs.create_feature_group(name="fg_manual_schema",




Metadata object representing a feature in a feature group in the Feature Store.

See Training Dataset Feature for the feature representation of training dataset schemas.




Default value of the feature as string, if the feature was appended to the feature group.



Description of the feature.



Whether the feature is part of the hudi precombine key of the feature group.



Name of the feature.



Data type of the feature in the online feature store.



Whether the feature is part of the partition key of the feature group.



Whether the feature is part of the primary key of the feature group.



Data type of the feature in the offline feature store.

Not a Python type

This type property is not to be confused with Python types. The type property represents the actual data type of the feature in the feature store.








Returns true if the feature has a complex type.