ADR-0007: Unified Feature Transformations and Feature Views

Status

Accepted

Context

In Feast, the OnDemandFeatureView name did not clearly convey that transformations execute at read time. The term "On Demand" was ambiguous about when the transformation occurs. Additionally, there were multiple feature view types (FeatureView, BatchFeatureView, StreamFeatureView, OnDemandFeatureView) with:

Excessive logic handling each type throughout the codebase (e.g., FeatureStore.apply(), get_online_features, get_historical_features).
Redundant fields across the different feature view classes.
Unclear transformation timing: when transformations occur, where they execute, and how materialization works varied by type.

Type	When Transformation Occurs	Where	Materialization
FeatureView	Undefined	Outside Feast	Feature Server or Batch
BatchFeatureView	Batch process	Offline Store (external)	Feature Server or Batch
StreamFeatureView	Streaming process	Stream Processor (external)	Stream Processor
OnDemandFeatureView	On Read	Feature Server	Feature Server
OnDemandFeatureView (writes)	On Write	Feature Server	Feature Server

Decision

Unify Batch, Streaming, and On-Demand feature views into a single FeatureView class with a @transform decorator that makes execution timing explicit.

Transformation Types

python

from enum import Enum

class FeastTransformation(Enum):
    NONE = 0       # No transformations (default)
    ON_READ = 1    # Current On Demand Feature View behavior
    ON_WRITE = 2   # Transformations at write time
    BATCH = 3      # Batch processing transformations
    STREAMING = 4  # Stream transformations

Proposed API

python

@transform(
    type=FeastTransformation.ON_WRITE,
    schema=[...],
    entity=[...],
    sources=[...],
    mode="python",           # pandas, substrate, etc.
    engine="feature_server", # Spark, Snowflake, etc.
    orchestrator=None,       # Airflow, KFP, etc.
)
def my_feature_view(inputs):
    outputs = {
        "my_feature": [v * 1.0 for v in inputs["input_feature_1"]],
    }
    return outputs

Key Decisions

Single class rather than defining a V2 class. A breaking change in stages is preferred to avoid rework for Java and Go servers.
Explicit transformation timing via enum rather than implicit behavior based on class type.
Staggered release: Ship a version supporting both old and new APIs with deprecation logging, then ship a breaking version.
Five clear primitives: Entities, DataSources, Fields, FeatureViews, and FeatureServices.

Current Implementation Status

The codebase currently uses a transformation() decorator and TransformationMode enum (with modes like PANDAS, PYTHON, SPARK, RAY, SQL, SUBSTRAIT) in sdk/python/feast/transformation/base.py. The legacy OnDemandFeatureView, StreamFeatureView, and BatchFeatureView classes still exist during the migration period.

Consequences

Positive

Clearer, more explicit API that makes transformation timing obvious.
Removes excessive handling of each feature view type throughout the codebase.
Eliminates redundant field definitions across multiple classes.
Establishes five clear primitives for the Feast data model.
FeatureViews can declare other FeatureViews as data sources, enabling computational graphs.

Negative

Requires a migration period with both old and new APIs supported.
Breaking change that needs careful coordination across Python, Java, and Go components.
Users must update existing feature view definitions during migration.

References

Original RFC: Feast RFC-043: Unify Feature Transformations and Feature Views
GitHub Issue: #4584
Related RFCs: RFC-021 (On-Demand Transformations), RFC-036 (Stream Transformations)
Implementation: sdk/python/feast/transformation/base.py