Feature Vectors#

Once you have trained a model, it is time to deploy it. You can get back all the features required to feed into an ML model with a single method call. A feature view provides great flexibility for you to retrieve a vector (or row) of features from any environment, whether you are either inside the Hopsworks platform, a model serving platform, or in an external environment, such as your application server. Harnessing the powerful RonDB, feature vectors are served at in-memory latency.

If you want to understand more about the concept of feature vectors, you can refer to here.

Retrieval#

You can get back feature vectors from either python or java client by providing the primary key value(s) for the feature view. Note that filters defined in feature view and training data will not be applied when feature vectors are returned. If you need to retrieve a complete value of feature vectors without missing values, the required entry are feature_view.primary_keys. Alternative, you can provide the primary key of the feature groups as the key of the entry. It is also possible to provide a subset of the entry, which will be discussed below.

# get a single vector
feature_view.get_feature_vector(
    entry = {"pk1": 1, "pk2": 2}
)

# get multiple vectors
feature_view.get_feature_vectors(
    entry = [
        {"pk1": 1, "pk2": 2},
        {"pk1": 3, "pk2": 4},
        {"pk1": 5, "pk2": 6}
    ]
)

// get a single vector
Map<String, Object> entry1 = Maps.newHashMap();
entry1.put("pk1", 1);
entry1.put("pk2", 2);
featureView.getFeatureVector(entry1);

// get multiple vectors
Map<String, Object> entry2 = Maps.newHashMap();
entry2.put("pk1", 3);
entry2.put("pk2", 4);
featureView.getFeatureVectors(Lists.newArrayList(entry1, entry2);

Required entry#

Starting from python client v3.4, you can specify different values for the primary key of the same name which exists in multiple feature groups but are not joint by the same name. The table below summarises the value of primary_keys in different settings. Considering that you are joining 2 feature groups, namely, left_fg and right_fg, the feature groups have different primary keys, and features (feature_*) in each setting. Also, the 2 feature groups are joint on different join conditions and prefix as left_fg.join(right_fg, <join conditions>, prefix=<prefix>).

For java client, and python client before v3.4, the primary_keys are the set of primary key of all the feature groups in the query. Python client is backward compatible. It means that the primary_keys used before v3.4 can be applied to python client of later versions as well.

Note:

"<rightFgId>" can be found by right_fg.id. "<joinIndex>" is the order or the feature group in the join. In the example, it is 1 because right_fg is in the first join in the query left_fg.join(right_fg, <join conditions>).

Missing Primary Key Entries#

It can happen that some of the primary key entries are not available in some or all of the feature groups used by a feature view.

Take the above example assuming the feature view consists of two joined feature groups, first one with primary key column pk1, the second feature group with primary key column pk2.

# get a single vector
feature_view.get_feature_vector(
    entry = {"pk1": 1, "pk2": 2}
)

// get a single vector
Map<String, Object> entry1 = Maps.newHashMap();
entry1.put("pk1", 1);
entry1.put("pk2", 2);
featureView.getFeatureVector(entry1);

This call will raise an exception if pk1 = 1 OR pk2 = 2 can't be found but also if pk1 = 1 AND pk2 = 2 can't be found, meaning, it will not return a partial or empty feature vector.

When retrieving a batch of vectors, the behaviour is slightly different.

# get multiple vectors
feature_view.get_feature_vectors(
    entry = [
        {"pk1": 1, "pk2": 2},
        {"pk1": 3, "pk2": 4},
        {"pk1": 5, "pk2": 6}
    ]
)

// get multiple vectors
Map<String, Object> entry2 = Maps.newHashMap();
entry2.put("pk1", 3);
entry2.put("pk2", 4);
Map<String, Object> entry3 = Maps.newHashMap();
entry3.put("pk1", 5);
entry3.put("pk2", 6);
featureView.getFeatureVectors(Lists.newArrayList(entry1, entry2, entry3);

This call will raise an exception if for example for the third entry pk1 = 5 OR pk2 = 6 can't be found, however, it will simply not return a vector for this entry if pk1 = 5 AND pk2 = 6 can't be found. That means, get_feature_vectors will never return partial feature vector, but will omit empty feature vectors.

If you are aware of missing features, you can use the passed features or Partial feature retrieval functionality, described down below.

Partial feature retrieval#

If your model can handle missing value or if you want to impute the missing value, you can get back feature vectors with partial values using python client starting from version 3.4 (Note that this does not apply to java client.). In the example below, let's say you join 2 feature groups by fg1.join(fg2, left_on=["pk1"], right_on=["pk2"]), required keys of the entry are pk1 and pk2. If pk2 is not provided, this returns feature values from the first feature group and null values from the second feature group when using the option allow_missing=True, otherwise it raises exception.

# get a single vector with 
feature_view.get_feature_vector(
    entry = {"pk1": 1},
    allow_missing=True
)

# get multiple vectors
feature_view.get_feature_vectors(
    entry = [
        {"pk1": 1},
        {"pk1": 3},
    ],
    allow_missing=True
)

Retrieval with transformation#

If you have specified transformation functions when creating a feature view, you receive transformed feature vectors. If your transformation functions require statistics of training dataset, you must also provide the training data version. init_serving will then fetch the statistics and initialize the functions with the required statistics. Then you can follow the above examples and retrieve the feature vectors. Please note that transformed feature vectors can only be returned in the python client but not in the java client.

feature_view.init_serving(training_dataset_version=1)

Passed features#

If some of the features values are only known at prediction time and cannot be computed and cached in the online feature store, you can provide those values as passed_features option. The get_feature_vector method is going to use the passed values to construct the final feature vector to submit to the model.

You can use the passed_features parameter to overwrite individual features being retrieved from the online feature store. The feature view will apply the necessary transformations to the passed features as it does for the feature data retrieved from the online feature store.

Please note that passed features is only available in the python client but not in the java client.

# get a single vector
feature_view.get_feature_vector(
    entry = {"pk1": 1, "pk2": 2},
    passed_features = {"feature_a": "value_a"}
)

# get multiple vectors
feature_view.get_feature_vectors(
    entry = [
        {"pk1": 1, "pk2": 2},
        {"pk1": 3, "pk2": 4},
        {"pk1": 5, "pk2": 6}
    ],
    passed_features = [
        {"feature_a": "value_a1"},
        {"feature_a": "value_a2"},
        {"feature_a": "value_a3"},
    ]
)

You can also use the parameter to provide values for all the features which are part of a specific feature group and used in the feature view. In this second case, you do not have to provide the primary key value for that feature group as no data needs to be retrieved from the online feature store.

# get a single vector, replace values from an entire feature group
# note how in this example you don't have to provide the value of 
# pk2, but you need to provide the features coming from that feature group
# in this case feature_b and feature_c 

feature_view.get_feature_vector(
    entry = { "pk1": 1 },
    passed_features = {
        "feature_a": "value_a", 
        "feature_b": "value_b", 
        "feature_c": "value_c"
    }
)

Feature Server#

In addition to Python/Java clients, from Hopsworks 3.3, a new feature server implemented in Go is introduced. With this new API, single or batch feature vectors can be retrieved in any programming language.