# Feature Stores

Feature stores allow data teams to serve data via an offline store and an online low-latency store where data is kept in sync between the two. It also offers a centralized registry where features (and feature schemas) are stored for use within a team or wider organization.

As a data scientist working on training your model, your requirements for how you access your batch / 'offline' data will almost certainly be different from how you access that data as part of a real-time or online inference setting. Feast solves the problem of developing train-serve skew where those two sources of data diverge from each other.

Feature stores are a relatively recent addition to commonly-used machine learning stacks.

### When to use it

The feature store is an optional stack component in the ZenML Stack. The feature store as a technology should be used to store the features and inject them into the process on the server side. This includes

* Productionalize new features
* Reuse existing features across multiple pipelines and models
* Achieve consistency between training and serving data (Training Serving Skew)
* Provide a central registry of features and feature schemas

### List of available feature stores

For production use cases, some more flavors can be found in specific `integrations` modules. In terms of features stores, ZenML features an integration of `feast`.

| Feature Store                                                                                | Flavor   | Integration | Notes                                                                    |
| -------------------------------------------------------------------------------------------- | -------- | ----------- | ------------------------------------------------------------------------ |
| [FeastFeatureStore](https://docs.zenml.io/stacks/stack-components/feature-stores/feast)      | `feast`  | `feast`     | Connect ZenML with already existing Feast                                |
| [Custom Implementation](https://docs.zenml.io/stacks/stack-components/feature-stores/custom) | *custom* |             | Extend the feature store abstraction and provide your own implementation |

If you would like to see the available flavors for feature stores, you can use the command:

```shell
zenml feature-store flavor list
```

### How to use it

The available implementation of the feature store is built on top of the feast integration, which means that using a feature store is no different from what's described on the [feast page: How to use it?](https://docs.zenml.io/stacks/stack-components/feast#how-do-you-use-it).

<figure><img src="https://static.scarf.sh/a.png?x-pxid=f0b4f458-0a54-4fcd-aa95-d5ee424815bc" alt="ZenML Scarf"><figcaption></figcaption></figure>