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docs: Feature Repo, Types and FAQ doc updates (#3049)
* docs: Feature Repo, Types and FAQ doc updates Signed-off-by: Achal Shah <achals@gmail.com> * cr Signed-off-by: Achal Shah <achals@gmail.com>
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# Data Types in Feast | ||
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Feast frequently has to mediate data across platforms and systems, each with its own unique type system. | ||
To make this possible, Feast itself has a type system for all the types it is able to handle natively. | ||
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Feast's type system is built on top of [protobuf](https://github.com/protocolbuffers/protobuf). The messages that make up the type system can be found [here](https://github.com/feast-dev/feast/blob/master/protos/feast/types/Value.proto), and the corresponding python classes that wrap them can be found [here](https://github.com/feast-dev/feast/blob/master/sdk/python/feast/types.py). | ||
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Feast supports primitive data types (numerical values, strings, bytes, booleans and timestamps). The only complex data type Feast supports is Arrays, and arrays cannot contain other arrays. | ||
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Each feature or schema field in Feast is associated with a data type, which is stored in Feast's [registry](registry.md). These types are also used to ensure that Feast operates on values correctly (e.g. making sure that timestamp columns used for [point-in-time correct joins](point-in-time-joins.md) actually have the timestamp type). | ||
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As a result, each system that feast interacts with needs a way to translate data types from the native platform, into a feast type. E.g., Snowflake SQL types are converted to Feast types [here](https://rtd.feast.dev/en/master/feast.html#feast.type_map.snowflake_python_type_to_feast_value_type). The onus is therefore on authors of offline or online store connectors to make sure that this type mapping happens correctly. | ||
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**Note**: Feast currently does *not* support a null type in its type system. |
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# Feature Repository | ||
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## Feature Repo | ||
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A feature repository is the collection of python files that define entities, feature views and data sources. Feature Repos also have a `feature_store.yaml` file at their root. | ||
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Users can collaborate by making and reviewing changes to Feast object definitions (feature views, entities, etc) in the feature repo. | ||
But, these objects must be applied, either through API, or the CLI, for them to be available by downstream Feast actions (such as materialization, or retrieving online features). Internally, Feast only looks at the registry when performing these actions, and not at the feature repo directly. | ||
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## Declarative Feature Definitions | ||
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When using the CLI to apply changes (via `feast apply`), the CLI determines the state of the feature repo from the source files and updates the registry state to reflect the definitions in the feature repo files. | ||
This means that new feature views are added to the registry, existing feature views are updated as necessary, and Feast objects removed from the source files are deleted from the registry. |
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