> ## Documentation Index
> Fetch the complete documentation index at: https://docs.cognite.com/llms.txt
> Use this file to discover all available pages before exploring further.

# Transformation external data sources

> Register and manage external data sources so Transformations can read Microsoft Fabric OneLake (beta REST API).

An **external data source** is a reusable, named connection that lets Transformations read from storage outside Cognite Data Fusion (CDF). For example, you can read Microsoft Fabric OneLake without staging a full copy in CDF RAW. Configure credentials and location once, then reference the source in transformation SQL by `externalId`.

This API is enabled per project. If requests return that the endpoint isn't enabled, contact Cognite Support. This is separate from the on-premises Cognite Fabric connector.

## Concepts

* **Source type** — The external system the source connects to. The supported value is `one_lake` for Microsoft Fabric OneLake.
* **Credentials** — Microsoft Entra ID application credentials: `clientId`, `tenantId`, and `clientSecret`. The client secret is write-only; it is stored encrypted and never returned when you read a data source.
* **Location** — Where in OneLake the source points: `workspaceId` and `containerId`, both as GUIDs.
* **Data set scoping** — Optional `dataSetId` that scopes access to the data source through CDF data set permissions.

## Usability checks

Before you use an external data source in a transformation, call the usability endpoint (`POST /transformations/externaldata/usability`) to verify that CDF can reach the storage location with the stored credentials. The response reports whether the connection and permissions succeed, so you can fix configuration issues before a transformation job fails.

## Key capabilities

* **List** external data sources configured for the project
* **Create or replace** external data sources, matched on `externalId`
* **Delete** external data sources by `id` or `externalId`
* **Verify usability** of a data source before using it in a transformation

<Info>
  The **stable** (`20230101`) API reference does not include external data sources. Use the **beta** or **alpha** API reference for concepts and endpoint documentation. Send `Cdf-Version: beta` on requests. For how preview APIs relate to stable, see [Beta API reference](/api-reference/concepts/20230101-beta/beta-api).
</Info>

## Rate and concurrency limits

External data source operations share the general CDF request budgets. For limit types and throttling behavior, see [API rate limits](/api-reference/concepts/20230101-beta/rate-limits).