# Retrieve Client Data
source: https://developer.mastercard.com/mastercard-processing-core/documentation/use-cases/data-management/query-cms-get-particular-data/retrieve-client-data/index.md

## Overview {#overview}

This use case describes how you fetch the client's data.

## Sequence diagram {#sequence-diagram}

![Sequence diagram](https://static.developer.mastercard.com/content/mastercard-processing-core/uploads/retrieve-client-data.png)

### Explanation {#explanation}

1. The server sends a `GET` request to the `/clients/{client_id}` endpoint to fetch data of the `client_id`.
2. The API returns the client's data stored in the CMS. Refer to [Endpoint](https://developer.mastercard.com/mastercard-processing-core/documentation/use-cases/data-management/query-cms-get-particular-data/retrieve-client-data/index.md#endpoint) to check the complete list of data returned by the endpoint.  
   Additionally, the API returns the `ETag` header with a value that uniquely identifies a particular client version in the CMS. To update the client's data, the `ETag` value must be passed in the `If-Match` header in the `PATCH` request to ensure idempotency. Tip: To fetch additional addresses or custom data stored in the CMS for the client, you must use dedicated API operations. Refer to [Retrieve addresses](https://developer.mastercard.com/mastercard-processing-core/documentation/use-cases/data-management/query-cms-get-particular-data/retrieve-addresses/index.md) or [Retrieve custom data](https://developer.mastercard.com/mastercard-processing-core/documentation/use-cases/data-management/query-cms-get-particular-data/retrieve-custom-data/index.md) use cases for more information. Refer to [Sandbox testing](https://developer.mastercard.com/mastercard-processing-core/documentation/sandbox-testing/data-management/query-cms-to-get-particular-data/retrieve-client-data/index.md) for information on how to execute the use case in the Sandbox environment.

## Endpoint {#endpoint}


API Reference: `GET /clients/{client_id}`