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

# Avoid mid-air collisions

> ETag support to prevent mid-air collisions

To prevent updating stale data - which can lead to data loss, we recommend the usage of
[ETag](https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/ETag).

Resources returned by Metriport's Medical API include an `eTag` property, which can later on be sent back to the API
when performing an update on said data.

The API will then compare that ETag with the one on the repository: if they don't
match it means data was updated by another process and the server will abort the update and respond with
[412 - Precondition Failed](https://developer.mozilla.org/en-US/docs/Web/HTTP/Status/412); otherwise it will proceed
updating the resource.

To send the ETag to an `update` endpoint, you can use the `If-Match` HTTP header, or include an `eTag` property
on the request body. The header takes precedence if both are informed.

The SDK implements this automatically, as long as the data obtained from a `get` or `list` function is updated and sent to
the respective `update` function. Example:

```typescript theme={null}
const patient = await metriportClient.getPatient("...");
// logic that updates the patient
...
// this update will automatically send the ETag if present on patient
await metriportClient.updatePatient(patient, facilityId);
```

If no ETag is provided the API will perform updates without checking for mid-air collisions.

This technique is also called
[Optimistic Locking](https://en.wikipedia.org/wiki/Optimistic_concurrency_control#Web_usage).

Note that currently we only support ETag to prevent mid-air collisions, not for caching purposes.