# Versioning
<a name="postVersions"></a>

If you enable versioning for a bucket, `POST` automatically generates a unique version ID for the object being added. Amazon S3 returns this ID in the response by using the `x-amz-version-id` response header. 

If you suspend versioning for a bucket, Amazon S3 always uses `null` as the version ID of the object stored in a bucket. 

For more information about returning the versioning state of a bucket, see [GET Bucket (Versioning Status)](API_GetBucketVersioning.md).

Amazon S3 is a distributed system. If you enable versioning for a bucket and Amazon S3 receives multiple write requests for the same object simultaneously, all versions of the object are stored. 

To see sample requests that use versioning, see [Sample Request](RESTObjectPOST.md#ExampleVersionObjectPost).