etag property

etag is used for optimistic concurrency control as a way to help prevent simultaneous updates of a model from overwriting each other.

It is strongly suggested that systems make use of the etag in the read-modify-write cycle to perform model updates in order to avoid race conditions: An etag is returned in the response to GetModel, and systems are expected to put that etag in the request to UpdateModel to ensure that their change will be applied to the model as intended.