An ETag is an entity tag, a string value associated with an instance of an entity. ETag values are a cache validation mechanism, allowing you to know when an entity instance is stale or out-of-date.
Smart APIs set and maintain ETag values via the ConcurrencyToken property. Whenever an instance of an entity is created or updated, its ConcurrencyToken property changes.
If your client has caching enabled, you will receive an ETag response header when you perform an HTTP GET request. You can also get ETag values
in the response body when requesting full metadata:
GET https://sam.acmeserver.com/SampleService/Sppid/v2/Pipes?$format=application/json;odata.metadata=full
in the form of the @odata.etag annotation.
ETags play a role in supporting optimistic concurrency checks when performing update and delete operations. Optimistic concurrency ensures that modifying an entity instance occurs only on the latest version of the data.
If you attempt to modify data that has changed since you got it, an error is returned. For details see Update/delete operations using optimistic concurrency.