WCF REST ETag属性

Question

I have come across ETag many times when reading about WCF REST services.

What are they and what are they used for?

Answer 1

The ETag value is an identifier.

It is explained pretty well on MSDN :

Entity Tags (or ETags) are a crucial aspect of the caching mechanisms in HTTP. An ETag is an opaque quoted string that may be returned along with a response in an ETag HTTP header. ETags are used to indicate whether or not a resource has changed state. If two requests for the same resource result in responses with the same ETag then it can be assumed that the state of the resource has not changed between the two requests.