An ETag is an opaque identifier assigned by a Web server to a specific version of a
resource found at a URL. If the resource content at that URL changes, a new and
different ETag is assigned. ETags can be quickly compared to determine if two
versions of a resource are the same or different. ETags can be used for:
- Cache validation, which allows a client to make conditional requests. The
Web server does not need to send a full response if the content has not
changed.
- Optimistic concurrency control, as a way to help prevent simultaneous
updates of a resource from overwriting each other.
Also documented in Javadoc and Doxygen.