Used in this manner ETags are similar to fingerprints, and they can be quickly compared to determine whether two representations of a resource are the same.The use of ETags in the HTTP header is optional (not mandatory as with some other fields of the HTTP 1.1 header).For example, for a user with a subscription to a monthly magazine that publishes a new issue on the first day of each month, consider the timeline shown in To provide access to all the content a customer is entitled to, keep a record of the date that each piece of content is published.
The ETag or entity tag is part of HTTP, the protocol for the World Wide Web.
It is one of several mechanisms that HTTP provides for web cache validation, which allows a client to make conditional requests.
Your app must also react to new, renewed, and lapsed subscriptions, and properly handle expired subscriptions.
Your app needs to determine which content the user has access to based on the period of time their subscription was active.
An ETag is an opaque identifier assigned by a web server to a specific version of a resource found at a URL.