Aside from cache-control and Expires, we have another option in our arsenal of HTTP goodies: Entity Tags, or ETags. An ETag is used to determine if the value of a resource has changed. If I update a customer record, the URI to the resource is the same, but the value is different, so I would expect the ETag to change. This becomes powerful when we’re using what is called a conditional GET. When making a GET request, we can specify additional headers, telling the service to send us the resource only if some criteria are met.
