Tim Bray refines how to use it. What you want to do is compute the ETag based on the underlying data resources that actually drive the page creation; the input to that process, not its output. This is often going to be a small number (sometimes one) of timestamp or version fields in a database row, or metadata from the underlying filesystem. It's also going to be application-dependent. So a framework that was really designed for the Web would expose the ETag generation to the application programmer in a way that let them be smart about conserving the resources that actually matter.
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