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First, the browser checks the local cache and finds the previous response.
Unfortunately, the browser can't use the previous response because the response has now expired.
From a performance optimization perspective, the best request is a request that doesn't need to communicate with the server: a local copy of the response allows you to eliminate all network latency and avoid data charges for the data transfer.
That’s the problem that validation tokens, as specified in the ETag header, are designed to solve.
The server generates and returns an arbitrary token, which is typically a hash or some other fingerprint of the contents of the file.
The client doesn't need to know how the fingerprint is generated; it only needs to send it to the server on the next request.
If the fingerprint is still the same, then the resource hasn't changed and you can skip the download.
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Fetching something over the network is both slow and expensive.