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Use Conditional Requests to Save Bandwidth

Store the ETag or Last-Modified value, send it back with If-None-Match, and handle the 304 so unchanged resources cost almost nothing.

curl -i -H 'If-None-Match: "abc123"' https://api.example.com/config

Context

Make a polling client revalidate with conditional headers instead of downloading unchanged payloads. A client polls an API every few seconds and downloads the full body every time even when nothing changed, wasting bandwidth and battery.

Steps

1. Store the ETag or Last-Modified value from the last successful response. Expected: The client remembers the validator per resource.
2. Send it back as If-None-Match or If-Modified-Since on the next request. Expected: The server can answer 304 when unchanged.
3. Treat 304 as reuse the stored body, not an error, and skip parsing. Expected: Unchanged responses cost a header round trip only.
4. Fall back to a full fetch when no validator is stored. Expected: First and cache-miss requests still work.

const res = await fetch(url, {
  headers: store.etag ? { 'If-None-Match': store.etag } : {},
});
if (res.status === 304) return store.body;
store.etag = res.headers.get('etag');
store.body = await res.json();
return store.body;

Verify

• The client remembers the validator per resource.
• The server can answer 304 when unchanged.
• Unchanged responses cost a header round trip only.
• First and cache-miss requests still work.

Limits

• Conditional requests still require a round trip and server-side comparison, so they reduce bytes, not latency.
• They do not help if the resource changes on every request.