Conditional GET with ETag

# 1) first request returns an ETag
curl -i https://api.example.com/config
#    ETag: "a1b2c3"

# 2) revalidate: send it back, get 304 if unchanged
curl -i https://api.example.com/config \
  -H 'If-None-Match: "a1b2c3"'

const first = await fetch('/config');
const etag = first.headers.get('ETag');

const next = await fetch('/config', {
  headers: { 'If-None-Match': etag }
});
if (next.status === 304) {
  // not modified — reuse cached body
}

import requests

r1 = requests.get('https://api.example.com/config')
etag = r1.headers['ETag']

r2 = requests.get('https://api.example.com/config',
                  headers={'If-None-Match': etag})
print(r2.status_code)  # 304 if unchanged (r2.content is empty)