ETags for conditional requests and caching

ETags enable efficient caching by allowing clients to make conditional requests that return 304 Not Modified when content hasn't changed. Rails automatically generates ETags based on response content, and fresh_when or stale? methods handle the conditional logic. When a client sends If-None-Match with a cached ETag, Rails compares it to the current response ETag and returns 304 with an empty body if they match. This saves bandwidth and server-side rendering time. ETags work particularly well for APIs serving relatively stable content like user profiles or configuration data. The challenge is ensuring ETag generation accounts for all dependencies—missing a dependency means clients receive stale data.