Your API Is Sending the Same Bytes Over and Over
Most APIs re-serialize and re-send identical payloads thousands of times a day. The client already has the data, the data hasn't changed, and yet every request burns a full round trip plus bandwidth plus database time. HTTP solved this in 1999 with conditional requests — and most APIs still don't use them.
This post shows you how to add ETag and Cache-Control to a REST API so clients can ask "has anything changed?" and get a tiny 304 Not Modified instead of the whole body.
The Two Headers That Do the Work
Cache-Control tells the client how long a response stays fresh. ETag is a fingerprint of the response body that lets the client revalidate once it goes stale. Together they let a client skip the network entirely while fresh, then revalidate cheaply afterward.
