# Caching with ETags — REST API Design

Source: https://www.geekswithgeeks.com/en/restapi/api-caching-etags

> Letting clients skip re-downloading data that hasn't changed.

## Cache-Control sets the rules

`Cache-Control: max-age=60` tells clients and proxies a response is fresh for 60 seconds — no request needed at all in that window. `no-store` says never cache it (sensitive data).

## ETags for revalidation

An `ETag` is a fingerprint of the current representation. The client resends it as `If-None-Match`; if unchanged the server replies `304 Not Modified` with **no body** — saving bandwidth while confirming freshness.

## The revalidation round-trip

The client always sends the request, but a match keeps the body off the wire.

```http
GET /orders/482 HTTP/1.1
If-None-Match: "a1b2c3"

HTTP/1.1 304 Not Modified
ETag: "a1b2c3"
```

**Quiz:** A client sends `If-None-Match` with an ETag that still matches the current resource. What should the server return?

- [ ] 200 OK with the full body
- [x] 304 Not Modified with no body
- [ ] 404 Not Found

*Answer:* 304 Not Modified with no body. A matching ETag means the client's cached copy is still valid, so the server confirms freshness with 304 and skips resending the body.