304 Not Modified

HTTP/1.1 304 Not Modified
Date: Mon, 12 Sep 2026 12:00:00 GMT
ETag: "33a64df551425fcc55e4d42a148795d9f25f89d4"
Cache-Control: max-age=3600

[NO BODY]

app.get('/data', (req, res) => {
  const currentETag = 'W/"123456"';
  // Express handles ETags automatically for res.send()
  // But you can manually check conditionals:
  if (req.headers['if-none-match'] === currentETag) {
    return res.status(304).end();
  }
  
  res.set('ETag', currentETag);
  res.json({ data: 'Hello World' });
});

from fastapi import FastAPI, Request, Response

app = FastAPI()

@app.get("/data")
def get_data(request: Request, response: Response):
    current_etag = '"123456"'
    if request.headers.get("if-none-match") == current_etag:
        return Response(status_code=304)
        
    response.headers["ETag"] = current_etag
    return {"data": "Hello World"}

func handler(w http.ResponseWriter, r *http.Request) {
    currentETag := "\"123456\""
    if match := r.Header.Get("If-None-Match"); match == currentETag {
        w.WriteHeader(http.StatusNotModified)
        return
    }
    w.Header().Set("ETag", currentETag)
    w.Write([]byte("Data"))
}

[HttpGet("data")]
public IActionResult GetData([FromHeader(Name = "If-None-Match")] string ifNoneMatch)
{
    var currentETag = "\"123456\"";
    if (ifNoneMatch == currentETag)
    {
        return StatusCode(304);
    }
    Response.Headers.Add("ETag", currentETag);
    return Ok(new { data = "Hello World" });
}

curl -I -H 'If-None-Match: "33a64df5"' http://example.com/styles.css