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HTTP server: Caching headers

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Cache headers let clients skip downloads they already have. This server sends cache-control, etag, and last-modified with a file, and answers a matching if-none-match revalidation with 304 Not Modified and no body.

Serve this script's own source file so the example is self-contained.
const filePath = import.meta.filename!;

Deno.serve(async (req) => {
  const stat = await Deno.stat(filePath);
A weak etag (W/ prefix) derived from mtime and size is cheap and good enough for cache validation. Hash the content instead if you need a strong validator, for example to support byte-range resumption.
  const mtime = stat.mtime ?? new Date();
  const etag = `W/"${mtime.getTime().toString(16)}-${stat.size.toString(16)}"`;

  const headers = {
    "etag": etag,
    "last-modified": mtime.toUTCString(),
Clients may reuse the file for 60 seconds, then must revalidate.
    "cache-control": "max-age=60, must-revalidate",
    "content-type": "text/plain; charset=utf-8",
  };
The client revalidates by echoing the etag in if-none-match. If it still matches, a bodyless 304 tells it to keep using its copy.
  if (req.headers.get("if-none-match") === etag) {
    return new Response(null, { status: 304, headers });
  }

  const file = await Deno.open(filePath, { read: true });
  return new Response(file.readable, { headers });
});
The first request downloads the file and the validators: curl -i http://localhost:8000/ HTTP/1.1 200 OK etag: W/"19eb80919f8-8bd" last-modified: Thu, 11 Jun 2026 18:54:24 GMT cache-control: max-age=60, must-revalidate content-type: text/plain; charset=utf-8 transfer-encoding: chunked date: Thu, 11 Jun 2026 18:54:53 GMT Revalidating with that etag transfers no body: curl -i http://localhost:8000/ -H 'if-none-match: W/"19eb80919f8-8bd"' HTTP/1.1 304 Not Modified etag: W/"19eb80919f8-8bd" last-modified: Thu, 11 Jun 2026 18:54:24 GMT cache-control: max-age=60, must-revalidate content-type: text/plain; charset=utf-8 date: Thu, 11 Jun 2026 18:54:53 GMT

Run this example locally using the Deno CLI:

deno run -N -R https://docs.deno.com/examples/scripts/http_server_static_caching.ts

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