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Francesco Di Donato
Francesco Di Donato

Posted on • Updated on

Web Caching - Last-Modified/If-Modified-Since

Less load on the server and less bandwidth usage for the same result? Where should I sign up? Nowhere, you just need to know the right headers.

Support code

Let's keep it simple - NodeJS, no dependencies. Build with me some endpoints, each using different headers, and find out how the browser behaves based on the headers received.

Go directly to the /no-headers endpoint or take a (very quick) look at the easiest server there is.

index.mj


import { createServer } from "http";
import noHeaders from "./src/index.mjs";

createServer((req, res) => {
  switch (req.url) {
    case "/no-headers":
      return noHeaders(req, res);
  }
}).listen(8000, "127.0.0.1", () =>
  console.info("Exposed on http://127.0.0.1:8000")
);


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src/utils.mjs


import fs from "fs/promises";
import path from "path";

export function to(promise) {
  return promise.then((res) => [res, null]).catch((err) => [null, err]);
}

export async function getView(name) {
  const filepath = path.resolve(
    process.cwd(),
    "src",
    "views",
    name + ".html"
  );
  return await to(fs.readFile(filepath, "utf-8"));
}

export async function getViewStats(name) {
  const filepath = path.resolve(process.cwd(), "src", "views", name + ".html");
  return await to(fs.stat(filepath));
}



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Add an HTML file at src/views/index.html. Its content is irrelevant.


No Headers - Endpoint

It simply reads the file and sends it to the requester. Apart from the Content-Type, no caching-related header is added.



// src/no-headers.mjs
import { getView } from "./utils.mjs";

export default async (req, res) => {
  res.setHeader("Content-Type", "text/html");

  const [html, err] = await getView("index");
  if (err) {
    res.writeHead(500).end("Internal Server Error");
    return;
  }

  res.writeHead(200).end(html);
};


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Start the server (node index.mjs), open /no-headers, and check the developer tools > network tab. Enable preserver log and hit refresh a few times.

Developer tool's network tab that shows the document never being cached, always being fetched.

Open any of them, and check the Request Headers - there is nothing related to caching, and the browser obeys.



HTTP/1.1 200 OK
Content-Type: text/html
Date: <date>
Connection: keep-alive
Keep-Alive: timeout=5
Transfer-Encoding: chunked


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Last-Modified - Endpoint

Spec

Create a new endpoint (to be registered at the url /last-modified). It reads the modification time of the file (mtime) and adds it formatted as UTC under the Last-Modified header.



// src/last-modified.mjs
import { getView, getViewStats } from "./utils.mjs";

export default async (req, res) => {
  res.setHeader("Content-Type", "text/html");

  const [stats, errStats] = await getViewStats("index");
  if (errStats) {
    res.writeHead(500).end("Internal Server Error");
    return;
  }

  const lastModified = new Date(stats.mtime);
  res.setHeader("Last-Modified", lastModified.toUTCString());

  const [html, errGet] = await getView("index");
  if (errGet) {
    res.writeHead(500).end("Internal Server Error");
    return;
  }

  res.writeHead(200).end(html);
};


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In fact, among the response headers to /last-modified, you find:



HTTP/1.1 200 OK
Last-Modified: Thu, 15 Nov 2023 19:18:46 GMT


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Anyway, if you refresh the page, the entire resource is still downloaded.

Yet something changed - the browser found Last-Modified, so it reuses the value for the If-Modified-Since Request Header. The serve receives that value and, if the condition is found to be not true (not modified since), returns the status 304 Not Modified.



import { getView, getViewStats } from "./utils.mjs";

export default async (req, res) => {
  res.setHeader("Content-Type", "text/html");

  const [stats, _] = await getViewStats("index");

  const lastModified = new Date(stats.mtime);
  lastModified.setMilliseconds(0); // IMPORTANT
  res.setHeader("Last-Modified", lastModified.toUTCString());

  const ifModifiedSince = new Headers(req.headers).get("If-Modified-Since");
  if (
    ifModifiedSince &&
    new Date(ifModifiedSince).getTime() >= lastModified.getTime()
  ) {
    res.writeHead(304).end();
    return;
  }

  // This is done ONLY IF it was not a 304!
  const [html, _] = await getView("index");

  res.writeHead(200, headers).end(html);
};


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By spec Last-Modified

Note:

  • The Response Header Last-Modified is always added, even in the case of 304 Not Modified.
  • The Request Header if-modified-since may not be present - definitely happens on the first call from a new client.

Most importantly, HTTP dates are always expressed in GMT, never in local time.

While formatting a date using toUTCString, you may observe that the resulting string loses information about milliseconds. However mtime retains millisecond precision - it may have a few milliseconds more than the value received from the client, which, after formatting, loses those milliseconds.

To ensure a valid comparison between the two values, it becomes necessary to remove the milliseconds from the mtime before performing the comparison.



lastModified.setMilliseconds(0);


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Finally, request the resource few times.
Developer tool's network tab that shows the document being cached after the first retrieval
Now, just go and update the HTML file. Then ask the browser to refresh and expect to receive a 200 OK Response.
Developer tool's network tab that shows the browser newly fetching the resource after it has been modified.


It's essential to recognize that the 304 response is consistently more lightweight than the 200 response. Beyond just the reduced data payload, it contributes to a decrease in server load. This optimization extends beyond mere HTML file reads and can apply to any intricate or resource-intensive operation.

Last-Modified is a weak caching header, as the browser applies a heuristic to determine whether to fetch the item from the cache or not. Heuristics vary between browsers.

Top comments (2)

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hassanbenhzaine profile image
Hassan Benhzaine

There is a mistake in this line :
"Open any of them, and check the Request Headers - there is nothing related to caching, and the browser obeys."

You mean't Response Headers not Request Headers

:)

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hassanbenhzaine profile image
Hassan Benhzaine

Very well explained, keep up the good work :)