Add dynamic LCP priority worker logic

assets/javascript/log.js

@@ -16,7 +16,7 @@ import {uuid} from './utils/uuid';
  * implementation. This allows you to create a segment or view filter
  * that isolates only data captured with the most recent tracking changes.
  */
-const MEASUREMENT_VERSION = 91;
+const MEASUREMENT_VERSION = 92;
 
 /**
  * A 13-digit, random identifier for the current page.
@@ -282,12 +282,32 @@ const trackINP = async () => {
 const trackLCP = async () => {
   onLCP(
     (metric) => {
+      let dynamicFetchPriority;
+
+      // If the LCP element is an image, send a hint for the next visitor.
+      const {element, lcpEntry} = metric.attribution;
+      if (lcpEntry?.url && lcpEntry.element?.tagName.toLowerCase() === 'img') {
+        const elementWithPriority = document.querySelector('[fetchpriority]');
+        if (elementWithPriority) {
+          dynamicFetchPriority =
+            elementWithPriority === lcpEntry.element ? 'hit' : 'miss';
+        }
+        navigator.sendBeacon(
+          '/hint',
+          JSON.stringify({
+            path: originalPathname,
+            selector: element,
+          })
+        );
+      }
+
       log.event(metric.name, {
         value: metric.delta,
         metric_rating: metric.rating,
         metric_value: metric.value,
-        debug_target: metric.attribution.element || '(not set)',
+        debug_target: element || '(not set)',
         debug_url: metric.attribution.url,
+        debug_dfp: dynamicFetchPriority,
         debug_ttfb: metric.attribution.timeToFirstByte,
         debug_rld: metric.attribution.resourceLoadDelay,
         debug_rlt: metric.attribution.resourceLoadTime,

test/worker.js

@@ -0,0 +1,93 @@
+import {strict as assert} from 'assert';
+import fetch from 'node-fetch';
+
+const urlWithLCPImage =
+  '/articles/my-challenge-to-the-web-performance-community/';
+
+const urlWithoutLCPImage = '/articles/cascading-cache-invalidation/';
+
+describe('worker', function () {
+  describe('priority hints', function () {
+    beforeEach(async () => {
+      // Delete the cache by passing an empty selector.
+      // TODO: figure out a better way to do this. There doesn't seem to
+      // currently be a way to clear KV store data locally via wrangler.
+      for (const url of [urlWithLCPImage, urlWithLCPImage]) {
+        await fetch(`http://127.0.0.1:3000/hint`, {
+          method: 'POST',
+          body: JSON.stringify({
+            path: url,
+            selector: '',
+          }),
+        });
+      }
+    });
+
+    it('should apply priority hints to prior LCP images', async () => {
+      await browser.url(urlWithLCPImage);
+
+      const fp1 = await browser.execute(() => {
+        return document.querySelector('img').getAttribute('fetchpriority');
+      });
+
+      // Wait until the hint has been sent.
+      await browser.executeAsync((done) => {
+        new PerformanceObserver((list) => {
+          for (const entry of list.getEntries()) {
+            if (entry.name.endsWith('/hint')) {
+              done();
+            }
+          }
+        }).observe({type: 'resource', buffered: true});
+      });
+
+      await browser.url('/__reset__');
+
+      // Wait until the SW has unregistered.
+      await browser.waitUntil(async () => {
+        return await browser.execute(() => {
+          return window.__ready__ === true;
+        });
+      });
+
+      await browser.url(urlWithLCPImage);
+
+      const fp2 = await browser.execute(() => {
+        return document.querySelector('img').getAttribute('fetchpriority');
+      });
+
+      assert.strictEqual(fp1, null);
+      assert.strictEqual(fp2, 'high');
+    });
+
+    it('should not apply priority hints to prior non-LCP images', async () => {
+      await browser.url(urlWithoutLCPImage);
+
+      const fp1 = await browser.execute(() => {
+        return document.querySelector('img').getAttribute('fetchpriority');
+      });
+
+      // No hint should be sent for this page, but we need to wait a bit
+      // to ensure that it doesn't happen.
+      await browser.pause(2000);
+
+      await browser.url('/__reset__');
+
+      // Wait until the SW has unregistered.
+      await browser.waitUntil(async () => {
+        return await browser.execute(() => {
+          return window.__ready__ === true;
+        });
+      });
+
+      await browser.url(urlWithoutLCPImage);
+
+      const fp2 = await browser.execute(() => {
+        return document.querySelector('img').getAttribute('fetchpriority');
+      });
+
+      assert.strictEqual(fp1, null);
+      assert.strictEqual(fp2, null);
+    });
+  });
+});

worker/index.js

@@ -1,4 +1,9 @@
 import {applyExperiment, getExperiment} from './lib/experiments.js';
+import {
+  addPriorityHints,
+  getPriorityHintKey,
+  logPriorityHint,
+} from './lib/performance.js';
 import {getRedirectPath} from './lib/redirects.js';
 import {matchesRoute} from './lib/router.js';
 
@@ -17,6 +22,18 @@ function getXIDFromCookie(cookie) {
   return cookie.match(/(?:^|;) *xid=(\.\d+) *(?:;|$)/) && RegExp.$1;
 }
 
+/**
+ * Removes a trailing `index.content.html` from a URL path.
+ * @param {string} path
+ * @returns {string}
+ */
+function normalizePath(path) {
+  if (path.endsWith('index.content.html')) {
+    return path.slice(0, -18);
+  }
+  return path;
+}
+
 /**
  * @param {Request} request
  * @param {Response} response
@@ -71,23 +88,31 @@ async function handleRequest({request, url, startTime, vars}) {
 
   const experiment = getExperiment(xid);
 
-  const response = await fetch(url.href, {
-    body: request.body,
-    headers: request.headers,
-    method: request.method,
-    redirect: request.redirect,
-    cf: {
-      cacheEverything: vars.ENV === 'production',
-      cacheTtlByStatus: {'200-299': 604800, '400-599': 0},
-    },
-  });
+  const [response, priorityHintsSelector] = await Promise.all([
+    fetch(url.href, {
+      body: request.body,
+      headers: request.headers,
+      method: request.method,
+      redirect: request.redirect,
+      cf: {
+        cacheEverything: vars.ENV === 'production',
+        cacheTtlByStatus: {'200-299': 604800, '400-599': 0},
+      },
+    }),
+    vars.PRIORITY_HINTS.get(
+      getPriorityHintKey(request, normalizePath(url.pathname))
+    ),
+  ]);
 
   const clone = new Response(response.body, response);
 
   setXIDToCookie(xid, clone);
   addServerTimingHeaders(clone, startTime);
 
   const rewriter = new HTMLRewriter();
+  if (priorityHintsSelector) {
+    addPriorityHints(rewriter, priorityHintsSelector);
+  }
   if (experiment) {
     applyExperiment(experiment, rewriter);
   }
@@ -104,6 +129,10 @@ export default {
     const startTime = Date.now();
     const url = new URL(request.url);
 
+    if (url.pathname === '/hint' && request.method === 'POST') {
+      return logPriorityHint(request, vars.PRIORITY_HINTS);
+    }
+
     // Return early if no route matches.
     // Note: this should never happen in production.
     if (!matchesRoute({request, url})) {

worker/lib/performance.js

@@ -0,0 +1,55 @@
+class PriorityHintsTransform {
+  #applied = false;
+  element(element) {
+    if (!this.#applied) {
+      element.setAttribute('fetchpriority', 'high');
+      this.#applied = true;
+    }
+  }
+}
+
+/**
+ * @param {string} experiment
+ * @param {HTMLRewriter} rewriter
+ */
+export function addPriorityHints(rewriter, selector) {
+  rewriter.on(selector, new PriorityHintsTransform());
+}
+
+/**
+ * @param {Request} request
+ * @param {string} path
+ * @returns {string}
+ */
+export function getPriorityHintKey(request, path) {
+  const device =
+    request.headers.get('sec-ch-ua-mobile') === '?1' ? 'mobile' : 'desktop';
+
+  // URL-encode the path because wrangler doesn't handle slashes when
+  // running locally (it treats them as directory separators).
+  return `${device}:${encodeURIComponent(path)}`;
+}
+
+/**
+ * @param {Request} request
+ * @param {Object} store
+ */
+async function storePriorityHints(request, store) {
+  const {path, selector} = await request.json();
+  const key = getPriorityHintKey(request, path);
+
+  const storedSelector = await store.get(key);
+  if (selector !== storedSelector) {
+    await store.put(key, selector);
+  }
+}
+
+/**
+ * @param {Request} request
+ * @param {Object} store
+ * @returns {Response}
+ */
+export async function logPriorityHint(request, store) {
+  await storePriorityHints(request, store);
+  return new Response(); // Empty 200.
+}

wrangler.toml

@@ -5,6 +5,10 @@ account_id = "833aa47653f1a867bf6f206ed576cd2f"
 compatibility_date = "2022-09-24"
 workers_dev = true
 
+kv_namespaces = [
+  { binding = "PRIORITY_HINTS", id = "7fdf8d6f35d042a5b21b77ab6a56028e", preview_id = "40156d62dc48468aa5e21fe01ed5b01e" }
+]
+
 [dev]
 host = "localhost:3001" # Origin server
 port = 3000 # Worker server
@@ -21,6 +25,7 @@ routes = [
   "philipwalton.com/shell*",
   "philipwalton.com/articles*",
   "philipwalton.com/about*",
+  "philipwalton.com/hint",
 ]
 [env.production.vars]
 ENV = "production"