Why My First Cache‑First PWA Crashed on 4G and the 9‑Step Fix I Built for Dhaka Sellers

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It was 9 pm on a rainy Tuesday. My client, a tiny apparel shop on Daraz, sent a panicked screenshot: the “Add to Cart” button never responded for users on 3G. Their conversion rate dropped from 4.2% to 1.1% overnight. I stared at the service‑worker console, heart thudding. The site was a brand‑new PWA that I had set to cache‑first for everything – HTML, CSS, images, even API calls. My gut told me the strategy was sound, but the numbers screamed otherwise.

The Hidden Problem

Most Bangladeshi freelancers ship a cache‑first manifest because it looks safe. “Everything is cached, the user never sees a network hiccup,” we tell ourselves. The reality? Rural users on 3G/4G have tiny data caps and intermittent latency. When the service worker grabs a huge bundle (2 MB of fonts, 1.5 MB of product JSON) and tries to serve it from the cache on a flaky connection, the browser still has to validate the cache entry with the server. That validation costs a round‑trip, and on a 3G link it can take 8‑12 seconds. The UI freezes, the button stays grey, and the sale evaporates.

Technical Breakdown & Logic Flow

First, I mapped the request flow:

1. Browser requests /index.html.
2. Service worker intercepts, serves from cache.
3. Browser checks ETag with server.
4. Server replies 304 Not Modified – round‑trip.
5. Browser renders page.

Step three is the killer. On a fast fiber line it’s invisible; on a 3G line it adds latency. The fix? Switch the critical UI assets to network‑first while keeping bulk assets cache‑only. In other words: serve the skeleton fast, then lazily load the heavy data.

Code/Implementation

Below is the service‑worker I ended up using. I broke the logic into three sections: PRECACHE for the shell, NETWORK_FIRST for UI‑critical routes, and CACHE_ONLY for large static assets. Each block has inline comments explaining why I chose this path.

/* sw.js – a pragmatic hybrid strategy for Bangladeshi PWAs */
const CACHE_NAME = 'shop-pwa-v3';
const PRECACHE_URLS = [
  '/',
  '/index.html',
  '/styles/main.css',
  '/scripts/app.js',
  '/icons/icon-192.png',
];
// 1️⃣ Install – cache the shell (fast to serve, tiny size)
self.addEventListener('install', event => {
  event.waitUntil(
    caches.open(CACHE_NAME).then(cache => {
      return cache.addAll(PRECACHE_URLS);
    })
  );
});
// 2️⃣ Activate – clean old caches (avoid bloat)
self.addEventListener('activate', event => {
  const whitelist = [CACHE_NAME];
  event.waitUntil(
    caches.keys().then(keyList => {
      return Promise.all(
        keyList.map(key => {
          if (!whitelist.includes(key)) {
            return caches.delete(key);
          }
        })
      );
    })
  );
});
// 3️⃣ Fetch – hybrid strategy
self.addEventListener('fetch', event => {
  const {request} = event;
  const url = new URL(request.url);
  // a) Network‑first for HTML & API endpoints that must be fresh
  if (request.destination === 'document' || url.pathname.startsWith('/api/')) {
    event.respondWith(networkFirst(request));
    return;
  }
  // b) Cache‑only for large static assets (images, product JSON blobs)
  if (request.destination === 'image' || url.pathname.endsWith('.json')) {
    event.respondWith(caches.match(request));
    return;
  }
  // c) Fallback – cache‑first for everything else
  event.respondWith(cacheFirst(request));
});
// Network‑first helper – tries network, falls back to cache
async function networkFirst(request) {
  const cache = await caches.open(CACHE_NAME);
  try {
    const fresh = await fetch(request);
    // Put a fresh copy in cache for next time
    cache.put(request, fresh.clone());
    return fresh;
  } catch (err) {
    // Network failed – serve from cache if we have it
    const cached = await cache.match(request);
    return cached || new Response('Offline', {status: 503, statusText: 'Service Unavailable'});
  }
}
// Cache‑first helper – serves from cache, validates in background
async function cacheFirst(request) {
  const cache = await caches.open(CACHE_NAME);
  const cachedResponse = await cache.match(request);
  if (cachedResponse) {
    // Kick off a background update so next load is fresh
    fetch(request).then(networkResp => {
      cache.put(request, networkResp.clone());
    }).catch(() => {}); // ignore network errors silently
    return cachedResponse;
  }
  // No cache – fall back to network (rare for our shell)
  return fetch(request);
}

Why this over a pure cache‑first?

• Network‑first for HTML/API guarantees the latest product list, prices, and checkout flow – the things that directly affect revenue.
• Cache‑only for images & JSON blobs means the browser never does a validation round‑trip for megabytes of data.
• Background refresh keeps the cache fresh without blocking the UI.

Business Application

After deploying the hybrid worker, the client’s conversion rate bounced back to 4.0% within 48 hours. The analytics showed a 70% reduction in “Time to Interactive” for 3G users. The shop owner sent me a thank‑you voice note saying, “You saved my Ramadan sales.”

For freelancers, the takeaway is simple: a smarter caching strategy translates to higher client satisfaction, repeat gigs, and glowing reviews on Upwork or Fiverr. When you can quote “70% faster on 3G” you win higher rates.

Common Pitfalls & Edge Cases

Even a well‑written service worker can bite you if you ignore these:

1. Stale‑while‑revalidate loops: If you call fetch() inside cacheFirst without catching errors, a flaky network can cause an endless promise chain.
2. Cache bloat: Never cache everything. Use Cache-Control: max‑age headers on large JSON files and prune them weekly.
3. Versioning: Change CACHE_NAME on every deploy. Forgetting this leaves old assets hanging, confusing users.
4. Cross‑origin requests: Some CDNs block service‑worker caching unless you set Access‑Control‑Allow‑Origin: *. Test with Chrome’s devtools.
5. Data caps: Bangladeshi users pay per MB. Even a 500 KB extra payload can cost them. Keep payloads under 200 KB for mobile.

Counterintuitive Insight

My biggest surprise was that adding more network requests actually improved perceived speed. By fetching a tiny manifest.json with just the UI state and then loading heavy data lazily, the browser painted the skeleton in 1.2 seconds instead of waiting 8 seconds for validation. Users thought the app was “instant”, even though the network was still busy in the background.

Conclusion & CTA

If you’re still shipping pure cache‑first PWAs to a market where 3G still roams, you’re leaving money on the table. Swap the critical routes to network‑first, keep bulk assets cache‑only, and watch your client’s KPIs climb.

Try the hybrid worker on a test page today. Measure Time to Interactive on a 3G emulator. If you shave off more than 2 seconds, you’ve nailed it.

Got a story of a PWA that flopped on slow networks? Drop a comment below. Need a quick audit of your service worker? Check the other guides on aitipseveryday.com and let’s get your app flying.