How to Build a Cache-First PWA: Strategies for Offline-First Experiences

Progressive Web Apps (PWAs) rely on caching strategically to provide a reliable offline experience. This article walks you through building a cache-first PWA using service workers, recommended caching strategies, and pitfalls to avoid for consistent behavior across devices.

"A well-designed cache-first PWA feels instantaneous and resilient—users stay productive even when connectivity is poor."

Core Principles

The cache-first approach serves resources from cache when available and falls back to network if missing. It prioritizes responsive UX and offline functionality, but requires robust cache management to avoid serving stale or inconsistent content.

Service Worker Patterns

Typical service worker lifecycle steps for a cache-first PWA:

1. Install: Pre-cache critical assets (shell) and open named caches.
2. Activate: Clean up old caches and take control of pages.
3. Fetch: Intercept network requests; attempt cache match first, then fetch and optionally update cache.

Example Service Worker (Cache-First Snippet)

const CACHE_NAME = 'app-shell-v3';
const PRECACHE = [ '/', '/index.html', '/styles.css', '/app.js' ];

self.addEventListener('install', event => {
  event.waitUntil(
    caches.open(CACHE_NAME)
      .then(cache => cache.addAll(PRECACHE))
  );
});

self.addEventListener('fetch', event => {
  event.respondWith(
    caches.match(event.request)
      .then(response => response || fetch(event.request).then(fetchRes => {
        // Optionally update cache
        return caches.open(CACHE_NAME).then(cache => {
          cache.put(event.request, fetchRes.clone());
          return fetchRes;
        });
      }))
  );
});

Cache Versioning

Use explicit cache names for versioning. During activation, compare cache names and remove old caches to prevent storage bloat. For complex apps, consider namespacing caches by resource type (shell, assets, API responses).

Sync and Update Strategies

• Background Sync: Use the Background Sync API to retry failed writes when connectivity returns.
• Stale-While-Revalidate: Serve cached content immediately, fetch fresh content in the background, and notify clients to refresh if necessary.
• Conflict Resolution: For offline writes, implement deterministic merge strategies or queue changes for server reconciliation.

Storage Considerations

Browsers enforce storage quotas. Store only necessary assets and leverage compression. For larger datasets, consider IndexedDB for structured offline storage and use caches for static assets.

Progressive Enhancement and UX

Provide indicators to users when they are offline and guarantee graceful degradation. Implement optimistic UI patterns for immediate feedback on actions taken offline.

Testing and Debugging

Use browser devtools to simulate offline mode, inspect caches, and test service worker lifecycle events. Validate behavior across browsers and mobile devices to ensure consistent offline experience.

Common Pitfalls

• Forgetting to update cache names on new deployments leading to stale content.
• Over-caching dynamic API responses and failing to handle sync/merge logic.
• Not handling storage quota errors gracefully, causing caching to silently fail.

Conclusion

Building a cache-first PWA requires careful cache management, explicit versioning, and user experience considerations for offline states. When done well, PWAs provide fast, resilient experiences that rival native apps while leveraging the web's reach.