Engineering an Offline-First PWA for Fitness
June 28, 2026 8 min read
When you are at the gym or the local calisthenics park, internet connection is often spotty. A fitness app that requires a constant cloud connection is a broken fitness app.
Local-First Architecture Flowzy is built differently. We use an offline-first architecture where the client is the source of truth.
1. IndexedDB & Local Storage
All workout logs, fatigue metrics, and nutrition data are written locally first. The UI reacts instantly (0ms latency).
2. Background Sync
When a network connection is detected, the Service Worker negotiates a payload sync with the Supabase cloud backend in the background.
3. App Shell Caching
The entire UI (glass elements, skill tree data, algorithms) is cached on your device. Loading the app takes less than 300ms, even on a cold boot.
This means you can track your entire workout in airplane mode, and never lose a single rep.