Building subscription functionality in a fitness app development requires more than integrating Apple StoreKit or Google Play Billing.

A successful payment doesn't automatically mean a user should receive permanent premium access. Subscriptions can expire, renew, enter a billing grace period, or be revoked. Users may also switch devices, reinstall the app, or restore earlier purchases.

For example, imagine a fitness app with three subscription tiers:

Plan Features

Free Workout tracking and basic progress history

Premium Personalized workout plans and advanced analytics

Pro All Premium features plus personalized coaching

When someone purchases Premium on an iPhone and later signs in on Android, the backend should determine whether they can access their paid features.

That's where purchase validation, subscription state management, and entitlements become essential.

In this tutorial, we'll design a subscription architecture using:

TypeScript and Node.js

PostgreSQL

Apple StoreKit and App Store…