Subscription Tracker

Problem

People lose track of recurring charges across apps and currencies. Parsing subscription details from screenshots or pasted text is messy, sync must work offline, and create/update endpoints need to stay correct under concurrent duplicate requests.

Approach

Built an Express/TypeScript API on PostgreSQL (Supabase) with an offline-first Android client that syncs on reconnect. Used Redis for caching, idempotency, rate limiting, and BullMQ jobs. Replaced a planned SMS-parsing pipeline with Gemini-based image/text extraction — name, price, and currency with confidence scoring and manual-entry fallback.

Architecture

• PostgreSQL + Express/TypeScript API with JWT auth and Zod-validated request bodies
• Idempotency keys and transaction-based concurrency protection on subscription create/update
• BullMQ jobs for trial/renewal reminders, currency-rate refresh, and expired-session cleanup with retry and exponential backoff
• Redis-cached currency rates with TTL and cached-fallback when upstream APIs fail
• Gemini extraction service for screenshots/pasted text, with regex fallback for low-confidence reads
• Jest/Supertest coverage across auth, CRUD, concurrency 409s, Redis idempotency, extraction, and job dedupe — 55 tests

Outcomes

• Verified under concurrent load with a custom script simulating 50 simultaneous duplicate requests — duplicate subscriptions eliminated
• Background reminders and rate refresh keep running with retry/backoff on failure
• Currency conversions stay available during upstream outages via cached fallback
• Open source backend and Expo Android app on GitHub