Fitsenpai

Overview

Fitsenpai is a fitness application used by 8,000+ people to track training and health metrics. As the sole full-stack engineer, I own the backend, the frontend, and everything in between.

Challenge

As a solo engineer on a production app handling real subscriptions, the product needed billing that couldn't silently double-charge a customer, background work that didn't block API responses, and a deploy process safe enough to ship without a dedicated release manager.

Architecture

A Next.js frontend backed by a Node.js and Express API, with PostgreSQL for persistence. Lemon Squeezy handles subscription billing, with webhook signature verification and idempotent handling to prevent duplicate charges. Background and deferred work runs through BullMQ and Redis to keep API response times fast.

Key decisions

• Integrated Lemon Squeezy for subscription billing with webhook signature verification and idempotent handling
• Moved deferred workloads to BullMQ and Redis to keep API response times fast
• Built automated test coverage with Jest, Vitest, and Supertest, focused on authentication and billing-critical paths
• Set up GitHub Actions CI/CD for zero-downtime deployments

Results

• Reduced rollout time by approximately 70%
• Zero-downtime deployments on every release
• No duplicate-charge incidents since the billing rework