SIM — Gym Edition

SIM (Sales and Inventory Management) built for a real gym, running on hardware I set up and maintain. Front desk check-in, subscriptions, retail checkout, staff attendance, and financial reporting — all in one offline-capable desktop app.

Front desk check-in: NFC tap or manual entry, with live reader status.

The Problem

Most gym software is either a subscription SaaS priced for a multi-location chain, or a spreadsheet-and-notebook system that breaks down the moment a member forgets their number at the desk. Neither fits a single-location gym that wants fast check-in, accurate billing, and reporting it can actually trust — without paying per-member SaaS fees or depending on an internet connection to let someone in the door.

What It Does

• NFC check-in — members tap a card at the front desk; a WebSocket gateway pushes reader events (scan, connect, error) to the UI in real time. Manual check-in by member number is available as a fallback.
• Subscriptions & packages — session-based or date-based expiry, freeze/unfreeze with custom re-expiry dates, renewal grace-period pricing, inline discounts, partial payments, and payment-method tracking, with payment status recomputed automatically as payments come in.
• Retail & cafe POS — a separate companion Electron terminal app for front-desk retail and cafe sales, with its own inventory (stock in/out, computed stock levels) and thermal receipt printing.
• Staff attendance — personnel clock in/out via card swipe, generating attendance reports without a separate timeclock system.
• Financial dashboard — revenue actuals and forecasts, daily breakdowns, average transaction value, and member counts, all computed from the same transaction data driving check-in and billing.
• Session confirmations queue — a review step for pending sessions before they’re logged, so front-desk mistakes don’t silently corrupt billing.

Revenue actuals vs. forecast, daily breakdown, and transaction averages.

The companion POS terminal app, used at the front desk for retail and cafe sales.

Why It’s Built This Way

SIM runs as an Electron desktop app talking to a local NestJS backend and PostgreSQL database — no cloud dependency for day-to-day operation. Auto-migration on startup keeps the schema current without manual DB work, and an auto-updater (with a separate release channel for the POS terminal) keeps the gym’s install current without me physically visiting.

Hand-written SQL migrations for full control over the schema, and direct integration with an ACR122 NFC reader and USB thermal printer via ESC/POS — the kind of hardware-facing work that doesn’t show up in a typical web app, and that surfaces its own class of bugs: printer memory corruption, timezone boundaries that only break at midnight, silent installer mismatches on auto-update.

Tech Stack

• NestJS backend with parameterized queries over pg
• Angular 21 admin console + a separate Angular POS terminal renderer
• Electron desktop shell (Mac/Windows/Linux), electron-builder, electron-updater
• PostgreSQL
• NFC hardware integration (pcsclite, nfc-pcsc) and ESC/POS thermal printing
• JWT auth, WebSocket gateway for real-time reader events
• GitHub Actions CI on a self-hosted runner, publishing to a private release channel