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04 Pilot 2026

Appointment & SMS Reminder System

Syncs Google Calendar and sends SMS automatically — for any appointment-based business.

AppointmentsSMSGDPR

Context

Any appointment-based business (clinics, salons, services) loses time and revenue to no-shows. They needed automatic SMS reminders, synced with the calendar the front desk already uses — with no extra work for staff.

What I built

  • Multi-tenant SaaS on a VPS (FastAPI + React), with a database per client and hourly Google Calendar sync (systemd).
  • Local NSSM agent (Windows service) — the only artifact installed at the client: it securely proxies between the VPS (HTTPS) and the Huawei modem (USB) to send SMS, with delivery-receipt (DLR) tracking.
  • Contact deduplication with Union-Find + path compression on the phone-number graph, so no duplicate SMS go out.

Technical highlights

Two SMS channels, automatic failover

a Huawei E3372 modem (the client’s own SIM, ~zero cost) via the on-prem NSSM agent as the primary, with automatic fallback to the SendSMS.ro HTTP API (paid) when the modem is unavailable — hybrid cloud (VPS) + on-prem sending, with delivery receipts (DLR) on both channels.

Multi-tenant with per-client isolation

a database per client, subdomain routing, and per-client theming — plus OS-level resource isolation (Linux cgroups), so one client can’t starve another’s CPU/RAM.

GDPR by design

data isolated per client (a separate database per tenant) and credentials redacted in the audit log.

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