Slotline: Appointment Booking
A booking platform with real-time availability and WhatsApp reminders that cut no-shows by a third for service businesses.
- Year
- 2024
- Timeline
- 9 weeks
- Category
- Booking System
- −34%
- No-show rate
- 0
- Double-bookings since launch
- 60s
- Average time to book
The Problem
Service studios juggled bookings across phone, DMs, and a paper diary. Double-bookings were common, no-shows costly, and staff wasted hours on reminder calls. They needed real-time self-serve booking and reminders that got read.
The Solution
Slotline: a booking system with real-time, conflict-free availability, staff and resource scheduling, and automated WhatsApp reminders. Customers book in seconds; staff see one live calendar.
My Role
Full-stack engineer. Designed the availability engine that prevents double-booking under concurrency, built the Next.js booking flow and admin calendar, and wired WhatsApp.
Tech Stack
- Next.js
- Node.js
- Express.js
- PostgreSQL
- Prisma ORM
- Redis
- WhatsApp Business API
- DigitalOcean
- Docker
How it fits together.
The high-level architecture: services, data stores, and the external systems they talk to.
Approach
The core is the availability engine: two people tapping the same slot must never both succeed. I used short-lived Redis locks plus a database uniqueness constraint as a backstop, so concurrency is safe under load and double-bookings dropped to zero. WhatsApp reminders far outperformed the old SMS/email, which moved the no-show number.
Highlights
- Real-time, conflict-free availability across staff and resources.
- WhatsApp confirmations, reminders, and one-tap reschedule.
- Timezone-aware scheduling with buffer and lead-time rules per service.
Screens
