SeatLock

A cinema booking system written in C++ that lets multiple users book tickets concurrently without double-booking a seat.

Demo

Problem Statement

How do we build a fast booking system that prevents double-bookings?

Strategy 1: Synchronous Approach

• Users buy tickets in first-come-first-served order at a register.
• This prevents double-booking but is slow, since every purchase blocks the next.

Strategy 2: Optimistic Concurrency

• Sell seats online with no lock while a purchase is in progress.
• Although this is faster, two users can end up racing for the same seat — one of them enters their card details only to find the seat was taken.

Strategy 3: Pessimistic Locking

• Sell seats online, but when a user starts checkout they're given a short-lived lock on the seat.
• This is the best of both worlds: it's fast, and no one wastes time entering payment details for a seat someone else just took.

SeatLock implements Strategy 3, using Redis as the lock store.

Getting Started

Requirements: a C++17 compiler (g++/clang++), Make, Docker, libhiredis

# install the hiredis client library
sudo apt install libhiredis-dev   # or: brew install hiredis

# start Redis (+ Redis Commander UI on :8081)
docker compose up -d

# build and run the server
make run

The app is served at http://localhost:8080.

Testing

Tests hit Redis directly, so make sure it's running first:

docker compose up -d
make test

This runs the concurrent booking test, which fires 100,000 concurrent booking requests at the same seat and asserts exactly one succeeds — the core guarantee the locking strategy is built to provide.