RushHour

A sandbox for system design: a ticket-selling service for large events, built to showcase the hard parts: contention over scarce inventory, caching, and load balancing.

This repo is deliberately a foundation. The plumbing runs and is testable by hand; the interesting decisions are left open as seams to be filled in.

Architecture

PostgreSQL is the durable source of truth, shared by every API instance. Overselling is prevented by a single atomic conditional UPDATE, the relational equivalent of a CAS (compare-and-swap):

UPDATE inventory SET available = available - @q WHERE event_id = @id AND available >= @q RETURNING available;

Redis is in the stack and reachable, but not on the hot path yet. It enters once the database is measured to be the bottleneck under load, not before.

src/RushHour.Api/
  Domain/              Event - the model (intentionally minimal)
  Abstractions/        IEventCatalog, IInventoryStore  <- the seams for design decisions
  Infrastructure/      PostgresEventCatalog, PostgresInventoryStore, DatabaseInitializer
  Endpoints/           /events, /events/{id}/reservations
  Program.cs           DI wiring; Postgres source of truth, Redis
deploy/nginx.conf      Load balancer config (round-robin over api1/api2)
docker-compose.yml     postgres + redis + 2 api instances + nginx
tests/requests.http    Manual smoke tests
tests/load-test.sh     Concurrency test (reveals behaviour under load)
docs/adr/              Where design decisions are recorded

Requirement

• Docker Desktop (for PostgreSQL, Redis, and the load balancer)

Optional

• .NET 8 SDK (only if you want to run the API outside Docker)

Run everything in Docker (load balancer + 2 instances + Postgres + Redis)

docker compose up --build

URL	What
http://localhost:8080	nginx load balancer (round-robins to api1/api2)
http://localhost:8081	api1 directly
http://localhost:8082	api2 directly
localhost:5432	PostgreSQL (user/pass/db: rushhour)
localhost:6379	Redis

Run the API locally (against the dockerized infra)

docker compose up -d postgres redis      # just the infrastructure
dotnet run --project src/RushHour.Api     # API on the host; port printed on startup