Realtime Events lets people create, join, leave, cancel and restore events while every open screen updates live, with the database as the single source of truth.
Several people act on the same events at the same time: one edits, another joins, the organizer cancels. Every open screen has to show the current state without a refresh, and no screen may show something the database does not hold.
The usual shortcut, pushing state over the socket and treating it as the truth, breaks the moment a message is lost or arrives out of order.
Event lifecycle, organizer permissions and joining rules sit in a pure domain layer. FastAPI only parses requests and calls use cases, so the rules are tested without a server.
Every change goes through the REST API and is committed first. Only then does one WebSocket channel notify every open client, which maps the notification into its own state. A notification a client cannot read makes it reload from the API.
The WebSocket messages are specified in an AsyncAPI 3.1 document, so the frontend and any other client build against the same contract.
The live channel runs inside one service instance. The documentation names the next step to scale it out, a shared broker such as Redis Pub/Sub or PostgreSQL LISTEN/NOTIFY, instead of pretending the limit is not there.
Ports and adapters. FastAPI is only the entry point; use cases work over the domain, and persistence, the live channel and migrations are adapters. SQLite runs locally and PostgreSQL in production.
Clients
Entry point
Application core
Use cases
Domain
Event lifecycle · Organizer permissions · Joining rules
No framework code
Adapters
A split list and detail view on desktop becomes one list on a phone. Joined and canceled events are marked, and the interface switches between English and German.
The service runs in production with a public demo: the frontend on Vercel, the API on Render and PostgreSQL behind Alembic migrations.
The backend has 100+ automated tests with coverage above 85%, enforced in CI, and the frontend keeps UI components apart from API and WebSocket logic, with its own unit tests.
Python · FastAPI · SQLAlchemy · Alembic · PostgreSQL · WebSockets · AsyncAPI · React · TypeScript · Vite · Tailwind
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