FastAPI Postgres Pub/Sub Chat

This sample pairs FastAPI with PostgreSQL LISTEN/NOTIFY to demonstrate real-time updates between two application containers. A minimal chat UI sends messages with a REST request, and both FastAPI instances broadcast the new message over WebSockets after Postgres notifies them.

Prerequisites

1. Download Defang CLI
2. (Optional) If you are using Defang BYOC authenticate with your cloud provider account
3. (Optional for local development) Docker CLI

Development

To run the application locally, you can use the following command:

docker compose -f compose.dev.yaml up --build

Once everything is running:

• Visit http://localhost:8000 for the first FastAPI service.
• Visit http://localhost:8001 for the second service.
• Send a chat message in either window. Both pages should update immediately, proving Postgres LISTEN/NOTIFY fans the event across containers.

Stop the stack with Ctrl+C, then run docker compose -f compose.dev.yaml down.

Configuration

For this sample, you can rely on the defaults. Override them with environment variables if needed:

Note that if you are using the 1-click deploy option, you can set these values as secrets in your GitHub repository and the action will automatically deploy them for you.

POSTGRES_PASSWORD

Database password (default chat_password).

defang config set POSTGRES_PASSWORD --random

SSL_MODE

Postgres SSL mode (default disable, should set to require in production).

defang config set SSL_MODE=require

Deployment

Note

Download Defang CLI

Defang Playground

Deploy your application to the Defang Playground by opening up your terminal and typing:

defang compose up

BYOC

If you want to deploy to your own cloud account, you can use Defang BYOC.

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Title: FastAPI Postgres Pub/Sub

Short Description: FastAPI sample that stores messages in Postgres and streams them to two app instances via LISTEN/NOTIFY.

Tags: FastAPI, PostgreSQL, WebSockets, PubSub

Languages: Python, SQL