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Python / FastAPI Basics Interview Questions

How do you containerise and deploy a FastAPI application with Docker?

Containerising FastAPI with Docker ensures consistent environments across development, staging, and production. Use a multi-stage build to keep the production image small, and run with Gunicorn + Uvicorn workers for production-grade concurrency.

# Dockerfile
FROM python:3.12-slim AS base
WORKDIR /app

# Install dependencies first (layer caching)
COPY requirements.txt .
RUN pip install --no-cache-dir -r requirements.txt

# Copy application code
COPY . .

# Create non-root user for security
RUN adduser --disabled-password --gecos "" appuser
USER appuser

EXPOSE 8000

# Production: Gunicorn manages multiple Uvicorn worker processes
CMD ["gunicorn", "main:app",\
     "--workers", "4",\
     "--worker-class", "uvicorn.workers.UvicornWorker",\
     "--bind", "0.0.0.0:8000"]


# docker-compose.yml
version: "3.9"
services:
  api:
    build: .
    ports:
      - "8000:8000"
    environment:
      - DATABASE_URL=postgresql+asyncpg://user:pass@db/mydb
      - SECRET_KEY=${SECRET_KEY}  # from .env
    depends_on:
      db:
        condition: service_healthy
    restart: unless-stopped

  db:
    image: postgres:16
    environment:
      POSTGRES_USER: user
      POSTGRES_PASSWORD: pass
      POSTGRES_DB: mydb
    healthcheck:
      test: ["CMD-SHELL", "pg_isready -U user -d mydb"]
      interval: 5s
      retries: 5
    volumes:
      - pgdata:/var/lib/postgresql/data

volumes:
  pgdata:

Worker calculation
CPUsRecommended workers (2*CPU+1)
13
25
49
817
Why use Gunicorn with UvicornWorker in production instead of just uvicorn --reload?
Why should the Docker container run as a non-root user?

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