Running PostgreSQL in a Dev Container with Flask/Django

Not familiar with Dev Containers? Read my first post about Dev Containers.

I recently added Dev Container support to a Flask+PostgreSQL sample and a Django+PostgreSQL sample. I really wanted to be able to run PostgreSQL entirely inside the Dev Container, since 1) I've had a hard time trying to setup PostgreSQL on my laptop in the past, and 2) I'd like to access the database when running the Dev Container inside Github Codespaces on the web. Fortunately, thanks to some Docker magic, I figured it out! 🐳

The first step is to create a docker-compose.yml file inside the .devcontainer folder:

version: "3"

services:
  app:
    build:
      context: ..
      dockerfile: .devcontainer/Dockerfile
      args:
        VARIANT: 3.9
        USER_UID: 1000
        USER_GID: 1000

    volumes:
      - ..:/workspace:cached

    # Overrides default so things don't shut down after the process ends
    command: sleep infinity

    # Runs app on the same network as the database container,
    # allows "forwardPorts" in devcontainer.json function
    network_mode: service:db

  db:
    image: postgres:latest
    restart: unless-stopped
    volumes:
      - postgres-data:/var/lib/postgresql/data
    environment:
      POSTGRES_USER: app_user
      POSTGRES_DB: app
      POSTGRES_PASSWORD: app_password

volumes:
  postgres-data:

That file declares an app service based off a Dockerfile (which we'll see next) as well as a db service based off the official postgres image that stores data in a mounted volume. The db service declares environment variables for the database name, username, and password, which must be used by any app that connects to that database.

The next step is the Dockerfile, also inside the .devcontainer folder:

FROM mcr.microsoft.com/vscode/devcontainers/python:0-3

RUN curl -fsSL https://aka.ms/install-azd.sh | bash

ENV PYTHONUNBUFFERED 1

RUN apt-get update && export DEBIAN_FRONTEND=noninteractive \
    && apt-get -y install --no-install-recommends postgresql-client

This file is particular to the needs of my sample, so it builds on a pre-built Python-optimized Dev Container from Microsoft and includes the azd tool. Your own file could start from any Docker image that includes Python. However, you will want the final command in the file that installs a PostgreSQL client.

The final file inside the .devcontainer folder is devcontainer.json, which is the entry point for Github Codespaces and the VS Code Dev Containers extension, and describes all the customizations. Here's a simplified version of mine:

{
    "name": "Python 3 & PostgreSQL",
    "dockerComposeFile": "docker-compose.yml",
    "service": "app",
    "workspaceFolder": "/workspace",
    "forwardPorts": [8000, 5432],
    "extensions": [
        "ms-python.python",
        "mtxr.sqltools",
        "mtxr.sqltools-driver-pg"
    ],
    "settings": {
        "sqltools.connections": [{
            "name": "Container database",
            "driver": "PostgreSQL",
            "previewLimit": 50,
            "server": "localhost",
            "port": 5432,
            "database": "app",
            "username": "app_user",
            "password": "app_password"
        }],
        "python.pythonPath": "/usr/local/bin/python"
    }
}

Let's break that down:

• dockerComposeFile points at the docker-compose.yml from earlier.
• service matches the name of the non-postgres service from that file.
• workspaceFolder matches the location of the volume from that file.
• forwardPorts instructs the Dev Container to expose port 8000 (for the Django app) and port 5432 (for the PostGres DB).
• extensions includes a really neat extension, SQLTools, which provides a graphical UI for the database tables and allows you to run queries against the tables.
• Inside settings, sqltools.connections specifies the same database name, username, and password that was declared in the docker-compose.yml.

The final step is to make sure that the app knows how to access the PostgreSQL database. In my sample apps, the DB details are set via environment variables, so I just use a .env file that looks like this:

DBNAME=app
DBHOST=localhost
DBUSER=app_user
DBPASS=app_password

...And that's it! If you'd like, here's a video where I run a Flask+PostgreSQL Dev Container inside Github Codespaces and also highlight the changes described above:

Check out the sample repos for full code, and let me know if you run into issues enabling a PostgreSQL DB in your own Dev Containers.