Database containers for local development in Docker - Commands & Configuration

Introduction

Running a database inside a container on your computer helps you test and develop apps without installing the database directly. It keeps your main system clean and lets you start or stop the database easily.

When you want to try out a new database version without changing your main system

When you need a database for your app but don't want to install it permanently

When you want to share the same database setup with your team easily

When you want to reset the database quickly by restarting the container

When you want to run multiple databases on one machine without conflicts

Config File - docker-compose.yml

docker-compose.yml

version: '3.8'
services:
  postgres-db:
    image: postgres:15
    restart: always
    environment:
      POSTGRES_USER: user123
      POSTGRES_PASSWORD: pass123
      POSTGRES_DB: myappdb
    ports:
      - "5432:5432"
    volumes:
      - pgdata:/var/lib/postgresql/data
volumes:
  pgdata:

This file tells Docker to run a PostgreSQL database container.

services: defines the database container named postgres-db.

image: uses the official PostgreSQL version 15 image.

environment: sets the username, password, and database name inside the container.

ports: maps the container's port 5432 to your computer's port 5432 so apps can connect.

volumes: keeps database data safe even if the container stops or restarts.

Commands

This command starts the PostgreSQL database container in the background so it runs without blocking your terminal.

Terminal

docker-compose up -d

Expected Output

Creating network "default" with the default driver

Creating volume "pgdata" with default driver

Creating postgres-db ... done

→

-d - Run containers in detached mode (in the background)

This command lists running containers so you can check if the database container is up and running.

Terminal

docker ps

Expected Output

CONTAINER ID   IMAGE          COMMAND                  CREATED          STATUS          PORTS                    NAMES

abcdef123456   postgres:15    "docker-entrypoint.s…"   10 seconds ago   Up 9 seconds    0.0.0.0:5432->5432/tcp   postgres-db

This command shows the logs of the database container to verify it started correctly without errors.

Terminal

docker logs postgres-db

Expected Output

PostgreSQL Database directory appears to contain a database; skipping initialization

2024-06-01 12:00:00.000 UTC [1] LOG:  starting PostgreSQL 15 on x86_64-pc-linux-gnu, compiled by gcc (GCC) 12.2.0, 64-bit

2024-06-01 12:00:00.001 UTC [1] LOG:  listening on IPv4 address "0.0.0.0", port 5432

2024-06-01 12:00:00.002 UTC [1] LOG:  listening on IPv6 address "::", port 5432

2024-06-01 12:00:00.003 UTC [1] LOG:  database system is ready to accept connections

This command stops and removes the database container and network when you no longer need the database running.

Terminal

docker-compose down

Expected Output

Stopping postgres-db ... done

Removing postgres-db ... done

Removing network default

Key Concept

If you remember nothing else from this pattern, remember: running a database in a container lets you develop and test safely without installing the database on your main system.

Common Mistakes

Not mapping the container port to the host port

Your app cannot connect to the database because the port is not accessible outside the container.

Always use the ports section in docker-compose.yml to map container port 5432 to host port 5432.

Not setting environment variables for user, password, and database

The database will use default credentials or fail to start properly, causing connection errors.

Set POSTGRES_USER, POSTGRES_PASSWORD, and POSTGRES_DB in the environment section.

Not using a volume for database data

Data will be lost when the container stops or is removed.

Use a named volume to persist database data across container restarts.

Summary

Use docker-compose.yml to define a PostgreSQL database container with user, password, and database name.

Start the database container in the background with 'docker-compose up -d'.

Check the container status with 'docker ps' and logs with 'docker logs postgres-db'.

Stop and remove the container with 'docker-compose down' when done.

Practice

1. What is the main benefit of using a database container for local development?

easy

A. You can run a database without installing it on your computer

B. It makes your computer run faster

C. It automatically writes your application code

D. It replaces the need for a database in production

Solution

1. Step 1: Understand what a database container does

   A database container runs a database inside a Docker container, so you don't need to install the database software directly on your computer.

2. Step 2: Identify the main benefit for local development

   This means you can start and stop the database easily without changing your computer setup.

3. Final Answer:

   You can run a database without installing it on your computer -> Option A

4. Quick Check:

   Database container = no local install needed [OK]

Hint: Containers run databases isolated, no install needed [OK]

Common Mistakes:

• Thinking containers speed up the computer
• Confusing containers with code generators
• Believing containers replace production databases

2. Which Docker command correctly starts a PostgreSQL database container exposing port 5432?

easy

A. docker build -p 5432 postgres

B. docker start -p 5432 postgres

C. docker create postgres -p 5432:5432

D. docker run -d -p 5432:5432 postgres

Solution

1. Step 1: Identify the correct command to run a container with port mapping

   The command to start a container and map ports is docker run -d -p hostPort:containerPort image. Here, -d runs it in the background.

2. Step 2: Check each option

   docker run -d -p 5432:5432 postgres uses docker run -d -p 5432:5432 postgres, which is correct. docker start -p 5432 postgres uses docker start which does not support -p. docker create postgres -p 5432:5432 uses docker create which creates but does not start the container. docker build -p 5432 postgres uses docker build which builds images, not runs containers.

3. Final Answer:

   docker run -d -p 5432:5432 postgres -> Option D

4. Quick Check:

   Run + port mapping = docker run -d -p [OK]

Hint: Use 'docker run -d -p host:container image' to start with ports [OK]

Common Mistakes:

• Using 'docker start' with port flags
• Confusing 'docker create' with 'docker run'
• Using 'docker build' to run containers

3. What will be the output of the command docker ps after running docker run -d -p 3306:3306 --name mysql-db mysql?

medium

A. No containers running

B. An error saying port 3306 is already in use

C. A running container named mysql-db with port 3306 mapped

D. A stopped container named mysql-db

Solution

1. Step 1: Understand what the command does

   The command runs a MySQL container in detached mode (-d), names it mysql-db, and maps port 3306 on the host to port 3306 in the container.

2. Step 2: Predict the output of docker ps

   docker ps lists running containers. Since the container runs detached and should start successfully, it will show a running container named mysql-db with port 3306 mapped.

3. Final Answer:

   A running container named mysql-db with port 3306 mapped -> Option C

4. Quick Check:

   Detached run + docker ps = running container shown [OK]

Hint: Detached run shows container in 'docker ps' list [OK]

Common Mistakes:

• Assuming 'docker ps' shows stopped containers
• Expecting errors without port conflicts
• Confusing 'docker ps' with 'docker ps -a'

4. You tried to start a PostgreSQL container with docker run -d -p 5432:5432 postgres but got an error that port 5432 is already in use. What should you do to fix this?

medium

A. Stop the service using port 5432 or map to a different host port

B. Change the container port to 5433

C. Remove the postgres image and try again

D. Use docker build instead of docker run

Solution

1. Step 1: Understand the port conflict error

   The error means something on your computer is already using port 5432, so Docker cannot map the container's port to it.

2. Step 2: Fix the conflict by stopping the service or changing host port

   You can stop the program using port 5432 or map the container port to a different host port like 5433 using -p 5433:5432.

3. Final Answer:

   Stop the service using port 5432 or map to a different host port -> Option A

4. Quick Check:

   Port conflict fix = stop service or change host port [OK]

Hint: Change host port or stop conflicting service to fix port error [OK]

Common Mistakes:

• Changing container port instead of host port
• Removing image does not fix port conflicts
• Using 'docker build' instead of 'docker run'

5. You want to reset your local MySQL database container to start fresh without losing your data volume. Which Docker command sequence is correct?

hard

A. docker rm mysql-db && docker run -d -p 3306:3306 --name mysql-db mysql

B. docker stop mysql-db && docker rm mysql-db && docker run -d -p 3306:3306 --name mysql-db mysql

C. docker restart mysql-db && docker run mysql

D. docker stop mysql-db && docker run -d -p 3306:3306 --name mysql-db mysql

Solution

1. Step 1: Stop and remove the existing container

   To reset the container, first stop it with docker stop mysql-db and then remove it with docker rm mysql-db. This removes the container but not the data volume.

2. Step 2: Run a new container with the same name and port mapping

   Start a new container with docker run -d -p 3306:3306 --name mysql-db mysql to get a fresh instance connected to the existing data volume.

3. Final Answer:

   docker stop mysql-db && docker rm mysql-db && docker run -d -p 3306:3306 --name mysql-db mysql -> Option B

4. Quick Check:

   Stop + remove + run = reset container without losing volume [OK]

Hint: Stop and remove container before rerunning to reset it [OK]

Common Mistakes:

• Restarting container does not reset it
• Removing container without stopping first
• Running new container without removing old one