Sidecar container pattern in Docker - Mini Project: Build & Apply

Implementing the Sidecar Container Pattern with Docker

📖 Scenario: You are working on a web application that needs to log its activity. Instead of adding logging code inside the main app container, you will use a sidecar container to handle logging separately. This keeps the main app simple and focused.

🎯 Goal: Build a Docker Compose setup with two containers: a main web app container and a sidecar container that collects logs from the app. You will create the necessary files and configurations step-by-step.

📋 What You'll Learn

Create a Docker Compose file with two services: webapp and logcollector

Use a shared volume to share logs between the containers

Configure the webapp container to write logs to the shared volume

Configure the logcollector container to read logs from the shared volume

Print the logs collected by the logcollector container

💡 Why This Matters

🌍 Real World

Many applications use sidecar containers to add features like logging, monitoring, or security without changing the main app container.

💼 Career

Understanding the sidecar pattern is important for DevOps roles to design scalable and maintainable containerized applications.

1

Create the Docker Compose file with the webapp service

Create a file named docker-compose.yml and define a service called webapp that uses the alpine image. Mount a volume named logdata to /var/log/app inside the container. The container should run the command sh -c "while true; do echo \"Log entry at $(date)\" >> /var/log/app/app.log; sleep 2; done" to simulate logging.

Docker

# Your code here

Need a hint?

Define the webapp service with the alpine image and mount the volume logdata to /var/log/app. Use a shell command to write logs every 2 seconds.

2

Add the logcollector service to the Docker Compose file

Add a new service called logcollector to the existing docker-compose.yml. Use the alpine image and mount the same volume logdata to /var/log/app. Set the command to sh -c "tail -f /var/log/app/app.log" to continuously read the log file.

Docker

version: '3.8'
services:
  webapp:
    image: alpine
    volumes:
      - logdata:/var/log/app
    command: sh -c "while true; do echo \"Log entry at $(date)\" >> /var/log/app/app.log; sleep 2; done"
  # Add the logcollector service below
volumes:
  logdata: {}
# Your code here

Need a hint?

Add the logcollector service with the same volume logdata mounted. Use the tail -f command to follow the log file.

3

Start the Docker Compose services

Run the command docker-compose up -d in the terminal to start both webapp and logcollector containers in detached mode.

Docker

# Run the command to start the containers
# Your code here

Need a hint?

Use docker-compose up -d to start the containers in the background.

4

View the logs collected by the logcollector container

Run the command docker-compose logs --follow logcollector to see the live logs collected by the logcollector container.

Docker

# Run the command to view logs from the logcollector container
# Your code here

Need a hint?

Use docker-compose logs --follow logcollector to continuously see the log entries generated by the webapp container and collected by the logcollector sidecar.