Sidecar container pattern in Docker - Practice Problems & Coding Challenges

Challenge - 5 Problems

🧠 Conceptual

intermediate

Purpose of Sidecar Containers

What is the main purpose of using a sidecar container in a Docker pod or deployment?

A To add auxiliary features like logging or monitoring alongside the main application container

B To replace the main application container with a backup container

C To run multiple unrelated applications in the same container

D To increase the CPU resources available to the main container by sharing cores

💡 Hint

Think about how sidecar containers help the main app without changing its code.

Sidecar containers run alongside the main container to provide supporting features like logging, monitoring, or proxying without modifying the main app.

💻 Command Output

intermediate

Output of Docker Compose with Sidecar

Given this docker-compose.yml snippet, what will be the output when running docker-compose up?

Docker

version: '3.8'
services:
  app:
    image: busybox
    command: sh -c "echo Main app running; sleep 5"
  sidecar:
    image: busybox
    command: sh -c "echo Sidecar running; sleep 5"

A

Main app running
Sidecar running

B Main app running

C Sidecar running

D Error: service 'sidecar' not found

💡 Hint

Both services run their commands concurrently when using docker-compose up.

Both the main app and sidecar services run their commands and print their messages concurrently.

❓ Configuration

advanced

Correct Sidecar Volume Sharing Configuration

Which Docker Compose configuration correctly shares a log directory between the main app and sidecar containers?

A

services:
  app:
    volumes:
      - ./logs:/var/log/app
  sidecar:
    volumes:
      - ./logs:/var/log/app

B

services:
  app:
    volumes:
      - logs:/var/log/app
  sidecar:
    volumes:
      - logs:/var/log/app
volumes:
  logs:

C

services:
  app:
    volumes:
      - logs:/var/log/app
  sidecar:
    volumes:
      - ./logs:/var/log/app
volumes:
  logs:

D

services:
  app:
    volumes:
      - ./logs:/var/log/app
  sidecar:
    volumes:
      - logs:/var/log/app
volumes:
  logs:

💡 Hint

Look for a named volume shared by both containers.

Using a named volume 'logs' shared by both containers ensures they access the same directory inside their containers.

❓ Troubleshoot

advanced

Sidecar Container Fails to Start

You have a sidecar container configured to collect logs, but it fails to start with the error: OCI runtime exec failed. What is the most likely cause?

A The sidecar container has no network access

B The main container is not running

C The sidecar container's command is invalid or missing

D The Docker daemon is stopped

💡 Hint

Check the command or entrypoint of the sidecar container.

An invalid or missing command in the sidecar container causes it to fail starting with runtime errors.

🔀 Workflow

expert

Sidecar Container Logging Workflow

In a Kubernetes pod using the sidecar pattern for logging, which sequence correctly describes the workflow for log collection?

A 1,3,2,4

B 2,1,3,4

C 3,1,2,4

D 1,2,3,4

💡 Hint

Think about the natural flow of logs from creation to storage.

The main container writes logs, the sidecar reads and forwards them, then the external system stores or analyzes them.