Sidecar container pattern in Docker - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the Docker Compose service to add a sidecar container that logs data.

Docker

services:
  app:
    image: myapp:latest
  sidecar:
    image: [1]

Drag options to blanks, or click blank then click option'

Afluentd

Bnginx

Cmysql

Dbusybox

Attempts:

3 left

💡 Hint

Common Mistakes

Choosing a main app image like nginx or mysql instead of a logging tool.

The sidecar container uses the fluentd image to collect and forward logs.

2fill in blank

medium

Complete the Docker Compose snippet to share a volume between app and sidecar containers.

Docker

services:
  app:
    volumes:
      - [1]:/data
  sidecar:
    volumes:
      - shared-data:/data
volumes:
  shared-data:

Drag options to blanks, or click blank then click option'

Ashared-data

Bapp-data

Clog-data

Dcache-data

Attempts:

3 left

💡 Hint

Common Mistakes

Using a volume name not declared under volumes:.

The volume shared-data is declared and shared between both containers at /data.

3fill in blank

hard

Fix the error in the Docker Compose sidecar service to correctly depend on the app service.

Docker

services:
  sidecar:
    image: fluentd
    depends_on:
      - [1]

Drag options to blanks, or click blank then click option'

Acache

Bdatabase

Capp

Dproxy

Attempts:

3 left

💡 Hint

Common Mistakes

Using unrelated service names like database or proxy.

The sidecar container should depend on the app service to start after it.

4fill in blank

hard

Fill both blanks to configure environment variables for the sidecar container.

Docker

services:
  sidecar:
    image: fluentd
    environment:
      - [1]=info
      - [2]=/var/log/app.log

Drag options to blanks, or click blank then click option'

ALOG_LEVEL

BAPP_MODE

CLOG_PATH

DDEBUG

Attempts:

3 left

💡 Hint

Common Mistakes

Using unrelated environment variable names like APP_MODE or DEBUG.

The environment variables LOG_LEVEL and LOG_PATH configure logging level and log file path.

5fill in blank

hard

Fill all three blanks to complete the Docker Compose sidecar service with restart policy and network.

Docker

services:
  sidecar:
    image: fluentd
    restart: [1]
    networks:
      - [2]
    depends_on:
      - [3]
networks:
  appnet:

Drag options to blanks, or click blank then click option'

Aalways

Bappnet

Capp

Don-failure

Attempts:

3 left

💡 Hint

Common Mistakes

Using wrong restart policies or network names not declared.

The sidecar restarts always, uses the appnet network, and depends on the app service.

Practice

1. What is the main purpose of the sidecar container pattern in Docker?

easy

A. To run containers only during the build process

B. To replace the main application container with a smaller one

C. To create multiple copies of the main application for load balancing

D. To run helper containers alongside the main application to add features

Solution

1. Step 1: Understand the sidecar pattern concept

   The sidecar pattern involves running a helper container alongside the main app container to provide additional features without modifying the main app.

2. Step 2: Identify the purpose of sidecar containers

   Sidecar containers commonly handle logging, monitoring, or proxy tasks to support the main app.

3. Final Answer:

   To run helper containers alongside the main application to add features -> Option D

4. Quick Check:

   Sidecar pattern = helper containers alongside main app [OK]

Hint: Sidecar means helper container runs with main app [OK]

Common Mistakes:

• Thinking sidecar replaces the main app container
• Confusing sidecar with load balancing
• Believing sidecar runs only during build

2. Which of the following is the correct way to define a sidecar container in a Docker Compose file?

easy

A. Define multiple services in the same docker-compose.yml file, where one service is the main app and another is the sidecar

B. Use the depends_on key to run containers sequentially

C. Run two containers with the same image name in separate Compose files

D. Use docker run --sidecar command to start a sidecar container

Solution

1. Step 1: Review Docker Compose structure for multiple containers

   Docker Compose allows defining multiple services in one file, each representing a container.

2. Step 2: Understand sidecar container definition

   Sidecar containers are defined as separate services alongside the main app service in the same Compose file.

3. Final Answer:

   Define multiple services in the same docker-compose.yml file, where one service is the main app and another is the sidecar -> Option A

4. Quick Check:

   Sidecar in Compose = multiple services in one file [OK]

Hint: Sidecar containers are separate services in one Compose file [OK]

Common Mistakes:

• Using a non-existent --sidecar flag in docker run
• Running containers in separate Compose files without linking
• Misusing depends_on for sidecar definition

3. Given this Docker Compose snippet, what will happen when you run docker-compose up?

version: '3'
services:
  app:
    image: myapp:latest
  logger:
    image: logcollector:latest
    depends_on:
      - app

medium

A. Only app container starts; logger is ignored

B. logger starts before app container

C. Both app and logger containers start; logger waits for app to start first

D. Docker Compose throws an error due to missing networks

Solution

1. Step 1: Analyze the Compose file services and dependencies

   There are two services: app and logger. The logger service depends on app.

2. Step 2: Understand depends_on behavior

   depends_on ensures app starts before logger, but both start when running docker-compose up.

3. Final Answer:

   Both app and logger containers start; logger waits for app to start first -> Option C

4. Quick Check:

   depends_on means start order, both containers run [OK]

Hint: depends_on controls start order, not exclusion [OK]

Common Mistakes:

• Thinking depends_on skips starting dependent container
• Assuming logger starts before app
• Expecting error due to missing networks (default network is created)

4. You have a sidecar container for logging, but logs are not appearing as expected. Which of these is the most likely cause?

medium

A. Docker Compose file version is set to '2' instead of '3'

B. The sidecar container does not share the same volume or network with the main app

C. The main app container image is missing

D. The sidecar container is defined as a separate Dockerfile

Solution

1. Step 1: Identify how sidecar containers share data

   Sidecar containers usually share volumes or networks with the main app to access logs or data.

2. Step 2: Check common causes for missing logs

   If volumes or networks are not shared, the sidecar cannot access the main app's logs, causing missing logs.

3. Final Answer:

   The sidecar container does not share the same volume or network with the main app -> Option B

4. Quick Check:

   Shared volumes/networks needed for sidecar access [OK]

Hint: Sidecar needs shared volumes or network to access data [OK]

Common Mistakes:

• Blaming Compose file version for logging issues
• Confusing missing image with logging problem
• Thinking sidecar must be in same Dockerfile

5. You want to add a monitoring sidecar container to your existing app without changing the app code. Which approach best follows the sidecar pattern?

hard

A. Add a new service in your Docker Compose file that runs the monitoring container sharing the app's network and volumes

B. Modify the app container image to include monitoring tools inside it

C. Run the monitoring container separately on another host

D. Add monitoring commands inside the app's Dockerfile entrypoint script

Solution

1. Step 1: Understand sidecar pattern goal

   The sidecar pattern adds features by running helper containers alongside the main app without modifying the app itself.

2. Step 2: Evaluate options for adding monitoring

   Adding a new service in Compose that shares network and volumes with the app fits the sidecar pattern perfectly.

3. Step 3: Exclude options that modify app or separate hosts

   Modifying app image or running monitoring separately breaks sidecar principles.

4. Final Answer:

   Add a new service in your Docker Compose file that runs the monitoring container sharing the app's network and volumes -> Option A

5. Quick Check:

   Sidecar = helper container alongside app, no app code change [OK]

Hint: Sidecar means add helper container, no app code change [OK]

Common Mistakes:

• Modifying app image instead of adding sidecar
• Running monitoring on separate host breaks sidecar
• Embedding monitoring commands inside app entrypoint