Container metrics collection in Docker - Mini Project: Build & Apply

Container Metrics Collection

📖 Scenario: You are managing a small set of Docker containers running different services on your local machine. You want to collect basic metrics like CPU and memory usage for each container to monitor their performance.

🎯 Goal: Build a simple Docker setup that collects and stores container metrics in a structured way for easy querying and monitoring.

📋 What You'll Learn

Create a Docker Compose file with two containers running simple services

Add a configuration to enable metrics collection for these containers

Write a command or script to query and display the metrics

Ensure the metrics data is stored or accessible for later analysis

💡 Why This Matters

🌍 Real World

Monitoring container resource usage is essential for maintaining healthy and efficient applications in production environments.

💼 Career

DevOps engineers and system administrators often need to collect and analyze container metrics to optimize performance and troubleshoot issues.

1

Create Docker Compose with two containers

Create a file named docker-compose.yml with two services: webapp running the image nginx:latest and db running the image mysql:5.7. Both should restart always.

Docker

# Your code here

Hint

Use the services key to define two containers with the exact names and images.

2

Enable metrics collection configuration

In the docker-compose.yml, add labels under both webapp and db services with the key com.example.metrics set to enabled.

Docker

version: '3.8'
services:
  webapp:
    image: nginx:latest
    restart: always
    # Add labels here
  db:
    image: mysql:5.7
    restart: always
    # Add labels here

Hint

Use the labels key under each service to add the metrics label.

3

Write a command to collect container metrics

Write a shell command that uses docker stats --no-stream --format to display the container name, CPU percentage, and memory usage for containers with the label com.example.metrics=enabled. Store the output in a variable called metrics.

Docker

version: '3.8'
services:
  webapp:
    image: nginx:latest
    restart: always
    labels:
      com.example.metrics: enabled
  db:
    image: mysql:5.7
    restart: always
    labels:
      com.example.metrics: enabled

# Write the docker stats command and assign to metrics variable
# Your code here

Hint

Use docker stats with --no-stream and --filter to get metrics only once for labeled containers.

4

Store metrics output to a file

Add a command to save the metrics variable content into a file named container_metrics.txt.

Docker

version: '3.8'
services:
  webapp:
    image: nginx:latest
    restart: always
    labels:
      com.example.metrics: enabled
  db:
    image: mysql:5.7
    restart: always
    labels:
      com.example.metrics: enabled

metrics=$(docker stats --no-stream --format "{{.Name}} {{.CPUPerc}} {{.MemUsage}}" --filter "label=com.example.metrics=enabled")

# Save metrics to a file
# Your code here

Hint

Use echo with double quotes and redirect output to the file.

Practice

1. What does the docker stats command primarily show?

easy

A. Logs of a specific container

B. List of all Docker images on the system

C. Live resource usage metrics of running containers

D. Docker daemon configuration settings

Solution

1. Step 1: Understand the purpose of docker stats

   This command is designed to display live resource usage statistics such as CPU, memory, and network for running containers.

2. Step 2: Differentiate from other Docker commands

   Other commands like docker images list images, docker logs show logs, and docker info shows daemon info, so they do not match the description.

3. Final Answer:

   Live resource usage metrics of running containers -> Option C

4. Quick Check:

   docker stats = live container metrics [OK]

Hint: Remember: stats = live resource usage [OK]

Common Mistakes:

• Confusing stats with logs
• Thinking stats shows images
• Assuming stats shows config

2. Which of the following is the correct syntax to get a single snapshot of container metrics without continuous updates?

easy

A. docker stats --no-stream

B. docker stats --single

C. docker stats --once

D. docker stats --snapshot

Solution

1. Step 1: Identify the flag for single snapshot

   The --no-stream flag tells docker stats to show metrics once and exit instead of streaming live updates.

2. Step 2: Verify other options

   Flags like --single, --once, and --snapshot are not valid Docker flags for this command.

3. Final Answer:

   docker stats --no-stream -> Option A

4. Quick Check:

   Single snapshot flag = --no-stream [OK]

Hint: Use --no-stream for one-time metrics [OK]

Common Mistakes:

• Using invalid flags like --single
• Omitting the flag and expecting single output
• Confusing with other Docker commands

3. What will be the output of the command docker stats --no-stream my_container if my_container is running?

medium

A. A continuous live stream of resource usage for all containers

B. Logs of my_container

C. An error saying container not found

D. A single snapshot of resource usage metrics for my_container

Solution

1. Step 1: Understand the command components

   docker stats --no-stream requests a single snapshot, and specifying my_container limits output to that container only.

2. Step 2: Predict the output

   Since my_container is running, the command outputs one snapshot of its resource usage metrics, not a stream or logs.

3. Final Answer:

   A single snapshot of resource usage metrics for my_container -> Option D

4. Quick Check:

   --no-stream + container name = single snapshot for that container [OK]

Hint: Add container name for focused single snapshot [OK]

Common Mistakes:

• Expecting continuous stream despite --no-stream
• Confusing stats output with logs
• Assuming error if container is running

4. You run docker stats --no-stream my_container but get an error: Error: No such container: my_container. What is the most likely cause?

medium

A. You need to add --all flag to see stopped containers

B. The container my_container is not running or does not exist

C. Docker daemon is not running

D. The --no-stream flag is invalid

Solution

1. Step 1: Analyze the error message

   The error clearly states the container my_container cannot be found, meaning it is either stopped or never created.

2. Step 2: Check other options

   The --no-stream flag is valid, Docker daemon errors would be different, and docker stats does not support --all to show stopped containers.

3. Final Answer:

   The container my_container is not running or does not exist -> Option B

4. Quick Check:

   Error 'No such container' means container missing [OK]

Hint: Check container existence before stats command [OK]

Common Mistakes:

• Assuming flag is wrong
• Thinking Docker daemon is down
• Trying unsupported flags like --all

5. You want to monitor CPU and memory usage of two specific containers named webapp and db with a single snapshot. Which command should you run?

hard

A. docker stats --no-stream webapp db

B. docker stats webapp db --no-stream

C. docker stats --no-stream --filter webapp --filter db

D. docker stats --filter name=webapp,db --no-stream

Solution

1. Step 1: Understand how to specify multiple containers

   Docker stats accepts container names as arguments separated by spaces to monitor multiple containers.

2. Step 2: Use the correct flag placement

   The --no-stream flag should come before container names; docker stats --no-stream webapp db places it correctly.

3. Step 3: Evaluate other options

   docker stats webapp db --no-stream incorrectly places the flag after container names, causing Docker to interpret --no-stream as a container name and resulting in an error. Options C and D use invalid or unsupported --filter syntax for this command.

4. Final Answer:

   docker stats --no-stream webapp db -> Option A

5. Quick Check:

   Multiple containers + --no-stream = docker stats --no-stream webapp db [OK]

Hint: List containers after --no-stream for snapshot [OK]

Common Mistakes:

• Using invalid --filter syntax
• Placing flags after container names incorrectly
• Trying to monitor stopped containers