Resource monitoring per container in Docker - Step-by-Step Execution

Process Flow - Resource monitoring per container

Start Docker Container

↓

Run 'docker stats'

↓

Collect CPU, Memory, Network, I/O

↓

Display live stats per container

↓

User observes resource usage

↓

Stop monitoring or container

This flow shows starting a container, running the docker stats command to collect live resource usage, and displaying it until stopped.

Execution Sample

Docker

docker run -d --name myapp nginx

docker stats myapp

Starts an nginx container named 'myapp' and then shows live resource usage stats for it.

Process Table

Step	Command	Action	Output/Result
1	docker run -d --name myapp nginx	Start container 'myapp' in background	Container ID returned, 'myapp' running
2	docker stats myapp	Start live resource monitoring for 'myapp'	Shows CPU %, Memory usage, Network I/O, Block I/O, PIDs
3	docker stats myapp (after 5 seconds)	Update resource stats	CPU 0.05%, Mem 10MiB / 500MiB, Net I/O 1kB / 0B, Block I/O 0B / 0B, PIDs 2
4	docker stats myapp (after 10 seconds)	Update resource stats	CPU 0.10%, Mem 12MiB / 500MiB, Net I/O 2kB / 0B, Block I/O 0B / 0B, PIDs 2
5	Ctrl+C	Stop monitoring	docker stats command exits
6	docker stop myapp	Stop container	'myapp' stopped
7	docker stats myapp	Try monitoring stopped container	Error: No such container: myapp

💡 Monitoring stops when user interrupts with Ctrl+C or container stops.

Status Tracker

Variable	Start	After Step 1	After Step 3	After Step 4	After Step 6	Final
Container State	Not running	Running	Running	Running	Stopped	Stopped
CPU %	0%	N/A	0.05%	0.10%	N/A	N/A
Memory Usage	0MiB	N/A	10MiB	12MiB	N/A	N/A
Network I/O	0B	N/A	1kB / 0B	2kB / 0B	N/A	N/A
Block I/O	0B	N/A	0B / 0B	0B / 0B	N/A	N/A
PIDs	0	N/A	2	2	N/A	N/A

Key Moments - 3 Insights

Why does 'docker stats' show an error after the container is stopped?

Because the container no longer exists or is not running, 'docker stats' cannot fetch resource data. See execution_table row 7.

Does 'docker stats' show a snapshot or live updates?

'docker stats' shows live updates of resource usage every second until stopped. See execution_table rows 2, 3, and 4.

What happens if you run 'docker stats' without specifying a container?

It shows stats for all running containers. Here we specified 'myapp' to focus on one container. This is implied in step 2.

Visual Quiz - 3 Questions

Look at the execution table, what is the CPU % of 'myapp' at step 4?

A 0.10%

B 0.05%

C 1%

D Not available

💡 Hint

Check the 'Output/Result' column at step 4 in the execution_table.

At which step does the container stop running?

A Step 3

B Step 6

C Step 7

D Step 2

💡 Hint

Look at the 'Container State' variable in variable_tracker after step 6.

If you do not press Ctrl+C, what will happen to the 'docker stats' command?

A It will exit immediately

B It will keep showing live stats continuously

C It will show stats once and stop

D It will stop after 10 seconds automatically

💡 Hint

Refer to execution_table steps 2 to 4 showing continuous updates.

Concept Snapshot

docker stats [container]
Shows live resource usage per container.
Includes CPU %, memory, network I/O, block I/O, and PIDs.
Runs continuously until stopped (Ctrl+C).
Only works on running containers.
Useful for monitoring container health in real time.

Full Transcript

This visual execution shows how to monitor resource usage per Docker container using the 'docker stats' command. First, a container named 'myapp' is started in detached mode. Then, 'docker stats myapp' is run to display live CPU, memory, network, and I/O usage. The stats update every second, showing changes in resource consumption. The monitoring continues until the user stops it with Ctrl+C. If the container is stopped, 'docker stats' cannot show data and returns an error. This helps users understand how to watch container resources live and what happens when containers stop.

Practice

1. What does the docker stats command show for each running container?

easy

A. The container's file system contents

B. The container's log output

C. Live resource usage like CPU, memory, network, and disk I/O

D. The container's environment variables

Solution

1. Step 1: Understand the purpose of docker stats

   The docker stats command is designed to show live resource usage statistics for running containers.

2. Step 2: Identify the resources monitored

   It monitors CPU, memory, network I/O, and disk I/O per container to help track performance.

3. Final Answer:

   Live resource usage like CPU, memory, network, and disk I/O -> Option C

4. Quick Check:

   docker stats = Live resource usage [OK]

Hint: Remember: docker stats shows live resource use per container [OK]

Common Mistakes:

• Confusing docker stats with docker logs
• Thinking it shows file system contents
• Assuming it shows environment variables

2. Which of the following is the correct command to show live resource usage for all running Docker containers?

easy

A. docker stats

B. docker monitor

C. docker inspect stats

D. docker top

Solution

1. Step 1: Recall the command for resource monitoring

   The command docker stats is the official Docker command to show live resource usage for containers.

2. Step 2: Verify other options

   docker monitor and docker inspect stats are invalid commands, and docker top shows running processes, not resource stats.

3. Final Answer:

   docker stats -> Option A

4. Quick Check:

   Correct command = docker stats [OK]

Hint: Use 'docker stats' to see live container resource use [OK]

Common Mistakes:

• Using docker top to check resource usage
• Trying docker monitor which doesn't exist
• Confusing docker inspect with stats

3. You run docker stats --no-stream on a container. What will you see?

medium

A. Continuous live updates of resource usage

B. A single snapshot of current resource usage

C. Only CPU usage without memory info

D. Error because --no-stream is invalid

Solution

1. Step 1: Understand the --no-stream option

   The --no-stream option tells docker stats to show resource usage only once, not continuously.

2. Step 2: Identify the output behavior

   It outputs a single snapshot of CPU, memory, network, and disk I/O usage at the moment the command runs.

3. Final Answer:

   A single snapshot of current resource usage -> Option B

4. Quick Check:

   --no-stream = single snapshot [OK]

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

Common Mistakes:

• Expecting continuous updates with --no-stream
• Thinking it shows only CPU usage
• Assuming --no-stream is invalid

4. You try to run docker stats --format '{{.CPUPerc}} but get no output. What is the likely problem?

medium

A. The format string is missing a closing single quote

B. The container is stopped, so no stats show

C. The --format option is not supported in docker stats

D. You need to add --no-stream to see output

Solution

1. Step 1: Check the format string syntax

   The command uses --format '{{.CPUPerc}} which is missing the closing single quote after the curly braces.

2. Step 2: Understand impact of missing quote

   Without the closing quote, the shell or Docker cannot parse the format string, so no output appears.

3. Final Answer:

   The format string is missing a closing single quote -> Option A

4. Quick Check:

   Format string syntax error = missing quote [OK]

Hint: Always close quotes in --format strings [OK]

Common Mistakes:

• Assuming stopped containers show no stats (they do not appear in stats)
• Thinking --format is unsupported
• Believing --no-stream is required for output

5. You want to monitor CPU and memory usage of a specific container named webapp and save the output once to a file. Which command should you use?

hard

A. docker stats --stream=false webapp > usage.txt

B. docker stats webapp --output usage.txt

C. docker monitor webapp > usage.txt

D. docker stats --no-stream --format 'table {{.Name}}\t{{.CPUPerc}}\t{{.MemUsage}}' webapp > usage.txt

Solution

1. Step 1: Choose the correct command and options

   To get a one-time snapshot, use --no-stream. To format output with container name, CPU, and memory, use --format with a table layout.

2. Step 2: Redirect output to a file

   Use shell redirection > usage.txt to save the output to a file named usage.txt.

3. Final Answer:

   docker stats --no-stream --format 'table {{.Name}}\t{{.CPUPerc}}\t{{.MemUsage}}' webapp > usage.txt -> Option D

4. Quick Check:

   One-time formatted output saved to file = docker stats --no-stream --format 'table {{.Name}}\t{{.CPUPerc}}\t{{.MemUsage}}' webapp > usage.txt [OK]

Hint: Use --no-stream and --format, redirect output to file [OK]

Common Mistakes:

• Using invalid docker monitor command
• Assuming --output option exists
• Using --stream=false instead of --no-stream