Container metrics collection in Docker - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ query_result

intermediate

Retrieve CPU usage percentage for all running containers

You want to see the CPU usage percentage of all running Docker containers using the docker stats command with JSON output. Which command will correctly show this information?

A docker stats --no-stream --format '{{.CPUPerc}}'

B docker stats --format '{{json .}}' | grep CPUPerc

C docker stats --no-stream --format '{{.MemUsage}}'

D docker stats --no-stream --format '{{json .}}' | jq '.CPUPerc'

💡 Hint

Use the --format option to select the CPU percentage field directly.

The docker stats command with --no-stream shows a snapshot of container metrics. Using --format '{{.CPUPerc}}' extracts the CPU usage percentage directly. Option A correctly uses these flags.

🧠 Conceptual

intermediate

Understanding memory usage metrics in Docker stats

Which metric from docker stats shows the current memory usage of a container?

A MemUsage

B MemLimit

C CPUPerc

D NetIO

💡 Hint

Look for the metric that shows how much memory the container is currently using.

MemUsage shows the current memory used by the container and the total memory available. MemLimit is the maximum memory allowed. CPUPerc is CPU usage, and NetIO is network input/output.

📝 Syntax

advanced

Correct JSON parsing of Docker stats output

You want to parse the JSON output of docker stats --no-stream --format '{{json .}}' to extract container names and CPU usage. Which jq command correctly extracts these fields as a JSON array of objects?

A jq '. | {Name, CPUPerc}'

B jq -s '[.[] | {Name: .Name, CPU: .CPUPerc}]'

C jq '[.Name, .CPUPerc]'

D jq -r '.Name, .CPUPerc'

💡 Hint

Use the -s option to read multiple JSON objects as an array.

The docker stats command outputs multiple JSON objects, one per line. Using jq -s reads all lines into an array. Then mapping each object to a smaller object with {Name: .Name, CPU: .CPUPerc} produces the desired array.

❓ optimization

advanced

Efficiently monitor memory usage of a specific container

You want to monitor the memory usage of a container named webapp continuously but only want to see the memory usage percentage updated every 2 seconds. Which command achieves this efficiently?

A docker stats --format '{{.MemPerc}}' --filter name=webapp --no-stream

B docker stats --format '{{.MemPerc}}' --filter name=webapp --interval 2

C docker stats --format '{{.MemPerc}}' --no-stream --filter name=webapp

D docker stats --format '{{.MemPerc}}' --filter name=webapp

💡 Hint

By default, docker stats updates every second unless --no-stream is used.

Option D runs docker stats filtered by container name webapp and continuously updates every second (default). To update every 2 seconds, you can use --interval 2 but this flag is not supported by docker stats. So the best is to run it continuously and handle timing externally.

🔧 Debug

expert

Diagnose why Docker stats JSON parsing fails

You run docker stats --no-stream --format '{{json .}}' and pipe it to jq to parse JSON. But jq reports an error: parse error: Invalid numeric literal at line 1, column 10. What is the most likely cause?

A The <code>jq</code> command is missing the <code>-r</code> flag to read raw input.

B The <code>docker stats</code> command does not support JSON output, so the output is invalid.

C The output contains multiple JSON objects without commas or array brackets, so jq expects a single JSON document.

D The container names contain special characters that break JSON formatting.

💡 Hint

Consider how multiple JSON objects are output and how jq expects input.

docker stats --format '{{json .}}' outputs multiple JSON objects, one per line, but not wrapped in an array or separated by commas. jq expects a single JSON document. To parse multiple JSON objects, use jq -s to read them as an array.