Docker logging drivers - Practice Problems & Coding Challenges

Challenge - 5 Problems

💻 Command Output

intermediate

Output of Docker container logs with json-file driver

You run a Docker container with the default logging driver (json-file). What will be the output of docker logs mycontainer if the container prints Hello World once and then exits?

Docker

docker run --name mycontainer alpine echo Hello World
docker logs mycontainer

A

Error: No logs available

B

Hello World

C

Container started
Hello World
Container exited

D Hello World (with JSON metadata lines)

💡 Hint

The default json-file driver stores logs in JSON but docker logs shows only the message content.

The json-file driver stores logs as JSON internally, but the docker logs command outputs only the container's stdout/stderr content, so you see just 'Hello World'.

🧠 Conceptual

intermediate

Understanding Docker logging driver options

Which Docker logging driver is designed to send container logs directly to the systemd journal?

A journald

B fluentd

C syslog

D json-file

💡 Hint

Systemd journal is a logging system used by many Linux distributions.

The 'journald' logging driver sends container logs to the systemd journal, integrating with the system's native logging.

❓ Troubleshoot

advanced

Diagnosing Docker logging driver misconfiguration

You configured a Docker container to use the 'fluentd' logging driver but see no logs in your Fluentd server. Which of the following is the most likely cause?

A The container logs are too large to send

B The container is using the default json-file driver instead

C The Fluentd server is not reachable at the configured address

D Docker does not support the fluentd driver

💡 Hint

Check network connectivity and configuration between Docker and Fluentd.

If the Fluentd server is unreachable, Docker cannot send logs, causing no logs to appear. The fluentd driver is supported and must be reachable.

🔀 Workflow

advanced

Configuring Docker container with syslog logging driver

You want to run a Docker container that sends logs to a remote syslog server at 192.168.1.100 on port 514 using UDP. Which command correctly sets this up?

A docker run --log-driver=json-file --log-opt syslog-address=udp://192.168.1.100:514 alpine echo test

B docker run --log-driver=fluentd --log-opt address=udp://192.168.1.100:514 alpine echo test

C docker run --log-driver=syslog --log-opt syslog-address=tcp://192.168.1.100:514 alpine echo test

D docker run --log-driver=syslog --log-opt syslog-address=udp://192.168.1.100:514 alpine echo test

💡 Hint

The syslog logging driver uses the option 'syslog-address' with protocol and IP.

Option D correctly uses the syslog driver with UDP protocol and the correct option name. Option D uses TCP instead of UDP. Option D uses the wrong driver. Option D uses json-file driver which ignores syslog options.

✅ Best Practice

expert

Choosing the best logging driver for high-volume production containers

For a high-volume production environment where you want centralized log management with minimal performance impact on containers, which Docker logging driver is generally recommended?

A fluentd

B json-file

C none

D local

💡 Hint

Centralized log management often uses external log collectors.

The fluentd driver sends logs to an external collector, reducing local disk usage and enabling centralized management. json-file stores logs locally which can impact disk and performance. none disables logging. local is for local optimized storage but not centralized.

Practice

1. What is the default Docker logging driver used if none is specified?

easy

A. none

B. syslog

C. journald

D. json-file

Solution

1. Step 1: Understand Docker default logging behavior

   Docker uses a default logging driver to store container logs unless changed.

2. Step 2: Identify the default driver

   The default driver is json-file, which stores logs as JSON files on the host.

3. Final Answer:

   json-file -> Option D

4. Quick Check:

   Default logging driver = json-file [OK]

Hint: Default is json-file unless changed in daemon config [OK]

Common Mistakes:

• Choosing syslog as default
• Thinking none disables logging by default
• Confusing journald with default

2. Which of the following is the correct syntax to set the logging driver to syslog when running a container?

easy

A. docker run --log-driver=syslog myimage

B. docker run --logging=syslog myimage

C. docker run --logdriver=syslog myimage

D. docker run --log syslog myimage

Solution

1. Step 1: Recall Docker run command options for logging

   The correct flag to specify logging driver is --log-driver.

2. Step 2: Match the correct syntax

   docker run --log-driver=syslog myimage uses --log-driver=syslog, which is the correct syntax.

3. Final Answer:

   docker run --log-driver=syslog myimage -> Option A

4. Quick Check:

   Use --log-driver to set logging driver [OK]

Hint: Use --log-driver=driver_name to set logging driver [OK]

Common Mistakes:

• Using --logging instead of --log-driver
• Missing the dash in --log-driver
• Using --log instead of --log-driver

3. Given the command:
docker run --log-driver=json-file --log-opt max-size=10m --log-opt max-file=3 alpine echo hello
What will happen when the container logs exceed 30 MB?

medium

A. Logs will grow indefinitely without rotation

B. Logs will be deleted immediately after 30 MB

C. Logs will be rotated and only the latest 3 files of 10 MB each are kept

D. Container will stop running when logs exceed 30 MB

Solution

1. Step 1: Understand log options max-size and max-file

   max-size=10m means each log file max is 10 MB; max-file=3 means keep 3 rotated files.

2. Step 2: Calculate total log size before rotation

   3 files x 10 MB each = 30 MB total before old logs are deleted.

3. Final Answer:

   Logs will be rotated and only the latest 3 files of 10 MB each are kept -> Option C

4. Quick Check:

   max-size and max-file control log rotation [OK]

Hint: max-size x max-file = total log size before rotation [OK]

Common Mistakes:

• Thinking logs delete immediately at 30 MB
• Assuming container stops on log size limit
• Ignoring max-file setting

4. You run a container with:
docker run --log-driver=none nginx
But you still see logs when running docker logs. What is the likely cause?

medium

A. The --log-driver=none flag is invalid and ignored

B. The container was started before the logging driver was set to none

C. Docker logs command shows logs from the host system

D. The container image nginx forces logging regardless of driver

Solution

1. Step 1: Understand effect of --log-driver=none

   This disables logging for the container started with this flag.

2. Step 2: Consider container start timing

   If the container was started before setting the flag, old logs remain accessible.

3. Final Answer:

   The container was started before the logging driver was set to none -> Option B

4. Quick Check:

   Logging driver applies only at container start [OK]

Hint: Logging driver must be set when container starts [OK]

Common Mistakes:

• Assuming --log-driver=none is invalid
• Thinking docker logs shows host logs
• Believing image controls logging driver

5. You want to configure Docker daemon to use the journald logging driver by default and limit log file size to 5 MB with 2 rotated files. Which daemon.json configuration is correct?

hard

A. {
"log-driver": "journald",
"log-opts": {
"max-size": "5m",
"max-file": "2"
}
}

B. {
"logging-driver": "journald",
"logging-opts": {
"max-size": "5m",
"max-file": "2"
}
}

C. {
"log-driver": "journald",
"log-opts": {
"max-size": 5,
"max-file": 2
}
}

D. {
"log-driver": "journald",
"log-opts": {
"max-size": "5MB",
"max-file": "2"
}
}

Solution

1. Step 1: Identify correct keys in daemon.json

   The keys are "log-driver" and "log-opts" (not "logging-driver" or "logging-opts").

2. Step 2: Check value formats for options

   max-size must be a string with units like "5m" (not number or "5MB" which is invalid).

3. Final Answer:

   { "log-driver": "journald", "log-opts": { "max-size": "5m", "max-file": "2" } } -> Option A

4. Quick Check:

   Use "log-driver" and string values with units [OK]

Hint: Use "log-driver" and string sizes like "5m" in daemon.json [OK]

Common Mistakes:

• Using wrong keys like logging-driver
• Using numeric values without units
• Using invalid units like MB instead of m