Docker logging drivers - Mini Project: Build & Apply

Docker Logging Drivers Setup

📖 Scenario: You are managing a Docker container for a simple web application. You want to control how Docker handles the logs generated by this container.

🎯 Goal: Learn how to configure Docker logging drivers for a container and verify the logging setup.

📋 What You'll Learn

Create a Docker container running the nginx image with a specific name

Set a logging driver configuration variable

Run the container with the logging driver option

Check the container's logging driver configuration

💡 Why This Matters

🌍 Real World

In real projects, controlling Docker logging helps manage log storage, format, and integration with monitoring tools.

💼 Career

Understanding Docker logging drivers is important for DevOps roles to ensure efficient log management and troubleshooting.

1

Create a Docker container named mynginx using the nginx image

Run the command to create and start a Docker container named mynginx using the nginx image in detached mode.

Docker

# Run the docker command to create and start the container
# Your code here

Hint

Use docker run -d --name mynginx nginx to start the container in the background.

2

Set a variable LOG_DRIVER to json-file

Create a shell variable called LOG_DRIVER and set its value to json-file.

Docker

docker run -d --name mynginx nginx
# Set the LOG_DRIVER variable to 'json-file'
# Your code here

Hint

Use LOG_DRIVER=json-file to set the variable.

3

Run a new container named mynginx2 using the nginx image with the logging driver set to $LOG_DRIVER

Run a Docker container named mynginx2 using the nginx image in detached mode, and set the logging driver to the value of the LOG_DRIVER variable using the --log-driver option.

Docker

docker run -d --name mynginx nginx
LOG_DRIVER=json-file
# Run the container mynginx2 with logging driver set to $LOG_DRIVER
# Your code here

Hint

Use docker run -d --name mynginx2 --log-driver $LOG_DRIVER nginx.

4

Check the logging driver of the container mynginx2

Run the command to inspect the container mynginx2 and display only the logging driver used by this container.

Docker

docker run -d --name mynginx nginx
LOG_DRIVER=json-file
docker run -d --name mynginx2 --log-driver $LOG_DRIVER nginx
# Inspect and print the logging driver of mynginx2
# Your code here

Hint

Use docker inspect --format='{{.HostConfig.LogConfig.Type}}' mynginx2 to see the logging driver.

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