Understanding and Configuring the Logging Driver on a Docker Host

Logging Drivers on Docker are pluggable components which all you to access and pull log data out of containers running on a Docker host. Many Logging Drivers exist for Docker, but the default and most common is known as "json-file". This is set as the default on a newly installed Docker host.

It is possible to change the default Logging Driver by editing the following file:

/etc/docker/daemon.json

Please note that a new file will be created if this does not already exist with custom settings 

sudo vi /etc/docker/daemon.json

Add the following code block in JSON format to set the Logging Driver to "json-file" for the entire Docker host. 

{
  "log-driver": "json-file",
  "log-opts": {
    "max-size": "15m"
  }
}

The "log-opts" switch is another way for us to control the Logging Driver, and the subsettings depend on the driver itself. Above we have set "log-opts" to "max-size" which sets a max log file size to 15MB's.

Once you have made the code changes to the daemon.json file, commit and save the file. You must restart the Docker services before the changes will be pharsed. To do this run the following command:

sudo systemctl restart docker

Depending on whether you are running Docker EE or CE edition will depend on the Logging Drivers available to you. 

In Docker EE you have the following drivers available: 

• syslog
• gelf
• fluentd
• awslogs
• splunk
• etwlogs
• gcplogs
• Logentries

However in Docker CE edition the options are more limited:

• local
• json-file
• journald

The information above demonstrates how to set the Logging Driver at a global level across the entire host. It is possible with Logging Drivers to use different drivers on a per-container basis.

To do this use the -log-driver switch with the docker run command. For example:

docker run --log-driver json-file --log-opt max-size=50m hello-world