Configure logging drivers

Docker includes multiple logging mechanisms to help you get information from running containers and services. These mechanisms are called logging drivers.

Each Docker daemon has a default logging driver, which each container uses unless you configure it to use a different logging driver.

In addition to using the logging drivers included with Docker, you can also implement and use logging driver plugins.

Configure the default logging driver

To configure the Docker daemon to default to a specific logging driver, set the value of log-driver to the name of the logging driver in the daemon.json file, which is located in /etc/docker/ on Linux hosts or C:\ProgramData\docker\config\ on Windows server hosts. The default logging driver is json-file. The following example explicitly sets the default logging driver to syslog:

{
  "log-driver": "syslog"
}

If the logging driver has configurable options, you can set them in the daemon.json file as a JSON array with the key log-opts. The following example sets two configurable options on the json-file logging driver:

{
  "log-driver": "json-file",
  "log-opts": {
    "max-size": "10m",
    "max-file": "3",
    "labels": "production_status",
    "env": "os,customer"
  }
}

Note: log-opt configuration options in the daemon.json configuration file must be provided as strings. Boolean and numeric values (such as the value for max-file in the example above) must therefore be enclosed in quotes (").

If you do not specify a logging driver, the default is json-file. Thus, the default output for commands such as docker inspect <CONTAINER> is JSON.

To find the current default logging driver for the Docker daemon, run docker info and search for Logging Driver. You can use the following command:

$ docker info --format '{{.LoggingDriver}}'

json-file

Configure the logging driver for a container

When you start a container, you can configure it to use a different logging driver than the Docker daemon’s default, using the --log-driver flag. If the logging driver has configurable options, you can set them using one or more instances of the --log-opt <NAME>=<VALUE> flag. Even if the container uses the default logging driver, it can use different configurable options.

The following example starts an Alpine container with the none logging driver.

$ docker run -it --log-driver none alpine ash

To find the current logging driver for a running container, if the daemon is using the json-file logging driver, run the following docker inspect command, substituting the container name or ID for <CONTAINER>:

$ docker inspect -f '{{.HostConfig.LogConfig.Type}}' <CONTAINER>

json-file

Configure the delivery mode of log messages from container to log driver

Docker provides two modes for delivering messages from the container to the log driver:

• (default) direct, blocking delivery from container to driver
• non-blocking delivery that stores log messages in an intermediate per-container ring buffer for consumption by driver

The non-blocking message delivery mode prevents applications from blocking due to logging back pressure. Applications are likely to fail in unexpected ways when STDERR or STDOUT streams block.

WARNING: When the buffer is full and a new message is enqueued, the oldest message in memory is dropped. Dropping messages is often preferred to blocking the log-writing process of an application.

The mode log option controls whether to use the blocking (default) or non-blocking message delivery.

The max-buffer-size log option controls the size of the ring buffer used for intermediate message storage when mode is set to non-blocking. max-buffer-size defaults to 1 megabyte.

The following example starts an Alpine container with log output in non-blocking mode and a 4 megabyte buffer:

$ docker run -it --log-opt mode=non-blocking --log-opt max-buffer-size=4m alpine ping 127.0.0.1

Use environment variables or labels with logging drivers

Some logging drivers add the value of a container’s --env|-e or --label flags to the container’s logs. This example starts a container using the Docker daemon’s default logging driver (let’s assume json-file) but sets the environment variable os=ubuntu.

$ docker run -dit --label production_status=testing -e os=ubuntu alpine sh

If the logging driver supports it, this adds additional fields to the logging output. The following output is generated by the json-file logging driver:

"attrs":{"production_status":"testing","os":"ubuntu"}

Supported logging drivers

The following logging drivers are supported. See the link to each driver’s documentation for its configurable options, if applicable. If you are using logging driver plugins, you may see more options.

Limitations of logging drivers

The docker logs command is not available for drivers other than json-file and journald.