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, or log driver for short.

As a default, Docker uses the json-file logging driver, which caches container logs as JSON internally. In addition to using the logging drivers included with Docker, you can also implement and use logging driver plugins.

Tip: use the local logging driver to prevent disk-exhaustion

By default, no log-rotation is performed. As a result, log-files stored by the default json-file logging driver logging driver can cause a significant amount of disk space to be used for containers that generate much output, which can lead to disk space exhaustion.

Docker keeps the json-file logging driver (without log-rotation) as a default to remain backward compatibility with older versions of Docker, and for situations where Docker is used as runtime for Kubernetes.

For other situations, the local logging driver is recommended as it performs log-rotation by default, and uses a more efficient file format. Refer to the Configure the default logging driver section below to learn how to configure the local logging driver as a default, and the local file logging driver page for more details about the local logging driver.

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 configuration file. Refer to the "daemon configuration file" section in the dockerd reference manual for details.

The default logging driver is json-file. The following example sets the default logging driver to the local log driver:

{
  "log-driver": "local"
}

If the logging driver has configurable options, you can set them in the daemon.json file as a JSON object with the key log-opts. The following example sets four 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"
  }
}

Restart Docker for the changes to take effect for newly created containers. Existing containers don't use the new logging configuration automatically.

[!NOTE]

log-opts 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 don't specify a logging driver, the default is json-file. 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 on Linux, macOS, or PowerShell on Windows:

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

json-file

[!NOTE]

Changing the default logging driver or logging driver options in the daemon configuration only affects containers that are created after the configuration is changed. Existing containers retain the logging driver options that were used when they were created. To update the logging driver for a container, the container has to be re-created with the desired options. Refer to the configure the logging driver for a container section below to learn how to find the logging-driver configuration of a container.

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 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, new messages will not be enqueued. 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 controls the size of the buffer used for intermediate message storage when mode is set to non-blocking. The default is 1m meaning 1 MB (1 million bytes). See function FromHumanSize() in the go-units package for the allowed format strings, some examples are 1KiB for 1024 bytes, 2g for 2 billion bytes.

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 (in the following example, 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.

Driver	Description
`none`	No logs are available for the container and `docker logs` does not return any output.
`local`	Logs are stored in a custom format designed for minimal overhead.
`json-file`	The logs are formatted as JSON. The default logging driver for Docker.
`syslog`	Writes logging messages to the `syslog` facility. The `syslog` daemon must be running on the host machine.
`journald`	Writes log messages to `journald`. The `journald` daemon must be running on the host machine.
`gelf`	Writes log messages to a Graylog Extended Log Format (GELF) endpoint such as Graylog or Logstash.
`fluentd`	Writes log messages to `fluentd` (forward input). The `fluentd` daemon must be running on the host machine.
`awslogs`	Writes log messages to Amazon CloudWatch Logs.
`splunk`	Writes log messages to `splunk` using the HTTP Event Collector.
`etwlogs`	Writes log messages as Event Tracing for Windows (ETW) events. Only available on Windows platforms.
`gcplogs`	Writes log messages to Google Cloud Platform (GCP) Logging.

Limitations of logging drivers#

Reading log information requires decompressing rotated log files, which causes a temporary increase in disk usage (until the log entries from the rotated files are read) and an increased CPU usage while decompressing.
The capacity of the host storage where the Docker data directory resides determines the maximum size of the log file information.