moby--moby/docs/admin/logging/fluentd.md

Fluentd logging driver

The fluentd logging driver sends container logs to the Fluentd collector as structured log data. Then, users can use any of the various output plugins of Fluentd to write these logs to various destinations.

In addition to the log message itself, the fluentd log driver sends the following metadata in the structured log message:

Field	Description
container_id	The full 64-character container ID.
container_name	The container name at the time it was started. If you use docker rename to rename a container, the new name is not reflected in the journal entries.
source	stdout or stderr

The docker logs command is not available for this logging driver.

Usage

Some options are supported by specifying --log-opt as many times as needed:

• fluentd-address: specify host:port to connect localhost:24224
• tag: specify tag for fluentd message, which interpret some markup, ex {{.ID}}, {{.FullID}} or {{.Name}} docker.{{.ID}}
• fail-on-startup-error: true/false; Should the logging driver fail container startup in case of connect error during startup. Default: true (backwards compatible)
• buffer-limit: Size limit (bytes) for the buffer which is used to buffer messages in case of connection outages. Default: 1M

Configure the default logging driver by passing the --log-driver option to the Docker daemon:

docker daemon --log-driver=fluentd

To set the logging driver for a specific container, pass the --log-driver option to docker run:

docker run --log-driver=fluentd ...

Before using this logging driver, launch a Fluentd daemon. The logging driver connects to this daemon through localhost:24224 by default. Use the fluentd-address option to connect to a different address.

docker run --log-driver=fluentd --log-opt fluentd-address=myhost.local:24224

If container cannot connect to the Fluentd daemon, the container stops immediately.

Options

Users can use the --log-opt NAME=VALUE flag to specify additional Fluentd logging driver options.

fluentd-address

By default, the logging driver connects to localhost:24224. Supply the fluentd-address option to connect to a different address.

docker run --log-driver=fluentd --log-opt fluentd-address=myhost.local:24224

tag

By default, Docker uses the first 12 characters of the container ID to tag log messages. Refer to the log tag option documentation for customizing the log tag format.

labels and env

The labels and env options each take a comma-separated list of keys. If there is collision between label and env keys, the value of the env takes precedence. Both options add additional fields to the extra attributes of a logging message.

fail-on-startup-error

By default, if the logging driver cannot connect to the backend it will fail the entire startup of the container. If you wish to ignore potential connect error during container startup supply the fail-on-startup-error flag.

docker run --log-driver=fluentd --log-opt fail-on-startup-error=false

buffer-limit

When fluent driver loses connection, or cannot connect at container startup, it will buffer the log events locally for re-transmission. Buffer limit option controls how much data will be buffered locally, per container. Specified in bytes.

docker run --log-driver=fluentd --log-opt buffer-limit=5242880

The above would result to use 5M buffer locally. Keep in mind that during possible connection errors all your containers will start buffering locally and thus might result in considerable memory usage.

Fluentd daemon management with Docker

About Fluentd itself, see the project webpage and its documents.

To use this logging driver, start the fluentd daemon on a host. We recommend that you use the Fluentd docker image. This image is especially useful if you want to aggregate multiple container logs on a each host then, later, transfer the logs to another Fluentd node to create an aggregate store.

Testing container loggers

1. Write a configuration file (test.conf) to dump input logs:

    <source>
      @type forward
    </source>
   
    <match docker.**>
      @type stdout
    </match>
   

2. Launch Fluentd container with this configuration file:

    $ docker run -it -p 24224:24224 -v /path/to/conf/test.conf:/fluentd/etc -e FLUENTD_CONF=test.conf fluent/fluentd:latest
   

3. Start one or more containers with the fluentd logging driver:

    $ docker run --log-driver=fluentd your/application