Fluentd logging driver

Estimated reading time: 4 minutes

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 a socket address to connect to the Fluentd daemon, ex fluentdhost:24224 or unix:///path/to/fluentd.sock
  • tag: specify tag for fluentd message, which interpret some markup, ex {{.ID}}, {{.FullID}} or {{.Name}} docker.{{.ID}}

To use the fluentd driver as the default logging driver, set the log-driver and log-opt keys to appropriate values in the daemon.json file, which is located in /etc/docker/ on Linux hosts or C:\ProgramData\docker\config\daemon.json on Windows Server. For more about +configuring Docker using daemon.json, see +daemon.json.

The following example sets the log driver to fluentd and sets the fluentd-address option.

 {
   "log-driver": "fluentd",
   "log-opts": {
     "fluentd-address": "fluentdhost:24224"
   }
 }

Restart Docker for the changes to take effect.

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=fluentdhost:24224

If container cannot connect to the Fluentd daemon, the container stops immediately unless the fluentd-async-connect option is used.

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. tcp(default) and unix sockets are supported.

docker run --log-driver=fluentd --log-opt fluentd-address=fluentdhost:24224
docker run --log-driver=fluentd --log-opt fluentd-address=tcp://fluentdhost:24224
docker run --log-driver=fluentd --log-opt fluentd-address=unix:///path/to/fluentd.sock

Two of the above specify the same address, because tcp is default.

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, env, and env-regex

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.

The env-regex option is similar to and compatible with env. Its value is a regular expression to match logging-related environment variables. It is used for advanced log tag options.

fluentd-async-connect

Docker connects to Fluentd in the background. Messages are buffered until the connection is established.

fluentd-buffer-limit

The amount of data to buffer before flushing to disk. Defaults the amount of RAM available to the container.

fluentd-retry-wait

How long to wait between retries. Defaults to 1 second.

fluentd-max-retries

The maximum number of retries. Defaults to 10.

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 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 *>
       @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/test.conf -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
    
Fluentd, docker, logging, driver