Docker Cookbook

280 | Chapter 9: Monitoring Containers

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Once the package is installed, you need to configure 
td-agent
, telling it to match cer‐
tain events and redirect them to a specific 
location
. For example, to match all Docker
events (which by default are tagged with 
docker.
) and redirect them
to 
stdout
, edit the 
td-agent
configuration file 
/etc/td-agent/td-agent.conf
and add the
following line:

type stdout

Then restart the service:
$ sudo service td-agent restart
You are now ready to start using Fluentd to manage your Docker logs. Let’s start an
Nginx container and use this logging driver:
$ docker run -d -p 80:80 --name nginx --log-driver=fluentd nginx
Now if you access Nginx in your browser and then check the 
td-agent
log file, you will
see the Docker logs:
$ tail -n 3 /var/log/td-agent/td-agent.log
...
2015-08-17 13:41:10 docker.dc3a645abfaa: {"log":"192.168.33.1 ...,\
"container_id":"dc3a645abfaa...",\
"container_name":"/nginx",\
"source":"stdout"}
You see that the logs are prefixed with 
docker.
. If you wanted to pre‐
fix the logs with something else, you could specify a different Go template (currently
{{.ID}}
, 
{{.FullID}}
, 
{{.Name}}
). For example, to prefix the logs with the name of
the container, use the 
log-opt
option like so:
$ docker kill nginx
$ docker rm nginx
$ docker run -d -p 80:80 --name nginx \
--log-driver=fluentd \
--log-opt fluentd-tag=docker.{{.Name}} nginx
The logs will become similar to the following:
$ tail -n 3 /var/log/td-agent/td-agent.log
...
2015-08-17 13:43:45 docker./nginx: {"container_id":"e4152ad9bdba...",\
"container_name":"/stupefied_franklin",\
"source":"stdout",\
"log":"192.168.33.1 ...}
In this example, you redirected the logs to only the Fluentd logs themselves. This is
not extremely useful or practical. In a production deployment, you would redirect the
logs to a remote data store like 
elasticsearch
, 
influxdb
, or 
mongoDB
, for example.