Docker GELF logging additional fields - docker

Question

I am trying to make my docker-compose file write its logging to a Graylog server, using the GELF protocol. This works fine, using the following configuration (snippet of docker-compose.yml):
logging:
driver: gelf
options:
gelf-address: ${GELF_ADDRESS}
The Graylog server receives the messages I log in the JBoss instance in my Docker container. It also adds some extra GELF fields, like container_name and image_name.
My question is, how can I add extra GELF fields myself? I want it to pass _username as an extra field. I have this field available in my MDC context.
I could add the information to the message by using a formatter (Conversion Pattern) in my CONSOLE logger, by adding the following to this logger:
%X{_user_name}
But this is not what I want, as it will be in the GELF message field, not added as seperate extra field.
Any thoughts?

Answer 1

It does seem impossible in the current docker-compose version (1.8.0) to include the extra fields.
I ended up removing any logging configuration from the docker-compose file and instead integrate the GELF logging in the docker container's application. Since I am using JBoss AS 7, I have used the steps as described here: http://logging.paluch.biz/examples/jbossas7.html
To log the container id, I have added the following configuration:
<custom-handler name="GelfLogger" class="biz.paluch.logging.gelf.jboss7.JBoss7GelfLogHandler" module="biz.paluch.logging">
<level name="INFO" />
<properties>
<property name="host" value="udp:${GRAYLOG_HOST}" />
<property name="port" value="${GRAYLOG_PORT}" />
<property name="version" value="1.1" />
<property name="additionalFields" value="dockerContainer=${HOSTNAME}" />
<property name="includeFullMdc" value="true" />
</properties>
Field dockerContainer is substituted by the HOSTNAME environment variable on the docker container and contains the containerId. The other placeholders are substituted by docker-compose environment variables.
By including the full MDC, I was able to put the username (and some other fields) as an additional GELF field. (For more information about MDC, see http://logback.qos.ch/manual/mdc.html)