Logs: stdout, drivers and rotation

Docker · lesson 18 of 31 · 3 min read

Send logs where Docker can see them, and stop the default driver quietly eating the disk.

Key points

Docker captures only stdout and stderr of PID 1. A log file inside the container is invisible to docker logs.
The default json-file driver has no size limit. One chatty service can fill a host disk overnight.
Cap it with --log-opt max-size=10m --log-opt max-file=3, or set the same defaults in daemon.json.
Log to the console and let the platform ship it. Emitting JSON lines makes the logs searchable with no parser to maintain.
docker logs --since 10m --tail 100 -f api is the one worth memorising - and logs die with docker rm.

Example

# logback-spring.xml - console only, no file appender in a container
#   <appender name="STDOUT" class="ch.qos.logback.core.ConsoleAppender"> ...

# cap the log for one container
docker run -d --name api \
  --log-opt max-size=10m --log-opt max-file=3 \
  myapp:1.0

# or set it for every container: /etc/docker/daemon.json
#   { "log-driver": "json-file",
#     "log-opts": { "max-size": "10m", "max-file": "3" } }

docker logs --since 10m --tail 100 -f api
docker inspect --format "{{.LogPath}}" api     # where the bytes live

Console appender, capped driver. A log file inside a container helps nobody.

This is a reading copy. The full lesson — with the visual explainer, the interactive lab and a Run button for the code — lives in the Docker course, and every lesson in it is listed on the Docker contents page.