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Following Log Files on Disk

Some containers do not write logs to sysout or syserr. Many folks have asked if Dozzle can also show logs that are written to files. Unfortunately, files in containers are not accessible to other containers and so Dozzle wouldn't have a way to access these files. Dozzle can only access logs written to sysout or syserr which is the same functionality as docker logs command.

If you are creating a service using Docker then make sure to write logs to streams. An application should not attempt to write to logfiles. Instead delegate the logging to Docker. The twelve factor app has a great principle around logging that explains the importance of this principle.

However, there are workarounds to be able to still access files using mounts.

Mounting Local Log Files with Alpine

Dozzle reads any output stream. This can be used in combination with alpine to tail a mounted file. An example of this is as follows:

docker run -v /var/log/system.log:/var/log/test.log alpine tail -f /var/log/test.log
version: "3"
    container_name: dozzle-from-file
    image: alpine
      - /var/log/system.log:/var/log/stream.log
      - tail
      - -f
      - /var/log/stream.log     
    network_mode: none
    restart: unless-stopped

In the above example /var/log/system.log is mounted from the host and used with tail -f to follow the file. tail is smart to follow log rotations. During my testing, using alpine only uses about 300KB of memory.

The second tab shows a docker-compose file which is useful if you want the log stream to survive server reboot.

Released under the MIT License. Open sourced and sponsored by Docker OSS.