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docker-debian-stack

This is somewhat inspired by the Phusion Docker image. This git repo is going to be my collection of simple, usable Docker images.

Let's get into the "why" of this stack:

Phusion has some good ideas

  • If you're running multiple processes in your container, you need a proper init program.
  • Some kind of syslog daemon is good to have.
  • Having a cron daemon running is also a good idea, depending on what packages you're setting up.

Parts of their ideas, I disagree with

  • I don't think running SSH in a container is ever a good idea.
  • Their init process is a custom-written init, done in Python.
    • This increases the size of the image, and prevents people from creating smaller utility images
  • They include runit, so you essentially have two init daemons running
  • I haven't gone through the code extensively, but I do know this:
    • docker stop sends a SIGTERM to PID 1
    • runit doesn't forward SIGTERM to processes it's monitoring.
    • So I'm not entirely sure how processes are getting TERM'd properly.

How I'm planning to address that

Instead of a custom init + runit, I'm just using s6. It's a set of process supervising programs.

There's a few benefits to using s6:

  • If s6 gets a SIGTERM, I know it sends a SIGTERM to everything it's supervising. It's right there in the documentation
  • I can statically compile s6, so I can reuse it for other base images, including small utility images. There's no external library dependencies and minimal RAM usage.
  • It's very easy to setup services - make a directory at /etc/s6/ and place run and finish scripts.
    • (Or just symlink finish to /bin/true).
  • I didn't write it, somebody who knows a lot about init did.

Caveats

  1. I'm pretty sure s6 has the ability to do dependency management, but I haven't figured that out yet.
    • In practice, I don't see this being a problem. I usually place dependent services into their own containers anyway.
  2. s6 requires your process to not background. It doesn't have any forking detection like systemd/upstart does.

TODO

  1. Add the syslog-ng service.
  2. Look into statically compiling the logstash-forwarder daemon.
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