graphviz describing multi-stage docker builds

I recently decided I should learn to use Graphviz more, as a great tool for making certain kinds of plots. Less than a week later a great use case surfaced.

🔗 Graphviz

Graphviz is a neat little tool I’ve only scratched the surface of. The syntax is super simple, the program is easy to install, and it works well. There are some basic examples on wikipedia that might be helpful.

🔗 Docker Multistage Builds

In something like 2016 or 2017 docker gained the ability to do “multi-stage” builds. The obvious use is to have a build stage that has your compiler, and a final stage that copies binaries directly from the build stage. That’s a simple case though; ultimately you can express any DAG by having a FROM (your base image) and various COPY statements (pulling binaries from other stages.)

I was explaining this to an engineer at work and how we have built something of a monster with our base image, in part because we have foundational tooling written in Go that we want in our base image, but we want to also have a Go compiler image that is… on top of our base image. I don’t want to include the whole Dockerfile here since it’s 115 lines of basically shell.

But I realized that it would be cool to generate a plot of our Dockerfile, if only to show what we’ve done. I pretty quickly whipped up two scripts that could produce a diagram of our work, one using sed that only supported the initial FROM, and the next using Perl to include copies. Here’s the output of the perl script; dashed lines mean COPY, solid lines mean FROM; the bottom nodes, with the -image suffix, are the final images we push.

The following is the code I used to generate the build:

perl -pE '
BEGIN { say "digraph {\n" }

our ($from, $to);
if (m/^FROM (.*) AS (.*)$/) {
   $from = $1;
   $to = $2;
   $_ = qq("$from" -> "$to"\n)
} elsif (m/COPY --from=(.*?)\s/) {
   $_ = qq("$1" -> "$to"[style=dashed]\n)
} else {
   undef $_
}

END { say "}" } 
' base/zr-ubuntu-18.04/Dockerfile |
   dot -Tsvg |
   fx

(The above uses my personal fx command for writing stuff to firefox over a pipe.)

I thought this was pretty cool! I was able to much more clearly describe what was going on in a multi-stage docker build than I would have been able to with words, even in person. And the result of the multi-stage docker image is also pretty great; in a single Dockerfile we are producing a mostly-vanilla base image, an image with all the stuff needed to do typical compilation, and a base image ready to do basic Go compilation.

(The following includes affiliate links.)

If you ever build any kind of visualization, do yourself a favor and read The Visual Display of Quantitative Information. It’s enjoyable, accessible, and beautiful. I read it a little over ten years ago and still enjoy flipping through it every now and then.

Apropos of nothing, I am still reading Crucial Conversations and still finding the insights helpful. In an earlier part of my life I would have called what it says, in brief “act like a mature adult” but obviously that’s not very constructive advice. Highly recommend.

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