Thoughtbot has an excellent and much desired article on getting Docker + Rspec + Serverspec wired up but I couldn’t find anything about images generated from Packer. Packer generates its own images and so we can’t just build_from_dir(.). Our images are already built at that point. We’re using Packer to run Chef and other things beyond what vanilla Docker can do.

The fix is really simple after I was poking around in pry looking at the serverspec API.

First of all, what am I even talking about? Serverspec is like rspec for your server. It has matchers and objects like

describe file('/etc/passwd') do
  it { should exist }

describe file('/var/run/unicorn.sock') do
  it { should be_socket }

So although we have application tests of varying styles and application monitors, serverspec allows us to test our server just like an integration test before we deploy. I had previously tried to go down this route with test kitchen to test our chef recipes but it was sort of picky about paths. Additionally, going with serverspec and docker doesn’t even require Chef. Chef has already been run at this point! What this means is fast tests. Just booting a docker image and running a command is fast.

# single test
$ time bundle exec rspec
1.415 total


So how does this work? Well, like I said the thoughtbot article is really good but I wanted to add to the ‘net about packer specifcally. The critical piece to make Serverspec work with a Docker image created from Packer is in your spec itself (spec/yer_image_name/yer_image_name_spec.rb).

# spec_helper and a lot of spec/ came from `serverspec-init1`

require 'spec_helper'
require "docker"

describe "yer_packer_image" do

  before(:all) do
    image = Docker::Image.get("yer_package_image")

    set :os, family: :debian   # this can be set per spec
    # describe package('httpd'), :if => os[:family] == 'redhat' do
    #   it { should be_installed }
    # end

    set :backend, :docker
    set :docker_image,

  it "has bash installed" do
    expect(package("bash")).to be_installed


See that image = Docker::Image.get("yer_package_image") bit in the before block? This is the difference between build my image (what the thoughtbot article uses) and run an existing image. Since packer builds the image, we can just reuse the one we have from our local store. Then later :docker_image, sets the image to use during the test. It knows about docker because of require "docker" from serverspec. I’ll mention what versions of these gems I’m using at the time of this post since this might bit-rot.

docker-api (1.26.2)
rspec (3.4.0)
serverspec (2.31.0)
specinfra (2.53.1)  # from serverspec

An idea that didn’t work

Ok this is cool! How about we have packer run our tests after the packer build. Unfortunately this is mostly useless. :( The tests will run but they won’t do anything if the tests fail.

Here’s the post-processor bit of our packer config. It just tells Packer to do things after it’s done building. The first bit is tag our image so we can push it out to our registry.

  "post-processors": [
        "type": "docker-tag",
        "repository": "your-company/yer_packer_image",
        "tag": "latest",
        "force": true
        "type": "shell-local",
        "inline": ["bundle exec rspec ../../spec/yer_packer_image"],
        "_useless": "don't do this"

The path structure is arbitrary above. We have a project we’re currently working on that I’ll explain in another blog post or talk. The only specifics about this file structure is that typically you’d want to do something like require 'spec_helper' but if you are building an image from a subdirectory and then running tests from another nested subdirectory then you’ll need to require_relative 'spec_helper'. I actually don’t know why this isn’t the default anyway.

But like I said, running tests with Packer as a post processor doesn’t do anything. You could run it with PACKER_DEBUG or something but I don’t like any of that. I’ll be following up with a more complete workflow as we figure this out. So you don’t need to do this last bit with the post-processors. I just wanted to leave a breadcrumb for myself later.