Creating DigitalOcean images with Packer

— February 17, 2015

tl;dr create a simple DigitalOcean Ubuntu 14.04 image that uses shell a provisioner to install Docker.

This post will guide you through the steps needed to create a simple DigitalOcean Ubuntu 14.04 image with the latest Docker and Kernel installed. This is in a way like creating our own DigitalOcean Docker application image. We’ll use a shell script as provisioner for this Packer template.

notes: we assume that Packer is already installed on the system, refer to the installation docs here. You could use CentOS or any other distro available at DigitalOcean in the same way. The script will need some package manager adaptation and that’s all.

Packer template

This template will create a new droplet on DigitalOcean, run the provision script, create a snapshot from the final state and destroy the droplet.

Create a packer template for our installation called docker-install-template.json and drop this JSON in it.

{
  "variables": {
    "do_api_token": "{{env `DIGITALOCEAN_API_TOKEN`}}"
  },

  "builders": [{
    "type": "digitalocean",
    "api_token": "{{user `do_api_token`}}",
    "size": "512mb",
    "region": "nyc3",
    "image": "ubuntu-14-04-x64",
    "droplet_name": "packer",
    "snapshot_name": "build-with-packer-{{timestamp}}"
  }],

  "provisioners": [{
    "type": "shell",
    "script": "install-script.sh"
  }]
}

Let me explain what this does. We are setting a variable named do_api_token that reads from the OS environment a variable named DIGITALOCEAN_API_TOKEN. Then creating a builder that for our case will obviously be DigitalOcean, setting the builder token from our custom variable and setting what type of droplet we’ll use to create this snapshot. You can read more about this builder in the packer digitalocean docs.

After the builder we have provisioners. Packer has the ability to use various types of provisioners, like for example, shell scripts, Ansible, Chef, Puppet, etc… For our case we will only use the “Shell” provisioner; it will install dependencies. You can read more about this provisioner here.

Notice that builders and provisioners are arrays. We can have multiple types of provisioners and multiple builders so the same image can be generated on various platforms.

Once our template is ready, we should validate it to see if there are no parsing errors.

packer validate docker-install-template.json

notes: this will not guarantee that we’ve done a fine job with the shell script. If the required variables are not set up in our system this validation will output an error message.

Script

For our script we’ll do some very simple things. As we intent to install Docker and have an up to date snapshot we should update installed packages, install the latest possible kernel and install the latest version of Docker.

Create a file with install-script.sh name and copy the following:

#!/usr/bin/env sh

# sleep timer for packer
sleep 30

# add additional repos
add-apt-repository "deb http://archive.ubuntu.com/ubuntu $(lsb_release -sc) main universe"
add-apt-repository "deb http://archive.ubuntu.com/ubuntu $(lsb_release -sc)-updates main universe"

# update, install curl and linux kernel 3.16
apt-get update --fix-missing
apt-get upgrade -y
apt-get install -y curl linux-headers-3.16.0-29 linux-headers-3.16.0-29-generic \
                   linux-image-3.16.0-29-generic linux-image-extra-3.16.0-29-generic

# install docker
curl -sSL https://get.docker.com/ubuntu/ | sudo sh

# print docker version
docker version

The sleep is to wait until the machine completely boots. Then we update the kernel to the latest, install docker and print the docker version.

Let’s try our script and template by creating the snapshot.

Creating the snapshot

Before creating our snapshot make sure to have the DigitalOcean token created. Remember that this token is only visible once. Export this token as DIGITALOCEAN_API_TOKEN and prepare to have your snapshot ready in a few minutes.

export DIGITALOCEAN_API_TOKEN="your_api_token"

Once the environment variable is set up we should be able to run the following command and wait till it’s done.

packer build docker-install-template.json

I’m not going to put the hole output, this is the final chunk…

    ...
    digitalocean: Client version: 1.5.0
    digitalocean: Client API version: 1.17
    digitalocean: Go version (client): go1.4.1
    digitalocean: Git commit (client): a8a31ef
    digitalocean: OS/Arch (client): linux/amd64
    digitalocean: Server version: 1.5.0
    digitalocean: Server API version: 1.17
    digitalocean: Go version (server): go1.4.1
    digitalocean: Git commit (server): a8a31ef
==> digitalocean: Gracefully shutting down droplet...
==> digitalocean: Creating snapshot: build-with-packer-1424129310
==> digitalocean: Waiting for snapshot to complete...
==> digitalocean: Destroying droplet...
==> digitalocean: Deleting temporary ssh key...
Build 'digitalocean' finished.

==> Builds finished. The artifacts of successful builds are:
--> digitalocean: A snapshot was created: 'build-with-packer-1424129310' in region 'New York 3'

This final chunk shows the output from the docker version command and the name of the snapshot with the format we gave it build-with-packer-{{timestamp}}

Now go to your DigitalOcean admin panel and create a new droplet using your newly created snapshot, remember that for the kernel update to work you must follow these instructions.

From my point of view Packer is a Fantastic tool. Being able to create a reproducible image from a template that will be ready to put into a new droplet is a time saver and an assurance that everything will work as it’s suppose to.

Remember to update your snapshots periodically for patches, etc…

Thanks for reading…

By @albertogg