Amir Chaudhry

thoughts, comments & general ramblings

From Jekyll site to Unikernel in fifty lines of code.

Mirage has reached a point where it’s possible to easily set up end-to-end toolchains to build unikernels! My first use-case is to be able to generate a unikernel which can serve my personal static site but to do it with as much automation as possible. It turns out this is possible with less than 50 lines of code.

I use Jekyll and GitHub Pages at the moment so I wanted a workflow that’s as easy to use, though I’m happy to spend some time up front to set up and configure things. The tools for achieving what I want are in good shape so this post takes the example of a Jekyll site (i.e this one) and goes through the steps to produce a unikernel on Travis CI (a continuous integration service) which can later be deployed. Many of these instructions already exist in various forms but they’re collated here to aid this use-case.

I will take you, dear reader, through the process and when we’re finished, the workflow will be as follows:

  1. You’ll write your posts on your local machine as normal
  2. A push to GitHub will trigger a unikernel build for each commit
  3. The Xen unikernel will be pushed to a repo for deployment

To achieve this, we’ll first check that we can build a unikernel VM locally, then we’ll set up a continuous integration service to automatically build them for us and finally we’ll adapt the CI service to also deploy the built VM. Although the amount of code required is small, each of these steps is covered below in some detail. For simplicity, I’ll assume you already have OCaml and Opam installed – if not, you can find out how via the Real Word OCaml install instructions.

Building locally

To ensure that the build actually works, you should run things locally at least once before pushing to Travis. It’s worth noting that the mirage-skeleton repo contains a lot of useful, public domain examples and helpfully, the specific code we need is in mirage-skeleton/static_website. Copy both the and files from that folder into a new _mirage folder in your jekyll repository.

Edit so that the two mentions of ./htdocs are replaced with ../_site. This is the only change you’ll need to make and you should now be able to build the unikernel with the unix backend. Make sure you have the mirage package installed by running $ opam install mirage and then run:

(edit: If you already have mirage, remember to opam update to make sure you’ve got the latest packages.)

$ cd _mirage
$ mirage configure --unix
$ make depend         # needed as of mirage 1.2 onward
$ mirage build
$ cd ..

That’s all it takes! In a few minutes there will be a unikernel built on your system (symlinked as _mirage/mir-www). If there are any errors, make sure that Opam is up to date and that you have the latest version of the static_website files from mirage-skeleton.

Serving the site locally

If you’d like to see this site locally, you can do so from within the _mirage folder by running unikernel you just built. There’s more information about the details of this on the Mirage docs site but the quick instructions are:

$ cd _mirage
$ sudo mirage run

# in another terminal window
$ sudo ifconfig tap0

You can now point your browser at and see your site! Once you’re finished browsing, $ mirage clean will clear up all the generated files.

Since the build is working locally, we can set up a continuous integration system to perform the builds for us.

Setting up Travis CI


We’ll be using the Travis CI service, which is free for open-source projects (so this assumes you’re using a public repo). The benefit of using Travis is that you can build a unikernel without needing a local OCaml environment, but it’s always quicker to debug things locally.

Log in to Travis using your GitHub ID which will then trigger a scan of your repositories. When this is complete, go to your Travis accounts page and find the repo you’ll be building the unikernel from. Switch it ‘on’ and Travis will automatically set your GitHub post-commit hook and token for you. That’s all you need to do on the website.

When you next make a push to your repository, GitHub will inform Travis, which will then look for a YAML file in the root of the repo called .travis.yml. That file describes what Travis should do and what the build matrix is. Since OCaml is not one of the supported languages, we’ll be writing our build script manually (this is actually easier than it sounds). First, let’s set up the YAML file and then we’ll examine the build script.

The Travis YAML file - .travis.yml

The Travis CI environment is based on Ubuntu 12.04, with a number of things pre-installed (e.g Git, networking tools etc). Travis doesn’t support OCaml (yet) so we’ll use the c environment to get the packages we need, specifically, the OCaml compiler, Opam and Mirage. Once those are set up, our build should run pretty much the same as it did locally.

For now, let’s keep things simple and only focus on the latest releases (OCaml 4.01.0 and Opam 1.1.1), which means our build matrix is very simple. The build instructions will be in the file _mirage/, which we will move to and trigger from the .travis.yml file. This means our YAML file should look like:

language: c
before_script: cd _mirage
script: bash -ex

The matrix enables us to have parallel builds for different environments and this one is very simple as it’s only building two unikernels. One worker will build for the Xen backend and another worker will build for the Unix backend. The _mirage/ script will clarify what each of these environments translates to. We’ll come back to the DEPLOY flag later on (it’s not necessary yet). Now that this file is set up, we can work on the build script itself.

The build script -

To save time, we’ll be using an Ubuntu PPA to quickly get pre-packaged versions of the OCaml compiler and Opam, so the first thing to do is define which PPAs each line of the build matrix corresponds to. Since we’re keeping things simple, we only need one PPA that has the most recent releases of OCaml and Opam.

#!/usr/bin/env bash
echo "yes" | sudo add-apt-repository ppa:$ppa
sudo apt-get update -qq
sudo apt-get install -qq ocaml ocaml-native-compilers camlp4-extra opam

[NB: There are many other PPAs for different combinations of OCaml/Opam which are useful for testing]. Once the appropriate PPAs have been set up it’s time to initialise Opam and install Mirage.

export OPAMYES=1
opam init
opam install mirage
eval `opam config env`

We set OPAMYES=1 to get non-interactive use of Opam (it defaults to ‘yes’ for any user input) and if we want full build logs, we could also set OPAMVERBOSE=1 (I haven’t in this example). The rest should be straight-forward and you’ll end up with an Ubuntu machine with OCaml, Opam and the Mirage package installed. It’s now trivial to do the next step of actually building the unikernel!

mirage configure --$MIRAGE_BACKEND
mirage build

You can see how we’ve used the environment variable from the Travis file and this is where our two parallel builds begin to diverge. When you’ve saved this file, you’ll need to change permissions to make it executable by doing $ chmod +x _mirage/

That’s all you need to build the unikernel on Travis! You should now commit both the YAML file and the build script to the repo and push the changes to GitHub. Travis should automatically start your first build and you can watch the console output online to check that both the Xen and Unix backends complete properly. If you notice any errors, you should go back over your build script and fix it before the next step.

Deploying your unikernel


When Travis has finished its builds it will simply destroy the worker and all its contents, including the unikernels we just built. This is perfectly fine for testing but if we want to also deploy a unikernel, we need to get it out of the Travis worker after it’s built. In this case, we want to extract the Xen-based unikernel so that we can later start it on a Xen-based machine (e.g Amazon, Rackspace or - in our case - a machine on Bytemark).

Since the unikernel VMs are small (only tens of MB), our method for exporting will be to commit the Xen unikernel into a repository on GitHub. It can be retrieved and started later on and keeping the VMs in version control gives us very effective snapshots (we can roll back the site without having to rebuild). This is something that would be much more challenging if we were using the ‘standard’ web toolstack.

The deployment step is a little more complex as we have to send the Travis worker a private SSH key, which will give it push access to a GitHub repository. Of course, we don’t want to expose that key by simply adding it to the Travis file so we have to encrypt it somehow.

Sending Travis a private SSH key

Travis supports encrypted environment variables. Each repository has its own public key and the Travis gem uses this public key to encrypt data, which you then add to your .travis.yml file for decryption by the worker. This is meant for sending things like private API tokens and other small amounts of data. Trying to encrypt an SSH key isn’t going to work as it’s too large. Instead we’ll use travis-senv, which encodes, encrypts and chunks up the key into smaller pieces and then reassembles those pieces on the Travis worker. We still use the Travis gem to encrypt the pieces to add them to the .travis.yml file.

While you could give Travis a key that accesses your whole GitHub account, my preference is to create a new deploy key, which will only be used for deployment to one repository.

# make a key pair on your local machine
$ cd ~/.ssh/
$ ssh-keygen -t dsa -C "travis.deploy" -f travis-deploy_dsa
$ cd -

Note that this is a 1024 bit key so if you decide to use a 2048 bit key, then be aware that Travis sometimes has issues. Now that we have a key, we can encrypt it and add it to the Travis file.

# on your local machine

# install the necessary components
$ gem install travis
$ opam install travis-senv

# chunk the key, add to yml file and rm the intermediate
$ travis-senv encrypt ~/.ssh/travis-deploy_dsa _travis_env
$ cat _travis_env | travis encrypt -ps --add
$ rm _travis_env

travis-senv encrypts and chunks the key locally on your machine, placing its output in a file you decide (_travis_env). We then take that output file and pipe it to the travis ruby gem, asking it to encrypt the input, treating each line as separate and to be appended (-ps) and then actually adding that to the Travis file (--add). You can run $ travis encrypt -h to understand these options. Once you’ve run the above commands, .travis.yml will look as follows.

language: c
before_script: cd _mirage
script: bash -ex
  - secure: ".... encrypted data ...."
  - secure: ".... encrypted data ...."
  - secure: ".... encrypted data ...."

The number of secure variables added depends on the type and size of the key you had to chunk, so it could vary from 8 up to 29. We’ll commit these additions later on, alongside additions to the build script.

At this point, we also need to make a repository on GitHub and add the public deploy key so that Travis can push to it. Once you’ve created your repo and added a README, follow GitHub’s instructions on adding deploy keys and paste in the public key (i.e. the content of

Now that we can securely pass a private SSH key to the worker and have a repo that the worker can push to, we need to make additions to the build script.

Committing the unikernel to a repository

Since we can set DEPLOY=1 in the YAML file we only need to make additions to the build script. Specifically, we want to assure that: only the Xen backend is deployed; only pushes to the repo result in deployments, not pull requests (we do still want builds for pull requests).

In the build script (_mirage/, which is being run by the worker, we’ll have to reconstruct the SSH key and configure Git. In addition, Travis gives us a set of useful environment variables so we’ll use the latest commit hash ($TRAVIS_COMMIT) to name the the VM (which also helps us trace which commit it was built from).

It’s easier to consider this section of code at once so I’ve explained the details in the comments. This section is what you need to add at the end of your existing build script (i.e straight after mirage build).

# Only deploy if the following conditions are met.
if [ "$MIRAGE_BACKEND" = "xen" \
            -a "$DEPLOY" = "1" \
            -a "$TRAVIS_PULL_REQUEST" = "false" ]; then

    # The Travis worker will already have access to the chunks
    # passed in via the yaml file. Now we need to reconstruct 
    # the GitHub SSH key from those and set up the config file.
    opam install travis-senv
    mkdir -p ~/.ssh
    travis-senv decrypt > ~/.ssh/id_dsa # This doesn't expose it
    chmod 600 ~/.ssh/id_dsa             # Owner can read and write
    echo "Host some_user"   >> ~/.ssh/config
    echo "  Hostname"          >> ~/.ssh/config
    echo "  StrictHostKeyChecking no"     >> ~/.ssh/config
    echo "  CheckHostIP no"               >> ~/.ssh/config
    echo "  UserKnownHostsFile=/dev/null" >> ~/.ssh/config

    # Configure the worker's git details
    # otherwise git actions will fail.
    git config --global ""
    git config --global "Travis Build Bot"

    # Do the actual work for deployment.
    # Clone the deployment repo. Notice the user,
    # which is the same as in the ~/.ssh/config file.
    git clone git@some_user:amirmc/www-test-deploy
    cd www-test-deploy

    # Make a folder named for the commit. 
    # If we're rebuiling a VM from a previous
    # commit, then we need to clear the old one.
    # Then copy in both the config file and VM.
    rm -rf $TRAVIS_COMMIT
    mkdir -p $TRAVIS_COMMIT
    cp ../mir-www.xen ../ $TRAVIS_COMMIT

    # Compress the VM and add a text file to note
    # the commit of the most recently built VM.
    bzip2 -9 $TRAVIS_COMMIT/mir-www.xen
    git pull --rebase
    echo $TRAVIS_COMMIT > latest    # update ref to most recent

    # Add, commit and push the changes!
    git add $TRAVIS_COMMIT latest
    git commit -m "adding $TRAVIS_COMMIT built for $MIRAGE_BACKEND"
    git push origin master
    # Go out and enjoy the Sun!

At this point you should commit the changes to ./travis.yml (don’t forget the deploy flag) and _mirage/ and push the changes to GitHub. Everything else will take place automatically and in a few minutes you will have a unikernel ready to deploy on top of Xen!

You can see both the complete YAML file and build script in use on my test repo, as well as the build logs for that repo and the deploy repo with a VM.

[Pro-tip: If you add *[skip ci] anywhere in your commit message, Travis will skip the build for that commit. This is very useful if you’re making minor changes, like updating a README.]*

Finishing up

Since I’m still using Jekyll for my website, I made a short script in my jekyll repository ( that builds the site, commits the contents of _site and pushes to GitHub. I simply run this after I’ve committed a new blog post and the rest takes care of itself.

#!/usr/bin/env bash
jekyll build
git add _site
git commit -m 'update _site'
git push origin master

Congratulations! You now have an end-to-end workflow that will produce a unikernel VM from your Jekyll-based site and push it to a repo. If you strip out all the comments, you’ll see that we’ve written less than 50 lines of code! Admittedly, I’m not counting the 80 or so lines that came for free in the *.ml files but that’s still pretty impressive.

Of course, we still need a machine to take that VM and run it but that’s a topic for another post. For the time-being, I’m still using GitHub Pages but once the VM is hosted somewhere, I will:

  1. Turn off GitHub Pages and serve from the VM – but still using Jekyll in the workflow.
  2. Replace Jekyll with OCaml-based static-site generation.

Although all the tools already exist to switch now, I’m taking my time so that I can easily maintain the code I end up using.

Expanding the script for testing

You may have noticed that the examples here are not very flexible or extensible but that was a deliberate choice to keep them readable. It’s possible to do much more with the build matrix and script, as you can see from the Travis files on my website repo, which were based on those of the Mirage site and Mort’s site. Specifically, you can note the use of more environment variables and case statements to decide which PPAs to grab. Once you’ve got your builds working, it’s worth improving your scripts to make them more maintainable and cover the test cases you feel are important.

Not just for static sites (surprise!)

You might have noticed that in very few places in the toolchain above have I mentioned anything specific to static sites per se. The workflow is simply (1) do some stuff locally, (2) push to a continuous integration service which then (3) builds and deploys a Xen-based unikernel. Apart from the convenient folder structure, the specific work to treat this as a static site lives in the *.ml files, which I’ve skipped over for this post.

As such, the GitHub+Travis workflow we’ve developed here is quite general and will apply to almost any unikernels that we may want to construct. I encourage you to explore the examples in the mirage-skeleton repo and keep your build script maintainable. We’ll be using it again the next time we build unikernel devices.

Acknowledgements: There were lots of things I read over while writing this post but there were a few particularly useful things that you should look up. Anil’s posts on Testing with Travis and Travis for secure deployments are quite succinct (and were themselves prompted by Mike Lin’s Travis post several months earlier). Looking over Mort’s build script and that of mirage-www helped me figure out the deployment steps as well as improve my own script. Special thanks also to Daniel, Leo and Anil for commenting on an earlier draft of this post.