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Reproducibility! You may not have noticed, but Robur has done a lot of work on several levels to provide an infrastructure to ensure the reproducibility of your applications (and our unikernels).

But before going into the details of such an infrastructure, we need to define what reproducibility is and what problems we are trying to solve.

I would like to say that I am only a user of this infrastructure (available here). Since I arrived at Robur (beyond the countless projects I want to do), one of my goals has been to:

  1. really deploy unikernels
  2. use them
  3. ensure their reproducibility
  4. use the artefacts produced in my unikernel infrastructure
  5. (bonus) monitor these unikernels (which will be the subject of another article)

I would like to give special thanks to Robur and its initial team (to which I have added myself) who not only created this infrastructure but also helped me in all the steps of deployment on my server. And if I had to add a small stone to the edifice, it would of course be this article which is meant to explain this infrastructure.

What does reproducibility mean?

Reproducibility is a deterministic build process where one ensures, from the sources and a build environment, to produce the same binary byte per byte.

In fact, the difficulty of reproducibility is not so much in the OCaml code or in the OCaml compiler (which itself remains very deterministic) but in the build environment. In particular, variables such as date, language or paths need to be normalized.

The OCaml compiler

Hannes remarks that "the OCaml compiler" story is a bit longer - and depends on the definition of reproducibility (with robur and orb, we e.g. always build in the same path, as the same user -- also intermediate build products (cma/cmxa/...) are not taken into consideration). There were reproducibility issues in the OCaml runtime, which have been fixed:

All these patches are related to values that cannot be set by the environment (such as the date), the use of temporary files (such as OCaml-generated assembly files), the non-deterministic behaviour of some Unix tools (such as sort) which may depend on the system language, or the absolute paths in artefacts which should be able to be reduced to relative paths (one might want to compile the same file in foo/ and then in bar/ without this altering the compilation). Finally, the production of *.cma seemed to correspond to non-deterministic behaviour of the system.

It is worth noting that Debian has been very forward-thinking on the issue of reproducibility and that most of these patches have been notified from the Debian bug tracker. Indeed, they have decided to make all Debian packages reproducible since 2013. The OCaml team has set this goal as well.

From my point of view, and the resulting patches, reproducibility is still a matter of details. Basically, the majority of these patches don't change what the compiler does globally, but specify the pipeline a bit more so that it is reproducible.

Nevertheless, if such an environment exists, what really interests the developer in this phase is not so much the result obtained but a tool that can:

  1. verify this reproducibility in their development
  2. be notified of sources that may alter the reproducibility of their software

From a "releaser" point of view, it is indeed this reproducibility that interests us since it is what we want to distribute. But, as I said, the difficulty is not here, since it is a matter of freezing a build context - of course, it is still a big job to freeze such a context.

But from the point of view of a developer - who is therefore iterating in their code - it is mainly a question of recognising what can alter the build process in order to really identify what it depends on.

In this respect, ensuring a deterministic build and normalising the elements that can intervene in the build process and thus creating a build context is only one step. The other step is to be able to identify the elements that can alter the reproduction process. Indeed, this build context must be exhaustive but can be extremely broad. Thus, reproducibility does not only concern the result but also the tools that can help to prove this reproducibility.

Such a tool would ensure that the project is replicable on a daily basis. It would provide a complete description of a build context. Unfortunately, whether as a developer or as a user, this context can change (for example, via an update). In this respect, the tool must be able to:

Why does reproducibility matter?

There are many reasons for wanting reproducibility and these reasons are not the same depending on which hat we wear. Indeed, reproducibility does not answer the same problems for a developer as for a releaser. Nevertheless, if we were to crystalize the interest in reproducibility into a more general concept, it would be trust.

As far as a developer is concerned, their job is, among other things, to fix bugs. The problem is to find the bug: where does it come from? This is where the archaeological work begins, starting with our code. However, bugs can exist outside our code. They can appear in the libraries we use. Worse, they can appear in the libraries that the libraries we use use... But the question becomes eminently complex when two developers try to find the same bug. This situation arises when one of them says: "It works for me". In this case, we are much more likely to consider that the bug does not come from our code but from the underlying dependencies we use: we can see that the software is not reproducible since from one computer to another, the behaviour is different for the same code. This is where the question arises: can we trust our workflow to build our software?

For a releaser, the problem can be even more serious. The release process consists of building the software and then distributing it. We talked about "freezing" this build context above. However, as a releaser, have we really taken into account all the dependencies (and security patches) in order to produce a single piece of software? One could legitimately ask the question whether our build computer would not be infected, or whether it really takes into account all security updates. The releaser must then make an exhaustive audit of what they need to build the software. If reproducibility is not assured, this means that this "frozen" context is not exhaustive and that there are in fact two larger contexts (of which the first is only a subset) that do not produce the same software. Where is the difference? Why is there a difference? What is the impact of this difference (about security)? All these questions bring us back to the trust that we place in this context, can I trust such a context to be sure that I have a deterministic production of my software?

Finally, reproducibility also concerns the user. Downloading software or an executable carries a risk - a risk of infection, piracy, etc. A user who downloads your software trusts you. Now the question is: where does this trust come from? It can come from the beers you share together to remake the world, or it can come from a transparent and repeatable process that the user can repeat to ensure that your distributed software is the software you built (no added malicious code, an exhaustive dependency graph that can be audited, etc.).

As we have said, reproducibility reaffirms the bond of trust that can exist in the software production chain from developer to user. One can legitimately say that all these steps are not fundamentally necessary (and one could even question the usefulness of these for an end user) but one must conceive the idea that the notion of security of a software and particularly the notion of trust that one grants to a software is above all and for something that is built.

It is certainly not something established (like saying: look! my software is secure!). It's about giving proofs that will never affirm that our software is secure but will give signals and reasons why you should trust us.


orb is the initial tool that will both launch the build and aggregate the information to define this famous context. From the latter, the user wants to restart the build and:

  1. show for a different context (an update), the software remains the same
  2. ensure that for the same context, the software remains the same
  3. notify differences if 2 contexts do not produce the same software

In detail, orb expects an OPAM package which is rather straightforward for OCaml software. Indeed, one can introspect the reproducibility at several levels. One could aggregate the hashes of all *.cmx and be sure that compiling N caml objects produces a deterministic result. But it is above all a notion of cursor that is at stake. As we said, the OCaml compiler is quite deterministic, should we really consume computation time for a result that we already know a priori?

Introspecting reproducibility at the level of caml objects would surely be a waste of time. Where compilation becomes complex is when it interacts with other elements:

Thus, the granularity of what can break the reproducibility of our software is not found in the compiler, nor in dune for example but more in the dependencies: the packages.

It is quite simple to use, only OPAM is needed. orb will create a temporary switch and will try to compile your package in this switch. Let's take the example of hxd:

$ opam pin add -y
$ orb build hxd
[ORB] using root "/home/dinosaure/.opam" and switch "orb228afdtemp"
[ORB] Switch orb228afdtemp created!
[ORB] Install start
[ORB] Install hxd
[ORB] Installed hxd
[ORB] tracking map got locks
[ORB] got tracking map, dropping states
[ORB] writing $PWD/
[ORB] cleaning up
[ORB] Switch orb228afdtemp removed
[ORB] cleaning up

This build created several files: build-environment, opam-switch, system-packages and This is our context and its expected output (the hash of the various objects installed). We can then rebuild from this context with orb rebuild:

$ orb rebuild .
[ORB] environment matches
[ORB] Switch orb228afdtemp created!
[ORB] now importing switch
[ORB] Switch orb228afdtemp imported!
[ORB] tracking map got locks
[ORB] got tracking map, dropping states
[ORB] writing $PWD/
[ORB] comparing with old build dir no
[ORB] It is reproducible!!!
[ORB] cleaning up
[ORB] Switch orb228afdtemp removed

As we expected, our software is reproducible in a specific context. It is quite expected that software released in OCaml is reproducible in a general way. hxd is even simpler as it is only OCaml and has very few dependencies.

So reproducibility from a defined state in a fixed time has been assured. But what is really interesting for us is to know if this reproducibility continues to exist even though the ecosystem evolves. Indeed, an update of cmdliner for example can lead to an alteration in the code produced and in this case, our reproducibility is no longer assured. But identifying these elements that evolve in spite of ourselves and that have an impact on our software requires meticulous and repetitive work.

orb helps us to make this work meticulous by aggregating for us the elements involved in the compilation of our software. Let's move on to the repetition.


What we are talking about here can be defined as a Continuous Integration infrastructure where this reproducibility check should take place on a daily basis. CI is a real problem in itself, but our use of it is quite simple, it can even be likened to a CRON task that would call orb daily.

The question is more about the deployment of such an infrastructure. This is where builder comes in. It is basically a daemon that will ask workers to run orb.

The communication between this daemon and the workers is done through the ASN.1 protocol (just like albatross). Again, Robur has done an excellent job that goes far beyond creating such software. Indeed, as said, the problem here is the deployment and distribution of builder and builder-worker incorporating the necessary elements to interface well with systemd (for Debian) or FreeBSD. And to enable the launch of orb, we simply use docker to launch Ubuntu, install orb and run the command.

$ curl -fsSL | gpg --dearmor > /usr/share/keyrings/
# echo "deb [signed-by=/usr/share/keyrings/] ubuntu-20.04 main" > /etc/apt/sources.list.d/robur.list
/ replace ubuntu-20.04 with e.g. debian-11 on a debian buster machine
$ sudo apt update
$ sudo apt install builder
$ sudo systemctl start builder
$ sudo systemctl start builder-worker

A local OPAM repository

In what defines your context, it has of course the OPAM deposit. For real use of orb and builder, the most interesting thing is to manage your own OPAM repository to put your software in before releasing it publicly.

One could simply use the OPAM tool, which makes it fairly easy to deploy a new repository. It would then be a matter of adding this repository to the platforms that builder can handle. But I think you are used to my taste for unikernels: so why not use a unikernel as an OPAM repository?

opam-mirror is a good project that allows, from a Git repository, to offer an OPAM repository as The unikernel is very simple, it is an HTTP server with a cache system that synchronizes with a Git repository. The idea is to create our own Git repository and then deploy our unikernel. We won't do all the steps and especially the build step, Robur has its own reproducibility infra in which opam-mirror is proposed: latest opam-mirror from

We're not going to play around with solo5-hvt either, but directly use albatross which is available from the Robur APT repository:

$ sudo apt install albatross
$ sudo systemctl start albatross_daemon

If you haven't done so, a unikernel needs a virtual "bridge" in order to connect to it. Usually, you add these lines to /etc/network/interfaces (a restart of the networking service is required afterwards):

auto service iface service
  inet manual up ip link add service-master address 02:00:00:00:00:01 type dummy
  up ip link set dev service-master up
  up ip link add service type bridge
  up ip link set dev service-master master service
  up ip addr add dev service
  up ip link set dev service up
  down ip link del service
  down ip link del service-master

Fortunately, we do not need to communicate with the outside world and the outside world does not need to communicate with our unikernel (we could, but that is not the point). The iptables rules are therefore not necessary!

Now we need to create a Git "server". In fact, from a Git perspective, only our SSH server is required. We just need:

  1. create a Git user
  2. add our public keys to .ssh/authorized_keys
  3. create a Git repository with git init --bare in the $HOME folder of Git
$ sudo adduser git
$ su git
$ cd
$ mkdir .ssh && chmod 700 .ssh
$ touch .ssh/authorized_keys
$ chmod 600 .ssh/authorized_keys
$ mkdir opam-repository.git
$ cd opam-repository.git
$ git init --bare

A question was asked about SSH and ocaml-git keys. Our workflow for unikernels is to generate a in-the-fly SSH key (in order to separate the use of our repository by our unikernels and our own use). By default, we generate an RSA SSH key with awa_gen_key (available via the OPAM package awa). This gives 2 lines:

  1. a seed to regenerate the private key from the Fortuna engine
  2. the public key in the format expected by SSH
$ awa_gen_key
seed is pUxPne3bfhuihCPH3VhJtjm2/NIM5ooQndXUc5Ey
ssh-rsa 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 awa@awa.local

The second line should be copied to the .ssh/authorized_keys in Git. The first one will be transmitted to our unikernel in this way (with albatross).

$ albatross-client-local create --net=service:service --block=tar:opam-mirror --mem=512 \
  mirror mirror.hvt \
  --arg="--ipv4=" \
  --arg="--ipv4-gateway=" \
  --arg="--remote=git@" \

That's it! All you have to do is add your packages in the OPAM format and the OPAM repository format. To do this, the root must contain at least a repo file and a package folder. Then, to understand the structure of an OPAM repository, you can take inspiration from opam-repository. Finally, the unikernel provides a gateway for it to resynchronise with the Git repository.

$ git clone git@localhost:opam-repository.git
$ cd opam-repository
$ echo 'opem-version: "2.0"' > repo
$ mkdir packages
$ touch packages/.gitkeep
$ git add repo packages
$ git commit -m "First commit"
$ git push
$ curl

And yes, there never really was a Git server per se. The idea has always been to "leave" the authentication management to another protocol (in this case SSH). There is however a way to have a non-secure Git server with git daemon but this method is deprecated for obvious reasons. This is one of the big changes in ocaml-git 3, the implementation of the Git protocol is the same whether it is via TCP/IP, SSH or HTTP.

How to launch a reproducible build?

Now we need to inform the templates to build our software from our new OPAM repository. We will use the /etc/builder/orb-build.template.ubuntu-20.04 template for now.

diff --git a/a b/b index b3b67a5..f5a99fa 100644 ---
a/a +++ b/b @@ -8,5 +8,5 @@ rm orb.deb
 DEBIAN_FRONTEND=noninteractive apt install --no-install-recommends --no-install-suggests -y dh-exec
orb build --disable-sandboxing --solver-timeout=600 --switch-name=/tmp/myswitch --date=1589138087 --out=. --repos=$repos %%OPAM_PACKAGE%%

Let's say we have released the bob software with a full description of its OPAM file in our repository. builder provides a builder-client tool to interact with the daemon. Thus, one can add a package to build and reproduce. The first argument is the name of the software, the second the name of the OPAM package.

$ builder-client orb-build bob bob

We can see that a docker image has been launched and is trying to compile our project now! builder will then try to allocate a worker every day to compile your project. It will keep track of the contexts and compare them to the result of the build. Thus, as we expected, it will ensure reproducibility and if this is no longer available, show us the difference that involved altering our result.

Hannes says: setup this way, orb uses the HEAD of opam-repository, thus it prepares rolling releases. the output of orb includes exact frozen packages (and git commits), so they are reproducible.


Finally, the last software of this infrastructure is a website to make the results available and more digestible through a nice web interface! The latter will allow to present several information such as:

This website is also available from the APT repository offered by Robur. Thus, it can be easily installed and deployed. One particular point should be noted however.

builder just tries to reproduce your software but as far as its distribution is concerned (notably on the website), it makes us specify where to upload the artefacts. So we need to change the way builder is launched a little.

In this, builder-web has a little user management (to protect access to the upload). So we also create a user for builder-web (with a password) and inform builder to use it.

$ sudo apt install builder-web
$ sudo builder-db migrate
$ sudo builder-db user-add dinosaure --unrestricted
Password: foo
$ sudo systemctl start builder-web

And finally, we must modify /usr/lib/systemd/system/builder.service with:

< ExecStart=/usr/bin/builder-server
> ExecStart=/usr/bin/builder-server --upload http://dinosaure:foo@localhost:3000/upload

That's it! You have a website available at http://localhost:3000/ on which there is an aggregate of all your builds - with all the information needed to prove/reproduce your software.


You will note that all this software is reproducible and available (beyond APT) on Robur's infra: In fact, the software on APT is the same as on the Robur's infra. The package repository at is fed by the daily builds, and all the above mentioned software (orb, builder, builder-web, albatross) are reproducible and are daily build -- thus subscribing to the package repository enables you to get the latest package with an update.

For my part, I deployed the infrastructure on my side in order to be able to distribute my famous Bob software (and make it reproducible). Then I made a small update to Contruno so that it could redirect TLS connections from to builder-web. So, my infrastructure is available here (and 2 unikernels, contruno and opam-mirror are involved).

In fact, once you understand how orb, builder, builder-worker and builder-web work, you can see that: 1) these solutions can be composed with other solutions which does not make them monolithic - this is my preferred approach 2) it is actually quite easy to deploy these tools - the only barrier is the lack of documentation and where to start but this article tries to address that issue 3) the result is nice! It didn't take me long to try to reproduce Bob (who, however, has a rather complex build because of Cosmopolitan)

For its usefulness, its simplicity and the result, I take the liberty of advertising it since it is a project that finally brings a lot to the development of software, its distribution and the release process.

It is a project that can also be articulated with albatross. I haven't tested it yet but imagine: you have packages about your unikernels (which is possible since mirage produces an OPAM file of your project) which you add to your local repository and then ask builder to ensure reproducibility. You could then ask albatross to get the Solo5 images from this infrastructure! And apparently this is possible - but I haven't tested it yet.

The issue of APT (and more generally, the distribution of software outside the OPAM world) becomes just as simple. Again, the process is not clearly identified but it is clear that Robur is taking advantage of its infrastructure to maintain an APT repository available on Debian and Ubuntu.

More generally, such an infrastructure helps to build trust between users and developers. This is all the more true in the case of Bob, which is ultimately just a big binary. As mentioned in the introduction, one can always wonder about the risks involved in downloading a binary and running it on one's computer. To this legitimate question, I answer: reproducibility