Module builds – how do they work?

Have you ever wondered how modular builds work? In my previous post called Building Modules for the F27 Server, I described the steps a packager needs to go through in order to produce a successful build in the Fedora infrastructure. But what exactly is going on at the backstage?


The build pipeline sees modules as build recipes for RPM packages. Each module has a list of SRPM (source RPM) packages that get build within the module. These packages are divided into build groups which are processed in batches one by one. Build groups are useful in situations when certain packages are needed in the buildroot to build some other ones within the same module. How these build groups work and what services are they part of?

All packages are build in Koji—the same service that builds packages for the traditional Fedora release. Interesting is that Koji doesn’t know anything about modules or build groups. And it uses the same mechanics in both modular and traditional releases. How it knows what to do? That’s where the Module Build Service (MBS) comes in.

MBS orchestrates package builds in Koji. This service makes sure that packages are built in a defined order by leveraging the concept of build groups, and reports the overall state of module builds to the user and other services. While MBS manages most of the steps of module builds, it relies on Koji to do the actual building, and to store the output such as binary packages and logs.

When a module build is finished, it is processed by other services to produce the final artifacts such as RPM repositories, container base images, cloud images, or bootable iso images. Artifacts are typically composed out of multiple modules.

Building a module

Everything starts when a module build gets submitted to MBS. At the time of writing this post, this action is done manually by the packager. They need to run the `mbs-build submit`command in their dist-git repository to get the build submitted. However, a new service called Freshmaker is being deployed. This service will automatically submit builds for every commit to dist-git.

Initial check

When a build is submitted to MBS, it enters the “init” state. During this state, MBS checks if the modulemd file is valid, and that all components of the modules are available. If everything is alright, the state switches to “wait”.

Modules in the “wait” state are waiting for a capacity in the build system. When a capacity is available, MBS switches the state to “build”. And that’s where the real building starts.

Preparing a buildroot

Before diving into describing the “build” state, we need to understand two concepts Koji uses during module builds: tags and tag inheritance. Tags are basically groups of Koji builds. A single build can be tagged into multiple tags. Tags can represent buildroots, releases, or any other groups of builds—like modules. MBS creates two tags for every module build: the buildroot, and the result. Buildroots are typically composed of multiple modules. Koji uses the tag inheritance mechanism to combine multiple tags into one. How is a module buildroot created?

Module buildroot is a tag in Koji representing the environment—a set of packages—where a module gets build. Constructing a buildroot is a first step of the “build” state. The buildroot is an empty tag that inherits all the modules specified as build dependencies in the modulemd that is being built. When this is done, MBS can submit package builds into Koji.

The first package that is built as a part of every module build is the `module-build-macros`. It contains a set of macros that are used when building other packages. When this build is finished, the result is tagged back to the buildroot tag—so it is available for all other components.

Building packages in build groups

Packages are built in batches called build groups. These build groups are defined in modulemd using a buildorder value. The buildorder value is an integer, every component (package) has it, and the default value is zero. Components with the same buildorder value are part of the same build group.

MBS schedules build groups one-by-one, starting with the lowest buildorder value. All packages within a build group are submitted at once and built in parallel. When all builds within the build group finish, the results are tagged back to the buildroot tag—so they are available for the next ones. This is repeated until all build groups are finished.

When all build groups are finished, MBS changes the module build state. If all component builds succeeded, the build state is set to “done”. In case there is one or more failures, the state is set to “failed”. Both of these states mean that the build phase has ended.

After the build

There is another state in MBS indicating that modules are ready to be part of a larger compose and consumed by the end-user. The state is called “ready”. Right now, MBS switches all states “done” to “ready” automatically. However, in the future, this will be probably switched by an external service that sees the bigger picture. Part of the bigger picture can be rebuilding all modular dependencies, additional integration testing, etc.

Next step – release

Modules in the “ready” state are not automatically available to the end user. To have a certain module available in a release, it needs to be added to a compose which is then used to produce the final artifacts such as RPM repositories, container base images, cloud images, or bootable iso images. I might write additional blog post describing this in the future.

This post has been mostly focused on the build phase. There are many other interesting services in the Factory 2.0 that make Modularity possible. Have a look at the Factory 2.0 Focus Documents for more information. You can also learn how to build modules for the F27 Server, see the F27 content plan and help us with building more modules, or browse the Modularity documentation to get more information about the effort.

Building Modules for Fedora 27

Let me start with a wrong presumption that you have everything set up – you are a packager who knows what they want to achieve, you have a dist-git repository created, you have all the tooling installed. And of course, you know what Modularity is, and how and why do we use modulemd to define modular content. You know what Host, Platform, and Bootstrap modules are and how to use them.

Why would I make wrong presumptions like that? First of all, it might not be wrong at all. Especially, if you are a returning visitor, you don’t want to learn about the one-off things every time. I want to start with the stuff you will be doing on regular basis. And instead of explaining the basics over and over again, I will just point you to the right places that will help you solve a specific problem, like not having a dist-git repository.

Updating a module

Let’s go through the whole process of updating a module. I will introduce some build failures on purpose, so you know how to deal with them should they happen to you.

Start with cloning the module dist-git repository to get the modulemd file defining the installer module.

$ fedpkg clone modules/installer
$ cd installer
$ ls
Makefile installer.yaml sources tests

I want to build an updated version of installer. I have generated a new, updated version of modulemd (I will be talking about generating modulemd files later in this post), and I am ready to build it.

$ git add installer.yaml
$ git commit -m "building a new version"
$ git push
$ mbs-build submit
asamalik's build #942 of installer-master has been submitted

Now I want to watch the build and see how it goes.

$ mbs-build watch 942

 40 components in the COMPLETE state
 3 components in the FAILED state
asamalik's build #942 of installer-master is in the "failed" state

Oh no, it failed! I reviewed the Koji logs using the links above, and I see that:

  • NetworkManager and python-urllib3 failed randomly on tests. That happens sometimes and just resubmitting them would fix it.
  • realmd however needs to be fixed before I can proceed.

After fixing realmd and updating the modulemd to reference the fix, I can submit a new build.

$ git add installer.yaml
$ git commit -m "use the right version of automake with realmd"
$ git push
$ mbs-build submit
asamalik's build #942 of installer-master has been submitted

To watch the build, I use the following command.

$ mbs-build watch 943

 42 components in the COMPLETE state
 1 components in the FAILED state
asamalik's build #943 of installer-master is in the "failed" state

Good news is that realmd worked this time. However, sssd failed. I know it built before, and by investigating the logs I found out it’s one of the random test failures again. Resubmitting the build will fix it. In this case, I don’t need to create a new version, just resubmit the current one.

$ mbs-build submit
asamalik's build #943 of installer-master has been resubmitted

Watch the build:

$ mbs-build watch 943
 43 components in the COMPLETE state
asamalik's build #943 of installer-master is in the "ready" state

Rebuilding against new base

The Platform module has been updated and there is a soname bump in rpmlib. I need to rebuild the module against the new platform. However, I’m not changing anything in the modulemd. I know that builds map 1:1 to git commits, so I need to create an empty commit and submit the build.

$ git commit --allow-empty -m "rebuild against new platform"
$ git push
$ mbs-build submit
asamalik's build #952 of installer-master has been submitted

Making sure you have everything

Now it’s the time to make sure you are not missing a step in your module building journey! Did I miss something? Ask for it in the comments or tweet at me and I will try to update the post.

How do I get a dist-git repository?

To get a dist-git repository, you need to have your modulemd to go through a Fedora review process for modules. Please make sure your modulemd comply with the Fedora packaging guidelines for modules. Completing the review process wil result in having a dist-git repository.

What packages go into a module?

Your module need to run on top of the Platform module which together with the Host and Shim modules form the base operating system. You can see the definition of Platform, Host, and Shim modules on github.

You should include all the dependencies needed for your module to run on the Platform module, with few exceptions:

  • If your module needs a common runtime (like Perl or Python) that are already modularized, you shoud use these as a dependencies rather than bundling them to your module.
  • If you see that your module needs a common package (like net-tools), you shouldn’t bundle them either. They should be split into individual modules.
  • To make sure your module doesn’t conflict with other modules, it can’t contain the same packages as other modules. Your module can either depend on these other modules, or such packages can be split into new modules.

To make it easier during the transition from a traditional release to a modular one, there is a set of useful scripts in the dependency-report-scripts repository, and the results are being pushed to the dependency-report repository. You are welcome to participate.

Adding your module to the dependency-report

The module definitions are in the modularity-modules organizations, in a format simlar to the Platform and Host definition. The scripts will take these repositories as an input together with the Fedora Everything repository to produce dependency reports and basic modulemd files.

How can I generate a modulemd file?

The dependency-report-scripts can produce a basic modulemd, stored in the dependency-report repository. The modulemd files stored in the dependency-report repository reference their components using the f27 branch for now. To produce a more reproducible and predictable result, I recommend you to use the script inside the dependency-report-scripts repository:


The result will be stored at the same place as the previous modulemd, that was using branches as references.

$ ls output/modules/MODULE_NAME/MODULE_NAME.yaml

Where do I get the mbs-build tool?

Install the module-build-service package from Fedora repositories:

$ sudo dnf install module-build-service

What can I build to help?

See the F27 Content Tracking repository and help with the modules that are listed, or propose new ones. You can also ask on #fedora-modularity IRC channel.

Where can I learn more about Modularity?

Please see the Fedora Modularity documentation website.



Flock 2017 was awesome!

Flock is a Fedora contributors’ conference happening every year, alternating between the US and Europe. And this year it was a more action-oriented than in previous years which means there were more workshops and ‘do-sessions’, and less talks.

I must admit I liked that change because I think that presenting a project online is easier than quick discussions and getting things done online. So using this even with many people together to do stuff as opposed to presentations seems pretty effective.

I was there to represent the Fedora Modularity effort – giving two and a half talks, collecting feedback and ideas, and learning from awesome people.


Day one


Matt Miller talked about the current state of Fedora. There are many changes going on, and it’s a good thing. We don’t want to be completely on fire, but also not super stable because we want to innovate. So the answer is: Just a little bit of fire. Matt had the following chart on his slides showing what our audience

And this chart is what I took from the keynote. Doing innovation means braking stuff. And that’s fine as long as we keep it under control.

Advertise your session

Everyone got a chance to sell their presentations or workshops to others before the conference began. There was a long line of people and everyone was introducing themselves and their presentation or workshop to others. Interesting idea, I liked the concept, there were just so many people I was loosing track a little bit. My proposal for next time: could we group them by topic? What do you think?

User Feedback on Modularity

This was a bit special event, as it was running six times during the conference! Two slots every day, an hour before lunch, and an hour after lunch. This fact alone felt like Modularity is pretty important to Fedora.

And it was also important to the Modularity people. We got a lot of feedback – both nice and useful. Every single one liked the overall concept of Modularity, majority also liked the UX.


One of the things I liked was that every day was a little bit different. Why? Because we had Martin Hatina – a DNF developer – with us. And he was fixing the code every day as people were discovering little bugs!

Factory 2.0, Fedora, and the Future

Mike Bonnet gave an intro into the Factory 2.0 project, giving an overview of all the new and old services that are part of that project and that are being worked on. It was a verygood presentation, also serving as an introduction to other, more detailed talks the team was giving, as he provided a high-level overview of everything that’s going on in Factory, and pointed us to the other talks for more details.

He gave an introduction into the following projects:

  • Module Build Service (MBS) – A service for building modules in Fedora.
  • Arbitrary Branching – Naming branches to follow upstream version of the component instead of a Fedora release.
  • ResultsDB – Test results from the infrastructure on a single place.
  • WaiverDB – Allowing tests to be waived, turning a fail into a pass.
  • Greenwave – Gating for content to move to a new state – like pushing updates automatically.
  • Bodhi – Package (and now also module) updates in Fedora. Gating on Greenwave decisions now deployed!
  • Freshmaker – Automatically triggers builds of content when its source gets updated.
  • ODCS – On-demand Compose Service, generates repos of signed pre-release content to build other content like containers, ostrees, and to run tests.

State of Fedora Server

Stephen Gallagher talked about the history, current state, and the future of Fedora Server Edition.

People could learn that previously, there was just a single Fedora ‘edition’ designed to run on a laptop and on a server. And that was fine then, but later on, both groups started to have different needs – like different defaults on a fresh system. On a laptop, you probably want to have a window manager – like Gnome – running. On a server, you might want a different GUI to manage it remotely, like Cockpit. And that’s why they split the Server Edition and the Workstation Edition apart.

And that is very nice! When you install a fresh Fedora Server, it has Cockpit running by default. That means you can just open your web browser on your laptop, log in to your server, and you’ll be able to see all the logs, CPU and memory consumption, and to perform basic configuration like managing your LVM volumes. Thanks to Cockpit apps you are (or will be in some cases) also able to manage other services like your containers or the FreeIPA server.

While the Server and Workstation split enabled this, there are still needs like having multiple versions available or independent lifecycles of some components. And that’s where Modularity comes in. Modularity enables parallel availability of content – so people will be able to choose different versions of software to install, or even the Server WG will be able to choose different default (even with a different lifecycle) for the Server as opposed to the Workstation. During the discussion, someone mentioned that Modularity is like customizing a laptop. You can choose the color, the CPU – but you can only fit one, etc. And I liked that. It was also nice to see people who are not ‘us’ to understand Modularity well enough to explain it to others.

Proposal: Splitting docs from the software

This was my first talk. I gave a short introduction into the problem of having the software and the documentation in a single source. This also applies to tests. Right now, to build a package, you need to have all the build requires for your software, for the documentation, and for the tests. That grows the build dependencies a lot which is a problem with Modularity where we want to be as flexible as we can – so having many build dependencies lowers the flexibility. That’s a problem especially for the base which hould have as little build dependencies as possible. Texlive or python-sphinx might be a nice example.

After the short introduction, we had a discussion about how we could solve that problem. I proposed two solutions: having a standard macro in SPEC that could switch the documentation off, or separating docs from the software on a source level. There are advantages to both: the first one is easier to maintain, as you have just a single thing. Also, the version of the docs will always match the version of the software. The drawbacks are that you need to rebuild the SRPM with the correct macros to get the right build requires. The second option is easier for build, because you have two separate things.

I was lucky enough to have Jason Tibbitts from the Fedora Packagibg Comittee in the discussion. And we decided to go with the second option – separating docs from the software on a source level for the critical components. However, we won’t force anyone to do that. Instead, we will explicitly mention in the packaging guidelines that this is absolutely fine, and recommended for some components, and then we’ll ask maintainers of the core components to do the change.

Day two

How to write dist-git tests in Fedora with Ansible

Stef Walter believes that lot of work in the future will be done by robots! And that includes testing our packages. Tests for the CI.. I mean.. robots can be described using Ansible.

I learned that you can work in two scenarios:

  • setting up the environment like VMs or containers for the tests to run in
  • setting up the environment like VMs or containers as part of the tests

Learn more:


Jan Kaluza makes automation happen! He presented the project he works on called Freshmaker, that automatically makes fresh things. It is a service that triggers rebuilds of content on a source change. It will make packagers’ life easier by removing many manual steps from their workflow.

The short-term goal for Freshmaker is to:

  • trigger container rebuilds when an RPM source changes
  • trigger module rebuilds when a modulemd changes

Gating on automated tests in Fedora – Greenwave

Matt Jia talked about Greenwave – a service that makes clever decisions about pushing content to another stage based on test results (consumed from ResultsDB and WaiverDB) and policy. This, for example, will enable pushing updates in Bodhi automatically, based on transparent criteria.

Atomic Host 101

OK, I’m lying. I wasn’t really here. But Dusty made an awesome job with the worshop and made it available online with all the instructions you need. And it’s completely offline! Once you download all the bits and pieces, you can play with it anywhere. So I tried it on the way back home. You should try it, too!

The most impressive thing was changing the OS from Fedora to CentOS with a single command, and everything worked fine after a reboot. Go for it:

The Future of fedmsg?

Jeremy Cline explained the current state of fedmsg, the problems they’re having with it, and a possible solution.

Right now, when using fedmsg, you basically need to subscribe everything to everything. And the messages are not guaranteed to be delivered.

The most interesting for me was a discussion about guaranteed vs. not guaranteed messaging. Basically, when you have a guaranteed message bus, it makes things very complicated. The sender needs to know about all the receivers. So you can no longer just consume the messages as you can now with fedmsg. Also, you can’t entirely trust the message broker, so you need to have a code in your app to handle this anyway. It seemed to me that building a reliable messaging bus might not be worth it.. but is it the right answer? I dont’ know.. but it’s pretty interesting problem and got me thinking about that for sure.

Evening Activity @ Wackenhammer’s Clockwork Arcade and Carousel

It was very relaxing, good fun, here’s a picture of me winning a 1000 tickets, and let’s move on!


Day three

The Modularity day! We had four hours in the afternoon. So let’s start.

Modularity – the future, building, and packaging

Join session of Langdon White, Ralph Bean, and me. We got a full room!

Langdon talked about the vision, why are we doing it, and some of the benefits it brings. He talked about the F26 Boltron Server we have built, the F27 Server MVP we are currently working on, about building Atomic as rolling updates separated from the other editions which is now work in progress, and about Workstation using Flatpaks in the future.

One of my favourites was a demonstration how to build container images with multiple versions of things without changing the dockerfile. See Langdon’s slides for the example. He also emphasized how we want to make packagers’ life as easy as possible, mostly by generating many of the things and running tests in the infrastructure.

Ralph gave us update on the Module Build Service (MBS) – a service that builds modules in Fedora. It is for developement and production. MBS runs in the Fedora infrastructure and orchestrates builds in Koji, but the same thing can also run on your laptop and use Mock to do your local builds. That means they share the code-base between the infra and the client tool which is a good thing.

We could also learn how MBS reuses components. There is a concept of build groups people can define in their modulemd. And basically all the builds get reused if their buildroot (which is the previous build group) haven’t changed. It’s very nicely explained in Ralph’s slides – so have a look!

And finally, in my part I talked about packaging. I explained the difference between traditional distribution, where packages have a branch for every release, and releases are built out of these branches; and the modular distro, where we have arbitrary branching, multiple version of content, and we use modulemd to describe the builds and the modules produced out of it.

I also talked about an idea of defining spins and editions using modulemd files. See my slides for more details and a bunch of graphics!

When to go fully modular?

This was a discussion about the future of Modularity. During the session I realized I should have advertised it as a Fedora Modularity WG meeting live. That would have been awesome. So maybe next time.

How many modules will be there?

A module could be described as a group of packages with additional metadata. But how bit is the group? Do we create a ‘text-editors’ module, do we create ‘vim’, ‘nano’, ’emacs’, and others? And should they include all the dependencies, or shoud the dependencies be shared between modules? The result is that we want to do the later, a module per application or service. And that they should share the dependencies.

But how to share the dependnecies? In a huge ‘shared-userspace’ module? Or using many small modules? And how to coordinate all of this so the modules won’t overlap? The outcome wasn’t that clear this time. The consensus was that creating smaller sensible modules as dependencies instead of ‘shared-userspace’ is probably better. But how small do we want to go? Do we want to push it to the extreme and have modules with a single package? And what about the overlaps and coordination?

Flatpaks and containers make this much easier, as they can overlap with each other by design. But installing RPMs directly on a system makes it difficult. We will need a service that will help packagers to coordinate. Something similar to the dependency-report I have prototoyped, but as a service, and focusing on the long-term maintenance over one off migration.

Is there an everything else module?

There are basically two main approaches how to modularize a distro:

  1. Build a huge ‘everything else’ module and separate individual modules out of it as we go. This way, we would have all the content.
  2. Modularize everything – including dependencies – from the start. We wouldn’t have all the content from the beginning, but it would be much cleaner. And we could get rid of some packages that are no longer maintained, but are being automatically rebuilt over and over again.

Someone also had an idea to include the traditional Fedora Everything repo in the modular distro to give users the content. However, it would conflict with the modularized packages, and could also have binary incompatibilities. That was clearly a bad idea. But what about the everything _else_ module?

I think most of the people would go with the option 2, modularize everything. And that’s what we do with the F27 Server.

How could we do Rawhide in Modularity?

That was an interesting question. Also, how do we do releases in the pure modularity world? Will we have a releases as we know them now? Or will individual modules (or module stacks) have their own releases? Would we have a rawhide as we know it today, for the whole distro, or will modules have their own ‘rawhide’ streams? We haven’t agreed on this one.

How do we deal with module EOL?

When every module and every RPM can have independent EOL (end of life), it can turn into a pretty messy situation when something can expire any time. And that’s not good. Matt Miller proposed we have a standardized EOL dates, for example every 6 months. That way, it would be basically the same as we have now, except some RPMs and modules could live longer. And that was the consensus in the end.

Let’s create a module

Tomas Tomecek did a workshop for packagers where they could build their module and see the whole process from the beginning to the end. It’s availabe on github: Modularity Workshop.

Let’s create tests for modules/containers

Petr Hracek and Jan Scotka explained people how to use the Meta Test Family to write tests for modules. It supports writing tests in any framework and running them in any environment. More information here: Meta Test Family.

Day four

What did we do / Demo day

People had a chance to present what they have achieved during the conference. It was interesting to see what other people learned and got done. As a side-effect, it helped me remember some of the things I have been part of myself. Even though there was a bit pressure in the end asking people to come and speak, I think it was just because people were not used to this sort of thin on previous Flocks. I believe that next year the pressure will disappear as more people will be getting ready for this during the week. I still think it was useful and I liked it.


It was awesome. I believe that just meeting people face to face is so important, as it makes the collaboration for the upcoming year so much easier when people know each other. Besides the sessions, I had many hallway (and pub) discussions. I liked seing other people to understand Modularity well enough they could explain it to others. I liked getting all the feedback and great ideas – especially the little ones, that were part of the hallway discussions. I liked the positive perception of Modularity on the UX demos. I learned a bunch of new stuff, met some new people I haven’t known in person before.

Thank you for having me there, it was very useful and pleasant experience!