WirePlumber: the new PipeWire session manager

Fedora Workstation 35 introduced a new session manager to PipeWire called WirePlumber. So what does a PipeWire session manager do and what makes WirePlumber special? To answer that question we talk to George Kiagiadakis. George is the main author of WirePlumber and can tell us what exciting things will be possible going forward with WirePlumber in place as the new official PipeWire session manager.

Christian Schaller: Hi George, thank you for taking the time to talk to us. Could you maybe start by telling us a little about yourself and how you got started with open source?

George Kiagiadakis: Hello Christian, thank you for having me in this interview. What can I say about myself… I am a senior software engineer, working at Collabora on multimedia projects such as PipeWire and GStreamer for 11 years now.

I got started in open source when I was back in high school, around 2003, when I got my hands on a DVD that contained an iso image of Mandrake 7.2. I was very curious to understand what that was, so I followed instructions and installed it without really understanding what I was doing. I rendered my Windows installation unbootable, obviously, so I forced myself to use Mandrake to connect to the internet and find instructions on how to bring Windows back… and I really liked it! Eventually I read more about this unfamiliar environment and the moment I realized that I could modify anything in this system… literally anything…I was thrilled! Unbelievable feeling. I have never looked back ever since….

Christian Schaller: When did you first hear about PipeWire and what is your impression of the project and the community around it so far?

George Kiagiadakis: I don’t fully remember when I first heard about PipeWire, but I remember that it was around summer of 2018 that I started looking into it…. At the time I was working on a project that involved PulseAudio with some cumbersome logic around it and I was not convinced at all that this was the right way to move forward in the future. So, I started looking at alternatives and I saw a lot of potential in PipeWire, which was at its early stages in the audio field back then. The next thing that I did was to book myself to attend the first PipeWire hackfest that we had that fall, where Wim explained to us how it all worked and it made perfect sense.

Regarding the project and its community, the first thing I can say is that I am glad to be part of it! It’s a relatively small community, but very active and passionate. Everyone is polite, patient and willing to help. I am also amazed by how fast things get done, especially when Wim is involved. Before we even finish discussing an issue, he most likely has fixed it….

Christian Schaller: And what was the origin of WirePlumber? What were the initial problems you felt needed to be solved?

George Kiagiadakis: WirePlumber was initially developed for use in the Automotive Grade Linux (AGL) project, where the problem that needed solving was mostly the problem of arbitrating between different audio streams. There is, for example, an application playing some music in your car and at the same time there is a GPS navigation application trying to assist you with directions. When the navigation app needs to speak to you, it needs to be clearly audible, so the music has to go in the background either by lowering its volume or by pausing it momentarily. Or when you receive a phone call, the music has to pause for sure. These are scenarios which are typical in many embedded devices. Our smartphones have similar logic, for instance.

Before PipeWire, solving that problem with PulseAudio was hard. It required developing custom components to define all the rules and logic and circumventing a lot of existing behavior. And it was so bad that people have even tried to work around it and use similar components for the rules, but on ALSA directly, while others have tried to use JACK, which is actually better because it allows an external application – the so-called session manager – to define all these rules without circumventing anything, but there are several limitations. There are actual production systems in automotive and elsewhere that use all these aforementioned solutions, but none is ideal.

With PipeWire, this is so much easier because, just like JACK, all this logic can be defined in this external application, the session manager. But PipeWire also lifts all the limitations that one would have with JACK, like the compatibility with PulseAudio applications, the fixed latency that can be an issue in some cases or JACK’s inability to use different audio formats or passthrough encoded audio to the hardware.

And so, after evaluating all these solutions at the time, it seemed like a great idea to use PipeWire. The only thing that was missing was a session manager that would be capable of implementing all this logic. And this is when the idea for WirePlumber was born.

Christian Schaller:  Do you have some good examples of things WirePlumber can do for people on the desktop that were not possible or hard to do before?

George Kiagiadakis: First of all, let me say that for bringing WirePlumber to the desktop we tried to make it behave exactly like pipewire-media-session, which in turn behaves like PulseAudio, so that the transition from one to the other does not change the experience that people are used to on their desktops. So, the very first thing that you will realize after switching to WirePlumber is that… nothing is different. But… behind the scenes, there is a difference.

The most interesting difference is that most of WirePlumber’s management logic is written in scripts, using the Lua language. These scripts can be modified and extended on the spot, much more easily than it would have been to modify the C code of pipewire-media-session. That means that the users now have the power to customize the behaviour of their PipeWire setup to suit better their needs. It is even possible to use the automotive / mobile logic on a desktop, if so desired. That does not mean, though, that modifying these scripts is a piece of cake or that everything works like a charm. There are still rough edges and a lot of things that can be improved to make this a great feature. I hope that with WirePlumber now being part of Fedora 35, we will be able to get more feedback on this feature and evolve it.

It is also worth mentioning that WirePlumber also has the ability to execute Lua scripts independently, outside the context of the session manager, and in addition it comes with a GObject-based library that offers a relatively easy API to control things in PipeWire. That allows power users to more easily script tasks that they often do in PipeWire and application developers to have their applications interact with PipeWire more easily.

And of course, the library is fully introspectable, which means that it is also possible to write scripts or applications in languages like Python or Javascript, utilizing WirePlumber’s library through GObject-introspection.

Christian Schaller: That sounds really cool, but to be clear are you saying that one can do applications like Ardour or Carla using these Python bindings? Or are you talking about applications that control the session more specifically?

George Kiagiadakis: No, this library is not meant for writing media applications like Ardour or Carla. It only provides functionality to control the session, so it is suitable to write things like a pavucontrol replacement or perhaps a script to automate tasks like switching some card profile, for example, but nothing that involves actual media streaming. We have other APIs to do that.

Christian Schaller: Any major future features planned for WirePlumber or would you say that it is fairly feature complete at this point?

George Kiagiadakis: There are no major things planned in the very near future, but that definitely doesn’t mean that WirePlumber is feature-complete. There are many things that I would like to improve on it and there are also several ideas for new features.

Something that bothers me right now is the configuration system and this is likely the first big thing that will change. Ideally this is something that should be unified with PipeWire, so that the entire stack can be configured similarly and it is also worth exploring potential integration of certain options into the settings dialogs of the various desktop environments.

Another area of interest is to explore bringing more of the policy that is used on embedded systems – the one I mentioned in the example with the car – onto the desktop. PulseAudio was already trying to do something like that, but I think that WirePlumber can achieve more because, with the scripting mechanisms that are provided, it is easier for desktop environments to implement their own customizations and behaviours that are tailored to the UX of the environment.

There are also several ideas for improving the experience of JACK application users when they use PipeWire as their JACK server. The typical case right now is that you need a JACK session manager to be running alongside WirePlumber in order to manage the JACK applications, while WirePlumber only deals with PipeWire and PulseAudio applications. I think we can do better, at least for some use cases.

Finally, I am certain that there will be more interesting ideas and features popping up as PipeWire evolves, especially into the video domain.

Christian Schaller: Any non-WirePlumber features you would love to work on in PipeWire? Like something you personally miss or would love to see improved?

George Kiagiadakis: There isn’t much to miss, to be honest, thanks to everyone’s huge effort to bring PipeWire on par with PulseAudio, feature-wise, especially the early adopters who have provided so much valuable feedback. But still, I do have a few things in mind that I would love to work on.

The first one is the network streaming support. I have used this feature countless times with PulseAudio and there was always some sort of annoyance, especially on wi-fi connections, since the underlying protocol used is not really meant for unreliable connections. PipeWire currently falls into the same mistakes, trying to be compatible, but I think it can do better.

Then there is also a pet feature that I would like to continue working on, which is the support for stream fade-in and fade-out. My colleague Julian and I had worked on this last year and we even had working cross-fade between applications, controlled by WirePlumber, but it was only partially merged and then set aside by other priorities.

Last but not least, I look forward to new video-related features, starting from improved integration with libcamera, to enable modern cameras to be first class citizens on the linux desktop. These are totally uncharted waters that I would love to dip my toes into….