Nitro: A tiny but flexible init system and process supervisor
(git.vuxu.org)180 points by todsacerdoti 13 hours ago
180 points by todsacerdoti 13 hours ago
I would like a comparison with runit, which is a very minimal but almost full-fledged init system. I see many similarities: control directories, no declarative dependencies, a similar set of scripts, the same approach to logging. The page mentions runit in passing, and even suggests using the chpst utility from it.
One contrasting feature is parametrized services: several similar processes (like agetty) can be controlled by one service directory; I find it neat.
Another difference is the ability to initiate reboot or shutdown as an action of the same binary (nitroctl).
Also, it's a single binary; runit has several.
Last year I decommed our last couple of servers that ran processes configured using runit. It was a sad day. I first learned to write runit services probably about 15 years ago and it was very cool and very understandable and I kind of just thought that's how services worked on linux.
Then I left Linux for about 5 years and, by the time I got back, Systemd had taken over. I heard a few bad things about it, but eventually learned to recognise that so many of those arguments were in such bad faith that I don't even know what the real ones are any more. Currently I run a couple of services on Pi Zeros streaming camera and temperature data from the vivarium of our bearded dragon, and it was so very easy to set them up using systemd. And I could use it to run emacsd on my main OpenSuse desktop. And a google-drive Fuse solution on my work laptop. "having something standard is good, actually", I guess.
The backlash against systemd was twofold. On one hand, when released and thrust upon distros via Gnome, it was quite rough around the edges, which caused both real problems and just understandable irritation. Fifteen years after, the kinks are ironed out, but it was sort of a long time. (Btrfs, released at about the same time, took even longer to stop being imprudent to use in production.)
On the other hand, systemd replaces Unix (sort of like Hurd, but differently). It grabs system init, logging, authentication, DNS, session management, cron, daemon monitoring, socket activation, running containers, etc. In an ideal Red Hat world, I suppose, a bare-metal box should contain a kernel, systemd, podman, IP tools, and maybe sshd and busybox. This is a very anti-Unix, mainframe-like approach, but for a big consulting firm, like Red Hat / IBM, it is very attractive.
All this lather about doing it the UNIX way, whilst neglecting to point out that the old tooling was far worse. "Do one thing well" implies it was done well to begin with.
DNS: Can you from memory recite how name lookups work on Linux? Ever had t otrack down problems with non-standard setups? `resolvectl` is not perfect, but it does let you control all of this stuff in one place, and with a nice, orderly view of what does what.
Init system: ever written the old sysV ones from scratch? Sure they're just shell script, but did you remember to make yours re-entrant? What about forking or master-slave processes? Hope you got your pid checking code just right...
Containers: Docker is quite robust nowadays but it's not like it follows the 'UNIX philosophy' either. And systemd/nspawn at least lets you do namespacing and cgroups reasonably well and in a straightforward way...
Mounts, etc.: let's not get into fstab and its idiosyncrasies.
Logging: let's hope you set up logrotate properly or you're in for a surprise.
And on it goes.
Systemd is not perfect. But what it replaces was god-awful and far worse.
Is following the "Unix way" a terminal value? I.e., is it desirable for itself, or is it just supposed to be a means to an end?
In discussions such as these, the Unix philosophy of "do one thing and do it well" is often being touted as a proxy for (and a necessary attribute of) "good design", as if all possible wisdom about the future of computing was available to the creators of UNIX in 1969.
> no declarative dependencies,
Is that a selling point? Could you explain why?
I've heard plenty of reasons why people find systemd distasteful as an init, but I've not heard much criticism of a declarative design.
I've gotten used to runit via Void Linux, and while it does the job of an init system, its UI and documentation leave something to be desired. The way logging is configured in particular was an exercise in frustration the last time I tried to set it up for a service.
I wouldn't mind trying something else that is as simple, but has sane defaults, better documentation, and a more intuitive UI.
I like using systemd but it also doesn't have great documentation either. I often find myself unable to grok things by only reading the official documentation and I have to resort to reading forum posts, other people's blogposts or Stack Overflow. To me documentation isn't good enough until it doesn't need any third party material.
This is by design.
runsvdir receiving TERM should only happen when stage 2 is triggered to end.
Once that happens, the individual runsv processes are still supervising their individual tasks and can be requested to stop through their respective control sockets. It's how standard stage 3 is implemented.
I'm always torn when I see anything mentioning running an init system in a container. On one hand, I guess it's good that it's designed with that use case in mind. Mainly, though, I've just seen too many overly complicated things attempted (on greenfield even) inside a single container when they should have instead been designed for kubernetes/cloud/whatever-they-run-on directly and more properly decoupled.
It's probably just one of those "people are going to do it anyway" things. But I'm not sure if it's better to "do it better" and risk spreading the problem, or leave people with older solutions that fail harder.
There's nothing inherently wrong with containers in the abstract: virtualization is a critical tool in computer science (some might it's difficult to define computer science without a virtual machine). There's not even anything wrong with this "less than a new kernel, more than a new libc" neighborhood.
The broken, ugly, malignant thing is this one godawful implementation Docker and its attic-dwelling Quasimodo cousin docker-compose.yml
It's trivial to slot namespaces (or jails if you also like the finer things BSD) into a sane init system, process id regime, network interface regime: its an exercise in choosing good defaults for all the unshare-adjacent parameters.
But a whole generation of SWEs memorized docker jank instead of Unix, and so now people are emotionally invested in it. You run compose to run docker to get Alpine and a node built on musl.
You can just link node to musl. And if you want a chroot or a new tuntap scope? man unshare.
Mostly just zombies and signal handlers.
And your software can do it, if it's written with the assumption that it will be pid1, but most non-init software isn't. And rather than write your software to do so, it's easier to just reach for something like tini that does it already with very little overhead.
I'd recommend reading the tini readme[0] and its linked discussion for full detail.
The main other problem is that the kernel doesn't register default signal handlers for signals like SIGTERM if the process is PID 1. So if your process doesn't register its own signal handlers, it's hard to kill (you have to use SIGKILL). I'm sure anyone who has used Docker a lot has run into containers that seem to just ignore signals -- this is the usual reason why.
> also, why can't the real pid1 do it? it sees all the processes after all.
How would the real PID 1 know if it _should_ reap the zombie? It's normal to have some zombie processes -- they're just processes whose exit statuses haven't been reaped yet. If you force-reaped a zombie you could break a program that just hasn't yet gotten around to checking the status of a subprocess it spawned.
From my experience in the robotics space, a lot of containers start life as "this used to be a bare metal thing and then we moved it into a container", and with a lot of unstructured RPC going on between processes, there's little benefit in breaking up the processes into separate containers.
Supervisor, runit, systemd, even a tmux session are all popular options for how to run a bunch of stuff in a monolithic "app" container.
My experience in the robotics space is that containers are a way to not know how to put a system together properly. It's the quick equivalent of "I install it on my Ubuntu, then I clone my whole system into a .iso and I call that a distribution". Most of the time distributed without any consideration for the open source licences being part of it.
I've always advocated against containers as a means of deploying software to robots simply because to my mind it doesn't make sense— robots are full of bare-metal concerns, whether it's udev rules, device drivers, network config, special kernel or bootloader setup, never mind managing the container runtime itself including startup, updating, credentials, and all the rest of it. It's always felt to me like by the time you put in place mechanisms to handle all that crap outside the container, you might as well just be building a custom bare metal image and shipping that— have A/B partitions so you copy an update from the network to the other partition, use grub chainloading, wipe hands on pants.
The concern regarding license-adherence is orthogonal to all that but certainly valid. I think with the ROS ecosystem in particular there is a lot of "lol everything is BSD/Apache2 so we don't even have to think about it", without understanding that these licenses still have an attribution requirement.
For workstations with GPUs and various kernel modules, rpm-ostree + GRUB + Native Containers for the rootfs and /usr and flatpaks etc on a different partition works well enough.
ostree+grub could be much better at handling failover like switches and rovers that then need disk space for at least two separate A/B flash slots and badblocks and a separate /root quota. ("support configuring host to retain more than two deployments" https://github.com/coreos/rpm-ostree/issues/577#issuecomment... )
Theoretically there's a disk space advantage to container layers.
Native Containers are bare-metal host images as OCI Images which can be stored in OCI Container Registries (or Artifact registries because packages too). GitHub, GitLab, Gitea, GCP, and AWS all host OCI Container/Artifact Registries.
From https://news.ycombinator.com/item?id=44401634 re bootc-image-builder and Native Containers and ublue-os/image-template, ublue-os/akmods, ublue-os/toolboxes w/ "quadlets and systemd" (and tini is already built-in to Docker and Podman) though ublue/bazzite has too many patches for a robot:
> ostree native containers are bootable host images that can also be built and signed with a SLSA provenance attestation; https://coreos.github.io/rpm-ostree/container/
SBOM tools can scan hosts, VMs, and containers to identify software versions and licenses for citation and attribution. (CC-BY-SA requires Attribution if the derivative work is distributed. AGPL applies to hosted but not necessarily distributed derivative works. There's choosealicense.com , which has a table of open source license requirements in an Appendix: https://choosealicense.com/appendix/ )
BibTeX doesn't support schema.org/SoftwareApplication or subproperties of schema:identifier for e.g. the DOI URN of the primary schema.org/ScholarlyArticle and it's :funder(s).
...
ROS on devices, ROS in development and simulation environments;
Conda-forge and RoboStack host ROS Robot Operating System as conda packages.
RoboStack/ros-noetic is ROS as conda packages: https://github.com/RoboStack/ros-noetic
gz-sim is the new version of gazebosim, a simulator for ROS development: https://github.com/conda-forge/gz-sim-feedstock
From https://news.ycombinator.com/item?id=44372666 :
> mujoco_menagerie has Mujoco MJCF XML models of various robots.
Mujoco ROS-compatibility: https://github.com/google-deepmind/mujoco/discussions/990
Moveit2: https://github.com/moveit/moveit2 :
> Combine Gazebo, ROS Control, and MoveIt for a powerful robotics development platform.
RoboStack has moveit2 as conda packages with clearly-indicated patches for Lin/Mac/Win: ros-noetic-moveit-ros-visualization.patch: https://github.com/RoboStack/ros-noetic/blob/main/patch/ros-...
...
Devcontainer.json has been helpful for switching between projects lately.
devcontainer.json can reference a local container/image:name or a path to a ../Dockerfile. I personally prefer to build a named image with a Makefile, though vscode Remote Containers (devcontainers extension) can build from a Dockerfile and, if the devcontainer build succeeds, start code-server in the devcontainer and restart vscode as a client of the code-server running in the container so that all of the tools for developing the software can be reproducibly installed in a container isolated from the host system.
It looks like it's bootc or bootc-image-builder for building native container images?
bootc-image-builder: https://github.com/osbuild/bootc-image-builder
> Supervisor, runit, systemd, even a tmux session are all popular options for how to run a bunch of stuff in a monolithic "app" container.
Did docker+systemd get fixed at some point? I would be surprised to hear that it was popular given the hoops you had to jump through last time I looked at it
It's only really fixed in podman, with the special `--systemd=always` flag. Docker afaik still requires manually disabling certain services that will conflict with the host and then running the whole thing as privileged— basically, a mess.
Not my favoured approach, but for early stage systems where proper off-board observability/alerting is not yet in place, tmux can function as a kind of ssh-accessible dashboard displaying the stdout of key running processes, and also allowing some measure of inline recovery— like if a process has crashed, you can up-arrow and relaunch it in the same environment it crashed out of.
Obviously not an approach that scales, but I think it can also work decently well as a dev environment, where you want to run "stock" for most of the components in the system, and just be syncing in an updated workspace and restarting the one bit being actively developed on. Being able to do this without having to reason about a whole tree of interlinked startup units or whatever does lower the barrier to entry somewhat.
One advantage is that if the process has some sort of console on it's stdin, you can do admin work easily. With init systems you now have to configure named pipes, worry about them blocking, have output in separate place, etc.
If I my plug my friend and colleague's work, https://nixos.org/manual/nixos/unstable/#modular-services has just landed in Nixpkgs.
This will be a game changer for porting to NixOS to new init systems, and even new kernels.
So, it's good time to be experimenting with things like Nitro here!
It will be interesting to compare this to dinit[1], which is used by chimera-linux.
Giving the readme a brief scan, it doesn't look like it currently handles service dependencies?
Nitro does not declaratively handle service dependencies, you cannot get a neat graph of them in one command.
You can still request other services to start in your setup script, and expect nitro to wait and retry starting your service when the dependent service is running. To get a nice graph, you can write a simple script using grep. OTOH it's easy to forget to require the shutdown of the dependent services when your service goes down, and there's no way to discover it using a nitro utility.
I used dinit in Artix Linux. It is lightweight and impressive (https://artixlinux.org/faq.php)
The name & function overlap with AWS Nitro is severe:
https://docs.aws.amazon.com/whitepapers/latest/security-desi...
S6 is way more complex and rich. Nitro or runit would be simpler alternatives; maybe even https://github.com/krallin/tini.
Thanks! Reading some of your other comments, it seems like runit or nitro may not have been a good choice for my usecase? (I'm using dependencies between services so there is a specific order enforced & also logging for 3 different services as well).
You seem to know quite a bit about init systems - for containers in particular do you have some heuristics on which init system would work best for specific usecases?
At Distrust, we wrote a dead simple init system in rust that is used by a few clients in production with security critical enclave use cases.
<500 lines and uses only the rust standard library to make auditing easy.
Yeah we only recently broke it out as a standalone repo/binary, as everyone historically vendored it, so docs will get love soon, but it will be part of the next stagex release built and signed by multiple parties deterministically as stagex/user-nit.
To run it all your need to know is put it in your filesystem as "/init" and then add this to your kernel command line for the binary you want nit to pivot to after bringing the system up:
nit.target=/path/to/binary
That's it. Minimum viable init for single application appliance/embedded linux use cases.
nit and your target binary are the only things you actually need to have in your CPIO root filesystem. Can be empty otherwise.
love to see new init projects. how does it stack up against runit (the last one i really familiarized myself with on void linux)?
Bring back the init wars! /S
Username relevant...
I got into Linux right before the init wars, and while they were hectic times they brought a lot of attention, discussions, and options to Linux.
I'd recommend changing names, nitro is already a semi-popular server engine for node.js https://nitro.build/
Any well-known generic word is very likely to already have been used by a bunch of projects, some of them already prominent. By now, the best project name is a pronounceable but unique string, for ease of search engine use. Ironically, "systemd" is a good name in this regard, as are "runit" or even "s6".
> Any well-known generic word is very likely to already have been used by a bunch of projects,
Are you sure? There are lots of words, and not so many projects that use words like these as their names.
Of the 118179 packages I see on this Ubuntu 18.04 system I can roughly roughly ask how many have names that are dictionary (wamerican) words:
comm -12 <(apt-cache dumpavail | awk -F': ' '/^Package:/{sub(/^lib/,"",$2); print $2}') /usr/share/dict/words | wc -l
An init system without the ability to specify dependencies? Without user/group configuration? Ordering must be manually configured? No parallel service launching? No resource management?
Please don't call this an init systern. It's a barebones process supervisor.