It isn't tied to systemd. The stage1 that is in the current prototype uses systemd to monitor and fork processes but we would love to see other stage1's that configure other process runners. For example configure and run a qemu-kvm filesystem as the container.
Also, even though it is using systemd to monitor and fork processes a design goal is to run on all Linux's that have a modern Kernel.
What about non-Linux platforms (FreeBSD, Mac OS X with a kext)?
One thing that I believe Docker has failed at is in taking a purely declarative approach to image definition; rather than specifying the packages that are assembled/inserted to create the container, Docker ships around non-portable Linux binaries.
I second that. At the begining Docker people were mentioning adding FreeBSD Jails support, what seemed to me an awesome thing, a platform independent containerization middleware, but recently they just seemt to forget about it and they're doing only linux-centric things - what a shame.
Yes, but the Docker Remote API allows for a great deal of implementation freedom -- including running on a different OS substrate. We're doing this with sdc-docker[1] to run Docker on top of SmartOS and in a SmartOS container, and the Docker folks have been incredibly supportive. Despite the rhetoric, Rocket appears to be much more bound to the OS platform than Docker -- and given @philips' comment that "part of the design difference is that rocket doesn't implement an API"[2], this binding appears to be deliberate.
It depends on how you look at it. The Docker Remote API provides an abstraction on the OS substrate and definitely binds you a lot more to Docker and their model. The "rocket doesn't implement an API" means all that it isn't really doing much more than kicking off the container and using the existing OS substrate to manage everything else.
I can see where for SmartOS & Windows the Docker approach is more flexible. If someone has already settled on Linux, but they have their own ideas about how to manage containers within Linux that have nothing to do with CoreOS, the Rocket model is going to leave them much more flexibility.
The app container specification has socket activation in it. This is going to essentially tie it to systemd. Otherwise you will need another daemon running to do the socket activation, but then that would seem to be a "fundamentally flawed" execution model.
Socket activation isn't mandated and completely optional. If an application detects it didn't get its sockets it can certainly just start listening instead if it wishes.
You may be shocked to discover that socket activation is actually a pretty old idea, and can even be found on non-Linux platforms. systemd embraces it in a pretty big way, but I can't see a problem getting the app containers to work with one of the other socket activation models.
And you may be surprised that I ran socket activation well over 15 years ago, so yes I'm well aware of the approach. The comment is more around the fact that in CoreOS's post they seems to harp on the security of a daemon process running as root that is responsible for spawning containers. What I'm saying is that with socket activation you will essentially have that again. Rocket can only work around it today because they have systemd as PID 1 running as root doing the socket activation.
Socket activation does not need systemd in any way. You can launch any socket-activated daemon with https://github.com/LEW21/sdlaunch - without any dependence on systemd.
Also, even though it is using systemd to monitor and fork processes a design goal is to run on all Linux's that have a modern Kernel.