I think this is how I most commonly recognize someone that is familiar with lxc +/- docker and containerization in general. A firm grasp of the fact that they're just beefy processes, isolated wiht namespaces and cgroups is the best, most succint way to describe docker (without even mentioning the benefits), but it also requires that the hearer knows what namespaces and cgroups are.

If I had to rank understanding in explanations of docker:

1. "Makes your application really portable"/other explanations that only cover the benefits of docker not how it works

2. "Lightweight VMs"

3. "LXC + some other stuff"

4. "processes + namespaces + cgroups (+/- image management tools, etc)"

100% agreed on the point you made, pretty sure docker+swarm and other orchestration efforts have basically lost the competition. Kubernetes just has the mind share, and even better than that -- it's actually good.

Kubernetes also has multiple competing container runtimes all rushing to fit the CRI (Container Runtime Interface). Just some of the stuff out there:

* cri-containerd - https://github.com/containerd/cri-containerd

* runv (hypervisor based) - https://github.com/hyperhq/runv

* clear containers (hypervisor based) - https://github.com/clearcontainers/runtime

* kata containers (hypervisor based, collaboration of runv + clear container) - https://katacontainers.io/

* frakti (combination of runv & docker, enabling switching @ runtime) - https://github.com/kubernetes/frakti

A bunch of the projects are in their infancy, but the CNCF/kubernetes and the community are doing the right thing investing in lots of them, and letting the good ones bubble to the top. I don't run a production kubernetes cluster but I've found cri-containerd to be really easy to use and haven't had any major problems with it, mostly small configuration things.

The last one, frakti, gets me really excited, because it lets you be flexible about which containers run in more protected environments and which don't. Also really exciting is frakti v2 (https://github.com/kubernetes/frakti/tree/containerd-kata), which is kata + containerd.

One important thing for me is that Docker also handles other operating systems than Linux through their respective hypervisors.

Docker might well die but first I will have to be able to locally build and test containers on Windows and macOS, before deploying them to the Kubernetes cluster–without fooling around with Virtualbox, Vagrant and the like.

Well they have multi-platform. Windows containers are intrinsically tied up with Docker (the current Windows containers docs start with a Docker EE Basic install).

My personal expectation is that Docker will be bought by Microsoft, probably this year or next ...

>Basically, Docker has almost no moat. They have mostly only brand recognition. Not discounting their efforts, but they are playing checkers and Google is playing chess.

Well how do you build images without docker?

That might be true of Docker. Kubernetes on the other hand is a way to deploy and manage distributed applications, which are by definition bigger than any single OS.

That already exists and has existed for several years now: SmartOS. imgadm(1M) and vmadm(1M) are core parts of the OS and do what Docker does, and more. Built not as an experiment, but to power a large scale commercial cloud business. And freeware / open source since before the project went live by virtue of OpenSolaris (now illumos).

Maybe even further. Mainframes have had similar for decades. Yes, perhaps more like actual VM's, but the namespacing, cgroup stuff, network overlays, etc, are very similar.

My mainframe folks, when talking about docker, universally yawn.

cgroup namespaces are a little younger and came arrived in 4.6 I think http://man7.org/linux/man-pages/man7/cgroup_namespaces.7.htm...

Could be as simple as the init() process. Have you tried a slimmed down guest distro like Alpine and compared startup times?

Mostly it's Docker-specific, but all of the things causing this latency tend to be what makes Docker useful in the first place. In particular, networking and mounts.

For starters, if you're just running the typical Docker set up to talk back and forth using a UNIX or TCP socket, all of _that_ networking and HTTP/JSON encoding+decoding will add some overhead. This is even worse if you're going through a proxy layer like the socket which Docker for Mac creates and forwards to the Linux VM. OK, so there's at least a few milliseconds of latency built in for this, the socket forwarding hop on my Docker for Mac seems to add ~5-15ms for instance.

Once the daemon receives the request, runc, the command that actually does the work to create a container without all that hoopla, is invoked.

Then, to actually start up a container quite a few mounts get set up either for devices (potentially including tty in the case of `run -t`) or for the union filesystem. Take a look at the output of `mount` in a container sometime, around 30-40 mounts (and all those overlay layers...) and a lot of that gets set up on the fly because each container has its own unique mount namespace and view of the world.

Then after all of that, you need to set up networking too (`docker run --net none` will skip this), otherwise containers can't talk to the Internet, to each other, or listen on ports of the host. Remember, each has its own network namespace, created from scratch. So, Docker's doing all manner of adding port proxies, modifying iptables, attaching containers to interfaces, and so on. Otherwise, `docker run alpine wget -qO- https://google.com` or `docker run --net=internalnet` wouldn't work out of the box.

The work isn't easily done concurrently since there are so many dependencies - e.g., you need to have a process in order to set a network namespace, in order to run that process, you need to have a root FS prepared to pivot root into, and so on.

Meanwhile to all of this happening the Docker daemon is doing god knows what, it could be pulling images, or running other containers already, supervising processes, etc. While many of those things are concurrent due to use of goroutines, they do eat up resources and generally too much work saturates and slows down the daemon. That's not really unique to Docker though, any program doing all this stuff at once would have that issue.

Anyway, that's why Docker takes way longer to start a process than good old execve() in your shell.

Pretty much. Usually you run your own init that starts the desired process. If you as a process have id (pid) 1 then you need to manage orphaned processes (and zombies) and general signal handling (e.g. SIGTERM).

There is a lot of cgroups and capabilities tweaking involved also. You can also use bpf (e.g. seccomp-bpf to restrict system calls etc.

In general you i) add the illusion of king in your little own world ii) provide necessary handling that the kernel expects from a king iii) restrict resources (e.g. memory, system calls,...) and facilitate communication with the rest of the worlds.