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How Container Filesystem Works: Building a Docker-Like Container from Scratch
131 points by lgunsch 3 days ago | 24 comments
  • aussieguy1234
    Layered file systems (multiple filesystems mounted on the same mount point) used to be used for making CD's and DVD's "writeable".
  • zoobab
    We had chroot since 1979, nobody managed to build a docker like wrapper for chroot which do not require netns?
    • vbezhenar
      Docker is a genius idea which looks obvious in retrospect, but someone need to invent it.

      Docker is more than just chroot. You also need: overlay file system; OCI registry and community behind it, to create thousands of useful images. And, of course, the whole idea of creating images layer by layer and using immutable images to spawn mutable containers.

      I don't actually think that you need network or process isolation. In terms of isolation, chroot is enough for most practical needs. Network and process isolations are nice to have, but they are not essential.

      • harrall
        I was a very early adopter of Docker and what sold me was Dockerfiles.

        A SINGLE regular text file that took regular shell commands and could build the same deployment from scratch every time and then be cleaned up in one command.

        This was UNHEARD of. Every other solution required learning new languages, defining “modules,” creating sets of scripts, or doing a lot of extra things. None of that was steezy.

        I was so sold on Dockerfiles that I figured that even if the Docker project died, my Dockerfiles would continue to live because other people would try copy the idea of Dockerfiles. Now it’s been 10 years and Docker and containerization has changed a lot but what hasn’t? Dockerfiles. My 10 year Dockerfiles are still valid. That’s how good they were.

      • akdev1l
        network isolation is very important too, that’s what lets people run 4 containers all listening on port 80

        process isolation is less prominent

        • vbezhenar
          You can bind your application to 127.0.0.2 for one container and to 127.0.0.3 for another container. Both can listen on port 80 and both can communicate with each other. And you can run another container, binding to 1.2.3.4:80 and using it as reverse-router. You can use iptables/nftables to prevent undesired connections and manually (or with some scripting) crafted /etc/hosts for named hosts to point to those loopback addresses. Or just DNS server. It's all doable.

          The only thing that you need is the ability to configure a target application to choose address to bind to. But any sane application have that configuration knob.

          Of course things are much easier with network namespaces, but you can go pretty far with host network (and I'd say it might be easier to understand and manage).

        • mikepurvis
          Process isolation is more about load management/balancing, which is more of a production concern than a development one.
      • tguvot
        i tried to build at work something like docker around 2003-2004. was trying to solve problem of distribution/updates/rollblacks of software on network appliances that we made. overlay filesystems back then were immature/buggy so it went nowhere. loopback mounted system was not sufficient (don't remember why)
      • lyu07282
        What I always wondered is why qcow2 + qemu never gave rise to a similar system, they support snapshots/backing-files so it should be possible to implement a system similar to docker? Instead what we got is just this terrible libvirt.
    • ronsor
      Chroot has significantly less isolation than Linux namespaces as used by Docker.
      • miladyincontrol
        This, better yet just use systemd-nspawn. Benefits of proper containers, configuration similar to any ol systemd service, super easy to use, simple to automate builds with mkosi.

        The one thing people really seem to miss on them is like, contrary to popular belief you dont need a whole OS container there, minimal distroless containers work just fine with systemd-nspawn similar to as they would on docker.

    • spullara
      Solaris Zones (follow on to Solaris Containers) was pretty amazing.

      https://en.wikipedia.org/wiki/Solaris_Containers

      • dboreham
        Quick note that all these things are pre-dated (by decades) by mainframe virtualization schemes such as MVS.
    • interroboink
      FreeBSD has had jails since version 4 (~year 2000), fwiw.

      Much of the technology was there, but Docker was able to achieve a critical mass, with streamlined workflows. Perhaps as much a social phenomenon as a technical one?

      • Yeroc
        I think the real genius of Docker was the image packaging. The pieces were there but delivery and scripting it all wasn't easy.
        • mikepurvis
          Indeed. Even to this day, debootstrap feels a bit arcane and unapproachable, particularly relative to `docker pull ubuntu`.
        • disagr
          [dead]
      • oftenwrong
        Don't discount the technical innovation required to integrate existing technologies in a novel and useful way. Docker was an "off the shelf" experience unlike any other solution at the time. You could `docker run ...` and have the entire container environment delivered incrementally on demand with almost no setup required. It did have a social factor in that it was easy for people to publish their own images and share them. Docker Hub was provided as a completely free distribution service. The way they made distribution effortless was no doubt a major factor in why it took off.

        https://www.youtube.com/watch?v=wW9CAH9nSLs

      • magicalhippo
        I used FreeBSD on my firewall in the early 2000s, and on my NAS from around 2007 till last year.

        The big pain with jails for me was the tooling. There was a number of non-trivial steps needed to get a jail that could host a networked service, with a lot that could go wrong along the way.

        Sure a proper sysadmin would learn and internalize these steps, but as someone who just used it now and again it was a pain.

        Way down the line things like iocage came along, but it was fragile and not reliable when I tried it, leading to jails in weird states and such.

        So I gave up and moved to Linux so I could use Docker.

        Super easy to spin up a new service, and fairly self-documenting as you just configure everything in a script or compose file so much less to remember.

        Initially in a VM on Bhyve, now on bare metal.

        It feels a bit sad though, as jails had some nice capabilities due to the extra isolation.

      • tkcranny
        Yeah it really was a social phenomena. Ten years ago conferences were swarmed with docker employees, swag, plenty of talks and excitement.

        The effort to introduce the concepts to the mainstream can’t be understated. It seems mundane now but it took a lot of grassroots effort and marketing to hit that critical mass.

      • jayd16
        There was clear incremental progress. Some of it can be seen in how mobile app isolation shook out as well.
    • aussieguy1234
      Bocker, docker in 100 lines of bash https://github.com/p8952/bocker