alt Hacker NewsIt doesn't work this way really?
It's called a layer because each layer on top depends on the layers below.
If you change the package defined in the bottom most layer, all 49 above it are invalid and need re-pulled or re-built.
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Layering in the container spec is achieved by overlaying each layer's filesystem (a tarball, I think) over each layer below it. If file "a" is modified in layers 3 and 5, the resulting container will have data for both versions but reading "a" in the container will return version 5.
Docker exploits this to figure out when it can cache a layer, but building a container is different than running one because changing the underlying file system can change what a command outputs. If you're running a container, changing one deeply buried layer doesn't change the layers above it because they're already saved.
> If you change the package defined in the bottom most layer, all 49 above it are invalid and need re-pulled or re-built.
I also initially thought that that was the case, but some tools are able to work around that [0] [1] [2]. I have no idea how it works, but it works pretty well in my experience.
[0]: https://github.com/hhd-dev/rechunk/
[1]: https://coreos.github.io/rpm-ostree/container/#creating-chun...
That’s mostly a Dockerism (and even Docker has `COPY --link` these days). The underlying tech supports independent layers.