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> Small differences in layer order and layer contents multiply to very large inefficiencies at scale. Can you provide an example of how layer order can cause a

• jacques_chester 8y ago
  Consider: FROM nodejs COPY /mycode /app RUN npm install /app Now suppose I change my app code. In a Dockerfile situation, the change to the `COPY` invalidates the `RUN npm install /app` layer, even if I didn't change anything that NPM would care about. An NPM buildpack can signal that there's nothing to change, allowing the overall lifecycle to skip re-building that layer. There's also the problem of efficient composition. Suppose I have this: RUN wget https://example.com/popular-shell-script.sh && \ go get https://git.example.com/something-else@abc123 && \ ./popular-shell-script.sh && \ rm ./popular-shell-script And this: RUN go get https://git.example.com/something-else@abc123 Both of the resulting images will contain the same `something-else` binary and in an ideal world of file-level manifests I could save on rebuilds and bandwidth consumption (NixOS has this, approximately). But I don't get to do that, because the layers have different overall contents and different digests. Buildpacks don't get you all the way to a file-centric approach, but because they follow a repeatable, controlled pattern of selecting the contents and order of layers, they greatly improve layer reuse between many images.