How deeply does Kubernetes in Action, Second Edition cover image layer?

Defines layer sharing, traces layers through a Dockerfile build, and explains container writes through copy-on-write.

Image layer

As explained in Kubernetes in Action, Second Edition

A container image layer is an independently stored filesystem change that composes a container image and can be shared across images.

A container image is built from layers rather than stored as one indivisible filesystem. A layer can be reused by multiple images. Docker stores a shared layer once, and a host downloading another image needs to fetch only the layers it does not already have. section 2.1.2

Layers from a Dockerfile

Start with `FROM node:23-alpine`, then use `COPY app.js /app.js`, `COPY html/ /html`, and `ENTRYPOINT ["node", "app.js"]`. Docker builds on the base image layer by layer: the two COPY directives add files, and the ENTRYPOINT directive specifies the startup command. Run `docker history kiada:latest` to see the resulting layers, their sizes, and the directives that created them. The newest layers appear first. section 2.2.2

$ docker history kiada:latest
IMAGE         CREATED   CREATED BY                           SIZE
afeb94f9465c  13m ago   ENTRYPOINT ["node" "app.js"]         0B       #1
<missing>     13m ago   COPY html/ /html                     533kB    #1
<missing>     13m ago   COPY app.js /app.js                  2.9kB    #1
<missing>     17h ago   CMD ["node"]                         0B        #2
<missing>     17h ago   ENTRYPOINT ["docker-entrypoint.sh"]  0B        #2
<missing>     17h ago   COPY docker-entrypoint.sh /usr/l...  388B      #2
<missing>     17h ago   RUN /bin/sh -c set -ex   && save...  7.18MB    #2
<missing>     17h ago   ENV YARN_VERSION=1.22.22             0B        #2
<missing>     17h ago   RUN /bin/sh -c ARCH= OPENSSL_ARC...  143MB     #2
<missing>     17h ago   ENV NODE_VERSION=23.4.0                        #2
<missing>     17h ago   RUN /bin/sh -c groupadd --gid 10...  8.9kB     #2
<missing>     9d ago    # debian.sh --arch 'amd64' out/...   74.8MB    #2

Compare the file-adding COPY layers with the 0B ENTRYPOINT layer, then look below them for layers inherited from the base image. section 2.2.2

Sharing an image layer does not mean containers share their changes. Each container uses the image’s read-only layers beneath its own read/write layer. If an application changes a file from a read-only layer, copy-on-write places a copy in that container’s read/write layer and changes it there. Other containers continue to see their own filesystem state. section 2.1.2

How Kubernetes in Action, Second Edition covers it

Cover of Kubernetes in Action, Second Edition

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Also explained in

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Learn Docker in a Month of Lunches, Second EditionAn image layer is an immutable filesystem change and associated data that forms part of a container image and can be shared by other images and containers.
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Podman in Action: Secure, rootless containers for Kubernetes, microservices, and moreAn image layer is an immutable filesystem change set that contributes to a container image and can be stacked with other layers.
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.NET in Action, Second EditionA container image layer is a separate immutable filesystem change produced by an image-build step and stored as part of the image.
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Build Python Web Apps with StreamlitA Docker image layer is an immutable filesystem change that contributes to the assembled contents of a Docker image.

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Related concepts

container imagepart of · A container image is a packaged filesystem and configuration artifact from which a container can be created and run.
Dockerfileproduced by · A Dockerfile is a text file containing declarative instructions for building a container image and specifying its contents and runtime metadata.
Dockerfile instructionproduced by · A Dockerfile instruction is a declarative directive that specifies a build operation or image metadata during container image construction.
run instructionproduced by · A Dockerfile RUN instruction executes a command during container image construction, producing filesystem changes that become part of the resulting image.
virtual filesystempart of · A virtual filesystem is a software abstraction that presents files and directories through filesystem-like organization and operations without requiring direct access to physical storage.
container registryused by · A container registry is a service or repository for storing, managing, and distributing container images and related artifacts.

This explainer was generated by Manning's knowledge graph from the text of Kubernetes in Action, Second Edition. The section links point to the parts of the book it draws on.