Docker really helps in managing filesystem layers and image caching through a layered architecture that would allow for efficient storage, versioning, and reuse of the different parts of an image. This is how Docker does that:
1. Layered Filesystem Structure: Docker images are built using layers. Each layer corresponds to a step in the Dockerfile, such as RUN, COPY, or ADD. Every command in the Dockerfile adds a new layer on top of the previous ones.
Read-only Layers: Once an image is created, all its layers are read-only and stacked from bottom to top. The final image represents the complete filesystem required for the container.
Writable Container Layer: At the time of creating a container from an image, Docker creates a writable layer on top of the read-only layers that form an image. Any change to the files in a container (creating, modifying, or deleting files occur in this writable layer without changing the image layers underlying it. Once a container is deleted, all changes made to this writable layer are lost.
2. Image Caching for Efficient Builds
Layer caching mechanism: Docker uses a layer caching mechanism to speed up the building of images and to reuse unchanged layers. The Docker will use the cached version of that layer if the Dockerfile command and its dependencies like files or environment variables have not changed.
Order and Cache Invalidation: Docker builds layers in the order defined in the Dockerfile. If a command changes, Docker rebuilds that layer and all layers following it. For optimal builds, place frequent changers towards the end of the Dockerfile, and less frequently changed commands like base dependencies up top.
3. Layer Storage and Efficiency
Union File System: Docker uses a union file system, such as OverlayFS or AUFS. This system combines multiple layers into a single, unified view. It enables efficient stacking, management, and access to the layered structure of Docker images.
Reduced Storage and Network Usage: With each layer being stored only once, Docker saves tremendous amounts of storage because layers are reused across images. Suppose two images share common layers. Then, no matter how many times we build, Docker will have to store every unique layer only once, thus cutting down both storage and network bandwidth for the image when shared.
4. Layer Reuse Across Builds
Shared Layers: When images are with the same layer or dependency, Docker caches that layer and allows all other images to use it. In this way, the building of images becomes faster and efficient.
Intermediate Containers: Docker will create an intermediate container for each command while building a Docker file. After finishing image building, Docker removes those intermediate containers, but their layers are always cached and reused for subsequent builds.
Conclusion
Docker leverages filesystem layers and caching to efficiently manage images. By avoiding modifications to unchanged layers, Docker simplifies the creation of versioned images. Caching accelerates image builds and minimizes storage usage, while shared components across images make Docker an exceptionally efficient platform for containerized applications.
