Cache Images are a way to preserve build optimizing layers across different host machines. These images can improve performance when using pack in ephemeral environments such as CI/CD pipelines.

Using Cache Images (--cache-image)

The --cache-image parameter must be in the following format

--cache-image <remote-image-location>

The --cache-images flag must be specified in conjunction with the --publish flag.

Examples

For the following examples we will use:

• The Dockerhub registry
• A sample Dockerhub user account named buildpack-examples
• Our samples repo

First we need to authenticate with docker to allow us to push images

    docker login
    ...

Next we trust the cnbs/sample-builder:bionic builder in order to allow access to docker credentials when publishing.

pack config trusted-builders add cnbs/sample-builder:bionic

To build an the index.docker.io/buildpack-examples/cache-image-example application image and the index.docker.io/buildpack-examples/maven-cache-image:latest cache image we may run the following

pack build index.docker.io/buildpack-examples/cache-image-example \
    --builder cnbs/sample-builder:bionic \
    --buildpack samples/java-maven \
    --path samples/apps/java-maven \
    --cache-image index.docker.io/buildpack-examples/maven-cache-image:latest \
    --publish

Now we may inspect both the application image, and the cache image by pulling them

# application image inspect
docker pull index.docker.io/buildpack-examples/cache-image-example
docker inspect index.docker.io/buildpack-examples/cache-image-example

#cache image inspect
docker pull index.docker.io/buildpacks-examples/maven-cache-image:latest
docker inspect index.docker.io/buildpacks-examples/maven-cache-image:latest

The cache image we produced may now be used by builds on other machines. Note these builds may also update the specified cache-image.

The following command will restore data for the samples/java-maven:maven_m2 layer from the cache image.

pack build index.docker.io/buildpack-examples/second-cache-image-example \
    --builder cnbs/sample-builder:bionic \
    --buildpack samples/java-maven \
    --path samples/apps/java-maven \
    --cache-image index.docker.io/buildpack-examples/maven-cache-image:latest \
    --publish

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