The design can be thought of as steps that aren't the finish-image-build step and the step that is.
finish-image-buildThe start-image and all optional steps are involved in creating and modifying a pair of configs called the prov.config and build.config. Both these configs live on the Cloud Build VM (the builder VM), The build.config instructs Daisy on which resources (disks, vm's) to create and which project to create them in. The prov.config file is used to instruct the provisioner, a binary that is executed on the “preload VM”, a VM whose bootdisk is exported as the customized image. The provisioner is the binary that executes all optional steps that a user will have specified. For example, run-script, install-gpu, seal-oem, and anthos-install are all steps that the provisioner has implemented.
finish-image-build stepThis step is implemented in two phases:
The preloading phase calls a command line tool called daisy and is implemented here preloader. The preloader generates a daisy config file based on the buildspec and calls daisy which creates all the GCP resources necessary for creating a custom COS image. A few disks and a VM are created, and the disks are mounted to the VM. One of these disks is the boot disk which is what will eventually be exported as the customized COS image. Another disk is called “cidata” which packages the provisioner and the provisioner's configs which will be used in the next step.
The provisioner phase calls a command line tool called the provisioner that gets pulled into the preload VM from the previously mentioned “cidata” disk. The provisioner is what executes the optional steps that you have specified in your cloudbuild.yaml file. It will run any scripts install any artifacts, or seal any partitions that were specified prior to this step. Once it finishes executing, the preloading phase will proceed to cleanup any left over resources and exit the build.