#Layered Builds

A build can take its source from another build's output. This is how you compose a stack of modules, each layering on top of a previous one:

flowchart LR Cloud[Debian cloud image] --> Base[base build<br/>installs guest tools] Base --> Web[web build<br/>adds nginx] Base --> DB[db build<br/>adds postgres] Web --> WebApp[webapp build<br/>adds your app]

vms:
    - name: builder
      source:
          buildRef:
              name: base-debian # name of the previous build
              vmName: builder # required if base has multiple VMs
      provisioners:
          - type: shell
            shell:
                inline: |
                    sudo apt-get install -y postgresql

The referenced build must already be in Succeeded state — otherwise the layered build fails immediately. So your build order matters: run the base build first and wait for it to succeed, then run the layered build that references it.