Skip to content

Building a Stemcell

(See What is a Stemcell? for an introduction to stemcells.)

To build a stemcell tarball for a supported IaaS-OS combination follow instructions in the bosh-linux-stemcell-builder's README.

Stemcell tarballs are currently specific to an IaaS-OS/CPI because they may:

In the future, BOSH team will investigate how to best consolidate stemcells into a single OS image. In the meantime, if you're developing a CPI for a new IaaS, you may consider reusing one of the officially generated stemcells, or making changes to the following projects:

Tarball Structure


This is an implementation detail. The tarball structure is subject to change without notice.

tar tvf light-bosh-stemcell-621.74-aws-xen-hvm-ubuntu-xenial-go_agent.tgz

Should result in:

-rw-rw-r--  0 ubuntu ubuntu      0 Aug  4 09:45 image
-rw-rw-r--  0 ubuntu ubuntu    710 Aug  4 10:06 stemcell.MF
-rw-r--r--  0 ubuntu ubuntu  50594 Aug  4 09:23 packages.txt
-rw-r--r--  0 ubuntu ubuntu  12543 Aug  4 09:22 dev_tools_file_list.txt
  • image: OS image in a format (raw, qcow, ova, etc.) understood by the CPI/IaaS.
  • stemcell.MF: YAML file with stemcell metadata.
  • packages.txt: Text file that includes list of packages installed. (Used to be included as stemcell_dpkg_l.txt)
  • dev_tools_file_list.txt: Text file that includes list of files removed by the agent if Agent's remove_dev_tools feature is enabled.



This is an implementation detail. The content of stemcell.MF is subject to change without notice.

  • name [String, required]: A unique name used to identify stemcell series.
  • operating_system [String, required]: Operating system in the stemcell. Example: ubuntu-xenial.
  • version [String, required]: Version of the stemcell. Example: 621.74.
  • sha1 [String, required]: The SHA1 of the image file included in the stemcell tarball.
  • bosh_protocol [String, optional]: Deprecated.
  • cloud_properties [Hash, required]: Describes any IaaS-specific properties needed to import OS image. These properties will be passed in to the create_stemcell CPI call.
  • api_version [Integer, optional]: Highest supported API version of the Agent in the stemcell. Defaults to 1.
  • stemcell_formats [Array of Strings, optional]: The list of stemcell formats that a CPI must support. The director will attempt to upload the stemcell to all CPIs that support any specified formats.

Name, operating system and version values will be visible via bosh stemcells command once a stemcell is imported into the Director.


tar -Oxzf bosh-stemcell-621.74-aws-xen-hvm-ubuntu-xenial-go_agent.tgz stemcell.MF
name: bosh-aws-xen-hvm-ubuntu-xenial-go_agent
version: '621.74'
bosh_protocol: 1
api_version: 3
sha1: 98b0844541831392cb2efc66292143a3332c705a
operating_system: ubuntu-xenial
- aws-raw
  name: bosh-aws-xen-hvm-ubuntu-xenial-go_agent
  version: '621.74'
  infrastructure: aws
  hypervisor: xen
  disk: 3072
  disk_format: raw
  container_format: bare
  os_type: linux
  os_distro: ubuntu
  architecture: x86_64
  root_device_name: "/dev/sda1"

Light Stemcells

A "light" stemcell represents a reference to an IaaS resource where the stemcell has already been imported. This helps solve IaaS limitations which restrict how base VM images can be imported, such as:

  • AWS only allowing imports from within running AWS VMs;
  • OpenStack disallowing Glance image upload; or
  • IaaSes taking a long time to import an image.

In these cases, a light stemcell tarball contains only metadata about the stemcell, but does not contain the actual image file with the OS disk image. In addition to the regular stemcell metadata, the stemcell.MF file should include a cloud_properties section with details about how the CPI may find the already-imported stemcell within the IaaS.

On AWS, for example, stemcells are imported into a specific region as an EC2 Amazon Machine Image which is referenced by an ami-* identifier. If you look at the stemcell.MF file of the light stemcell tarball, you'll see a list of regions and their corresponding AMI. When a stemcell is uploaded, the create_stemcell call will return matching AMI ID without doing any IaaS API calls.

tar -Oxzf light-bosh-stemcell-621.74-aws-xen-hvm-ubuntu-xenial-go_agent.tgz stemcell.MF
name: bosh-aws-xen-hvm-ubuntu-xenial-go_agent
version: '621.74'
bosh_protocol: '1'
api_version: 3
sha1: da39a3ee5e6b4b0d3255bfef95601890afd80709
operating_system: ubuntu-xenial
- aws-light
    us-gov-west-1: ami-1431a975
    ap-northeast-1: ami-0ddb32f9e2cb016f3
    ap-northeast-2: ami-04f416b078c7eb965
    ap-south-1: ami-0f04da873c8883a56
    ap-southeast-1: ami-0628f639a2c1abd77
    ap-southeast-2: ami-06f24628e83df3ca7
    ca-central-1: ami-0b8196ea9d0c10b00
    eu-central-1: ami-07ebdd782c27da598
    eu-west-1: ami-0f7e184ff7b50cd36
    eu-west-2: ami-01713a432b5494aa6
    eu-west-3: ami-059850a6db5f0f1f0
    sa-east-1: ami-0559933d31a7cbdf3
    us-east-1: ami-0cdc0ee47ff314116
    us-east-2: ami-05e20eb5a19355a32
    us-west-1: ami-0eb351fd3b5bb07e0
    us-west-2: ami-0147f5edb0c3600ab
    cn-northwest-1: ami-0855153be65a20e35
    cn-north-1: ami-01db1b9ef2de116fb


The process of building light stemcells will depend on your IaaS. Typically, you will have an automation pipeline which does the following:

  1. Watch for new versions of heavy stemcells;
  2. Import it into your IaaS; and
  3. Patch the stemcell tarball to remove the heavy image and add your IaaS image reference.
  4. For IaaS operators: publish a reference to the tarball for (get in touch).

If you're getting started in this process, you may want to refer to the following examples:

While the import step is highly IaaS-specific, there are a couple general recommendations:

  • You may want to reuse the process that the CPI uses internally with create_stemcell. If not reusing the same code, you should follow the exact same steps. There should be no noticeable difference between an IaaS base image created from a "light stemcell builder" vs an operator importing the stemcell on a director themselves.
  • Your IaaS may support alternative methods for transferring images once they're imported. As long as the process does not change the underlying stemcell image, you may feel free to use it. For example, in AWS we import the image into a single region, then use the CopyImage AWS API call to copy the image to all other regions.


There are two test suites each stemcell is expected to pass before it's considered to be production-ready:

  • shared Stemcell Tests which verify that proper packages and configurations are installed
  • shared BOSH Acceptance Tests (BATS) (provided by the BOSH team) which verify high level Director behavior with the stemcell being used