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Ignition Templates

Ignition is a provisioning tool for Flatcar Container Linux.

As a network boot server for Flatcar Container Linux, sabakan provides a template system for Ignition. For each machine role, administrator can upload Ignition template to sabakan.

Writing templates

An ignition template is defined by a YAML file like this:

version: "2.3"
include: ../base/base.yml
passwd: passwd.yml
files:
  - /etc/hostname
remote_files:
  - name: /tmp/some.img
    url: "{{ MyURL }}/v1/assets/some.img"
    mode: 0755
systemd:
  - name: chronyd.service
    enabled: true
    mask: false
networkd:
  - 10-node0.netdev

Field descriptions:

  • version: Ignition specification version. Current supported versions are:
    • "2.2" (default)
    • "2.3"
  • include: Another ignition template to be included.
  • passwd: A YAML filename that contains YAML encoded ignition's passwd object.
  • files: List of filenames to be provisioned.
    The file contents are read from files under files/ sub directory.
  • remote_files: List of remote files to be provisioned.
    mode is optional. It should be an octal UNIX file permission bits.
  • systemd: List of systemd unit files to be provisioned.
    The unit contents are read from files under systemd/ sub directory.
    If enabled is true, the unit will be enabled.
    If mask is true, the unit will be masked.
  • networkd: List of networkd unit files to be provisioned.
    The unit contents are read from files under networkd/ sub directory.

Rendering specifications

Files pointed by the template YAML are rendered by text/template. Strings in passwd YAML and url for remote_files are also rendered as templates.

. in the template is set to the Machine struct of the target machine.
For example, {{ .Spec.Serial }} will be replaced with the serial number of the target machine.

Following additional template functions are defined and can be used:

  • MyURL: returns the URL of the sabakan HTTP server.
  • MyURLHTTPS: returns the URL of the sabakan HTTPS server.
  • Metadata: takes a key to retrieve metadata value saved along with the template.
  • json: renders the argument as JSON.
  • add, sub, mul, div: do arithmetic on parameters.

For example, the following template may be replaced with 6 when Machine.Spec.Rack is 3.

{{ add .Spec.Rack 3 }}

Uploading templates to sabakan

sabactl ignitions set -f YAML ROLE ID will upload an ignition template YAML for machines whose role is ROLE. ID is a version string such as 1.2.3.

Sabakan keeps up to 10 old ignition templates for each role.
When a new ignition template has some defects, administrators can revert it to old one by deleting the new template.

Templates may have meta data. To associate meta data, create a JSON file containing a JSON object, and specify it with --meta FILENAME as follows:

$ cat >meta.json <<'EOF'
{
  "key1": "value1",
  "key2": [1, 2, 3]
}
EOF
$ sabactl ignitions set --meta meta.json -f cs.yaml cs 1.0.0

The meta data can be referenced by text/template function Metadata as described above.

Example

testdata/ directory contains a complete set of ignition templates.