Conditional Tinkerbell Template Actions

[chrisdoherty4]

Overview

Tinkerbell defines a Template object that contains Actions. Actions represent an activity that contributes to the provisioning of a machine (for the primary Tinkerbell use-case). Actions are flexible as they are OCI images that can be developed and maintained by third parties. This flexibility contributes to the flexibility of Templates.

Template's themselves, however, aren't particularly flexible. For use-cases such as CAPI/CAPT where the same template is used to provision the same kind of node (such as control plane nodes) where its necessary to perform different dependent on the hardware it can be difficult to model using Template's.

Proposal

Provide control flow type capabilities in Template's that enables toggling of individual actions. This could work in a similar fashion to Github Action's if statement.

# Github Action Example
jobs:
  job_name:
     if: EXPRESSION

The semantics of if are to run the job if the EXPRESSION evaluates to true.

We could create something similar for Tinkerbell Template actions (note the historical concept of a 'task' has been removed for simplicity as it will be removed in future versions of Tinkerbell).

actions:
- name: "write-file"
  if: EXPRESSION

Rationale

Adding expression capabilities with if in Tinkerbell Templates adds complexity in the form of maintenance. Its non-trivial and the Go standard library doesn't offer expression evaluation. Leveraging third party libraries for evaluating expressions would be ideal.

The particular CAPI/CAPT example used is quite specific to CAPI/CAPT. Its possible that a CAPT solution could be created that decouples templates from CAPT TinkerbellMachineTemplate objects and alleviates that specific problem as there's nothing inherently preventing a user in Tinkerbell core from creating a different template for a specific kind of machine today.

[chrisdoherty4]

After developing the initial v1alpha2 controller, @jacobweinstock and I discussed this and felt it would be quite useful long term. Implementation will be some time after the agreed functionality of v1alpha2 is implemented.