cel Condition

This Tekton Condition can be used in a Pipeline to gate execution of a Task on the successful evaluation and result of a CEL expression, passed as an argument.

Usage

Install the Condition:

KO_DOCKER_REPO=gcr.io/<my-project> ko apply -f condition.yaml

Use the Condition in a Pipeline:

apiVersion: tekton.dev/v1alpha1
kind: Pipeline
metadata:
  name: say-hello-if-300-is-even
spec:
  tasks:
  - name: say-hello
    conditions:
    - conditionRef: 'cel'
      params:
      - name: expression
        value: '300 % 2 == 0'
    taskSpec:
      steps:
      - image: busybox
        script: echo hello

When the Pipeline is run, the Task will only run and echo hello if the number 300 is even (which it always is).

Limitations

• There are no custom methods provided. Some could be added in the future.

• At this time, the expression must be fully specified upfront, and can't reference any other Pipeline parameters (i.e., expression's value can't be dependent on the resolved value of another parameter). This limits its usefulness.

• Tekton will spawn a Pod just to evaluate the expression, when it's perfectly safe to evaluate the expression in the control plane.

CLI example

This can also be run as a simple Go binary:

$ go run . '3 == 3'
2020/06/11 20:23:22 evaluating "3 == 3"
2020/06/11 20:23:22 result is true

$ go run . '3 == 4'
2020/06/11 20:23:28 evaluating "3 == 4"
2020/06/11 20:23:28 result is false
exit status 1

$ go run . 'true'
2020/06/11 20:23:35 evaluating "true"
2020/06/11 20:23:35 result is true

$ go run . 'false'
2020/06/11 20:23:38 evaluating "false"
2020/06/11 20:23:38 result is false
exit status 1

$ go run . 'TOTALLY INVALID SYNTAX'
2020/06/11 20:23:49 evaluating "TOTALLY INVALID SYNTAX"
2020/06/11 20:23:49 ERROR: <input>:1:9: Syntax error: mismatched input 'INVALID' expecting <EOF>
 | TOTALLY INVALID SYNTAX
 | ........^
exit status 1