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Jason Hall 2020-06-11 20:13:49 -04:00
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# `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.

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apiVersion: tekton.dev/v1alpha1
kind: Condition
metadata:
name: cel
spec:
parameters:
- name: expression
default: 'true'
check:
image: ko://github.com/imjasonh/cel-condition
args: ['$(params.expression)']

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module github.com/imjasonh/cel-condition
go 1.14
require github.com/google/cel-go v0.5.1

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package main
import (
"log"
"os"
"github.com/google/cel-go/cel"
"github.com/google/cel-go/common/types"
)
func main() {
e := os.Args[1]
log.Printf("evaluating %q", e)
env, err := cel.NewEnv(cel.Declarations())
if err != nil {
log.Fatalln(err)
}
ast, iss := env.Compile(e)
if iss.Err() != nil {
// Syntax error.
log.Fatalln(iss.Err())
}
prg, err := env.Program(ast)
out, _, err := prg.Eval(map[string]interface{}{})
if err != nil {
// Error evaluating.
log.Fatalln(err)
}
if !types.IsBool(out) {
log.Fatalf("result (%v) is not boolean (type: %s)", out.Value(), out.Type().TypeName())
}
if out != types.Bool(true) {
log.Fatal("result is false")
}
log.Println("result is true")
}