Tekton Triggers CEL

or how to convert a git ref to a branch name

Tekton Triggers is a Tekton project which allows to receive webhooks and react appropriately to those. You can follow along my article on the codecentric blog to see what is possible.

Tekton triggers includes a conecpt which is called Interceptors that provide a way to implement cross cutting concerns. To give you a quick example: If you integrate Github with Tekton you hopefully want to validate if your project is calling the tekton installation since you don’t want anybody stumbling on your tekton installation to run pipelines. How hard can it be? Just have a look at the documentation and see that you need to exchange secrests and need to run HMAC on the encoded request. Long story short: Just integrate the GitHub interceptor and you are done.

And now another simple problem. When gitlab triggers a webhook the branch name is included in the request but contained in a git reference. This is great but what if I just need the branch name? One way to handle it is to create a task that gets the reference as a parameter and returns just the branch name as an out parameter. This is quite a big overhead so maybe there is a simpler solution.

Common Expression Language Interceptor

The common expression language is a simple expression language based on protobuf types. The implementation used in the context of tekton is the go implementation called cel-go. If you integrate the CEL interceptor in the EventListener you can manipulate the request and find the results in the trigger bindings in a field called extensions.

Find a short example here where I configured the interceptor to return the branch name:

    - name: gitlab-push-events-trigger
        - name: "verify-gitlab-payload"
            name: "gitlab"
            kind: ClusterInterceptor
            - name: secretRef
                secretName: "gitlab-secret"
                secretKey: "secretToken"
            - name: eventTypes
                - "Push Hook"
        - ref:
            name: cel
            - name: "overlays"
                - key: branch_name
                  expression: "body.ref.split('/')[2]"
        - name: git-revision
          value: $(body.checkout_sha)
        - name: git-repository-url
          value: $(body.repository.git_http_url)
        - name: git-branch
          value: $(extensions.branch_name)

The branch is extracted in the last two lines so these can be sent over to the triggered pipeline.


As you can see we simply changed a request field to make it easily consumable from our pipeline.