EventListener

EventListener is a Kubernetes custom resource that allows users a declarative way to process incoming HTTP based events with JSON payloads. EventListeners expose an addressable “Sink” to which incoming events are directed. Users can declare TriggerBindings to extract fields from events, and apply them to TriggerTemplates in order to create Tekton resources. In addition, EventListeners allow lightweight event processing using Event Interceptors.

Syntax

To define a configuration file for an EventListener resource, you can specify the following fields:

  • Required:
    • apiVersion - Specifies the API version, for example triggers.tekton.dev/v1alpha1.
    • kind - Specifies the EventListener resource object.
    • metadata - Specifies data to uniquely identify the EventListener resource object, for example a name.
    • spec - Specifies the configuration information for your EventListener resource object. In order for an EventListener to do anything, the spec must include:
    • triggers - Specifies a list of Triggers to run
    • serviceAccountName - Specifies the ServiceAccount that the EventListener uses to create resources
  • Optional:
    • serviceType - Specifies what type of service the sink pod is exposed as
    • replicas - Specifies the number of EventListener pods
    • podTemplate - Specifies the PodTemplate for your EventListener pod

ServiceAccountName

The serviceAccountName field is required. The ServiceAccount that the EventListener sink uses to create the Tekton resources. The ServiceAccount needs a role with the following rules:

kind: Role
apiVersion: rbac.authorization.k8s.io/v1
metadata:
  name: tekton-triggers-example-minimal
rules:
# Permissions for every EventListener deployment to function
- apiGroups: ["triggers.tekton.dev"]
  resources: ["eventlisteners", "triggerbindings", "triggertemplates"]
  verbs: ["get"]
- apiGroups: [""]
  # secrets are only needed for Github/Gitlab interceptors, serviceaccounts only for per trigger authorization
  resources: ["configmaps", "secrets", "serviceaccounts"]
  verbs: ["get", "list", "watch"]
# Permissions to create resources in associated TriggerTemplates
- apiGroups: ["tekton.dev"]
  resources: ["pipelineruns", "pipelineresources", "taskruns"]
  verbs: ["create"]

If your EventListener is using ClusterTriggerBindings, you’ll need a ServiceAccount with a ClusterRole instead.

Triggers

The triggers field is required. Each EventListener can consist of one or more triggers. A Trigger consists of:

  • name - (Optional) a valid Kubernetes name
  • interceptors - (Optional) list of interceptors to use
  • bindings - A list of TriggerBindings reference to use or embedded TriggerBindingsSpecs to use.
  • template - The name of TriggerTemplate to use
triggers:
  - name: trigger-1
    interceptors:
      - github:
          eventTypes: ["pull_request"]
    bindings:
      - ref: pipeline-binding
      - name: message-binding
        spec:
            params:
              - name: message
                value: Hello from the Triggers EventListener!
    template:
      name: pipeline-template

Also, to support multi-tenant styled scenarios, where an administrator may not want all triggers to have the same permissions as the EventListener, a service account can optionally be set at the trigger level and used if present in place of the EventListener service account when creating resources:

triggers:
  - name: trigger-1
    serviceAccount: 
      name: trigger-1-sa
      namespace: event-listener-namespace
    interceptors:
      - github:
          eventTypes: ["pull_request"]
    bindings:
      - ref: pipeline-binding
      - ref: message-binding
    template:
      name: pipeline-template

The default ClusterRole for the EventListener allows for reading ServiceAccounts from any namespace.

ServiceType

The serviceType field is optional. EventListener sinks are exposed via Kubernetes Services. By default, the serviceType is ClusterIP which means any pods running in the same Kubernetes cluster can access services’ via their cluster DNS. Other valid values are NodePort and LoadBalancer. Check the Kubernetes Service types documentations for details.

For external services to connect to your cluster (e.g. GitHub sending webhooks), check out the guide on exposing EventListeners.

Replicas

The replicas field is optional. By default, the number of replicas of EventListener is 1. If you want to deploy more than one pod, you can specify the number to this field.

PodTemplate

The podTemplate field is optional. A PodTemplate is specifications for creating EventListener pod. A PodTemplate consists of: - tolerations - list of toleration which allows pods to schedule onto the nodes with matching taints. This is needed only if you want to schedule EventListener pod to a tainted node. - nodeSelector - key-value labels the node has which an EventListener pod should be scheduled on.

spec:
  podTemplate:
    nodeSelector:
      app: test
    tolerations:
    - key: key
      value: value
      operator: Equal
      effect: NoSchedule

Logging

EventListener sinks are exposed as Kubernetes services that are backed by a Pod running the sink logic. The logging configuration can be controlled via the config-logging-triggers ConfigMap present in the namespace that the EventListener was created in. This ConfigMap is automatically created and contains the default values defined in config-logging.yaml.

To access logs for the EventListener sink, you can query for pods with the eventlistener label set to the name of your EventListener resource:

kubectl get pods --selector eventlistener=my-eventlistener

Labels

By default, EventListeners will attach the following labels automatically to all resources it creates:

Name Description
triggers.tekton.dev/eventlistener Name of the EventListener that generated the resource.
triggers.tekton.dev/trigger Name of the Trigger that generated the resource.
triggers.tekton.dev/eventid UID of the incoming event.

Since the EventListener name and Trigger name are used as label values, they must adhere to the Kubernetes syntax and character set requirements for label values.

Interceptors

Triggers within an EventListener can optionally specify interceptors, to modify the behavior or payload of Triggers.

Event Interceptors can take several different forms today:

Webhook Interceptors

Webhook Interceptors allow users to configure an external k8s object which contains business logic. These are currently specified under the Webhook field, which contains an ObjectReference to a Kubernetes Service. If a Webhook Interceptor is specified, the EventListener sink will forward incoming events to the service referenced by the Interceptor over HTTP. The service is expected to process the event and return a response back. The status code of the response determines if the processing is successful - a 200 response means the Interceptor was successful and that processing should continue, any other status code will halt Trigger processing. The returned request (body and headers) is used as the new event payload by the EventListener and passed on the TriggerBinding. An Interceptor has an optional header field with key-value pairs that will be merged with event headers before being sent; canonical names must be specified.

The incoming request URL (received by the EventListener) is provided in the Eventlistener-Request-URL header provided to the Webhook interceptor.

When multiple Interceptors are specified, requests are piped through each Interceptor sequentially for processing - e.g. the headers/body of the first Interceptor’s response will be sent as the request to the second Interceptor. It is the responsibility of Interceptors to preserve header/body data if desired. The response body and headers of the last Interceptor is used for resource binding/templating.

Event Interceptor Services

To be an Event Interceptor, a Kubernetes object should:

  • Be fronted by a regular Kubernetes v1 Service over port 80
  • Accept JSON payloads over HTTP
  • Accept HTTP POST requests with JSON payloads.
  • Return a HTTP 200 OK Status if the EventListener should continue processing the event
  • Return a JSON body back. This will be used by the EventListener as the event payload for any further processing. If the Interceptor does not need to modify the body, it can simply return the body that it received.
  • Return any Headers that might be required by other chained Interceptors or any bindings.

Note: It is the responsibility of Interceptors to preserve header/body data if desired. The response body and headers of the last Interceptor is used for resource binding/templating.

---
apiVersion: triggers.tekton.dev/v1alpha1
kind: EventListener
metadata:
  name: listener-interceptor
spec:
  serviceAccountName: tekton-triggers-example-sa
  triggers:
    - name: foo-trig
      interceptors:
        - webhook:
            header:
              - name: Foo-Trig-Header1
                value: string-value
              - name: Foo-Trig-Header2
                value:
                  - array-val1
                  - array-val2
            objectRef:
              kind: Service
              name: gh-validate
              apiVersion: v1
              namespace: default
      bindings:
        - ref: pipeline-binding
      template:
        name: pipeline-template

GitHub Interceptors

GitHub Interceptors contain logic to validate and filter webhooks that come from GitHub. Supported features include validating webhooks actually came from GitHub using the logic outlined in GitHub documentation, as well as filtering incoming events.

To use this Interceptor as a validator, create a secret string using the method of your choice, and configure the GitHub webhook to use that secret value. Create a Kubernetes secret containing this value, and pass that as a reference to the github Interceptor.

To use this Interceptor as a filter, add the event types you would like to accept to the eventTypes field. Valid values can be found in GitHub docs.

The body/header of the incoming request will be preserved in this Interceptor’s response.

---
apiVersion: triggers.tekton.dev/v1alpha1
kind: EventListener
metadata:
  name: github-listener-interceptor
spec:
  serviceAccountName: tekton-triggers-github-sa
  triggers:
    - name: github-listener
      interceptors:
        - github:
            secretRef:
              secretName: github-secret
              secretKey: secretToken
            eventTypes:
              - pull_request
        - cel:
            filter: "body.action in ['opened', 'synchronize', 'reopened']"
      bindings:
        - ref: github-binding
      template:
        name: github-template

GitLab Interceptors

GitLab Interceptors contain logic to validate and filter requests that come from GitLab. Supported features include validating that a webhook actually came from GitLab, using the logic outlined in GitLab documentation, and to filter incoming events based on the event types. Event types can be found in GitLab documentation.

To use this Interceptor as a validator, create a secret string using the method of your choice, and configure the GitLab webhook to use that secret value. Create a Kubernetes secret containing this value, and pass that as a reference to the gitlab Interceptor.

To use this Interceptor as a filter, add the event types you would like to accept to the eventTypes field.

The body/header of the incoming request will be preserved in this Interceptor’s response.

apiVersion: triggers.tekton.dev/v1alpha1
kind: EventListener
metadata:
  name: gitlab-listener-interceptor
spec:
  serviceAccountName: tekton-triggers-example-sa
  triggers:
    - name: foo-trig
      interceptors:
        - gitlab:
            secretRef:
              secretName: foo
              secretKey: bar
            eventTypes:
              - Push Hook
      bindings:
        - ref: pipeline-binding
      template:
        name: pipeline-template

Bitbucket Interceptors

The Bitbucket interceptor provides support for hooks originating in Bitbucket server, providing server hook signature validation and event-filtering. Bitbucket cloud is not currently supported by this interceptor, as it has no secret validation, so you could match on the incoming requests using the CEL interceptor.

To use this Interceptor as a validator, create a secret string using the method of your choice, and configure the Bitbucket webhook to use that secret value. Create a Kubernetes secret containing this value, and pass that as a reference to the bitbucket Interceptor.

To use this Interceptor as a filter, add the event types you would like to accept to the eventTypes field. Valid values can be found in Bitbucket docs.

The body/header of the incoming request will be preserved in this Interceptor’s response.

---
apiVersion: triggers.tekton.dev/v1alpha1
kind: EventListener
metadata:
  name: bitbucket-listener
spec:
  serviceAccountName: tekton-triggers-bitbucket-sa
  triggers:
    - name: bitbucket-triggers
      interceptors:
        - bitbucket:
            secretRef:
              secretName: bitbucket-secret
              secretKey: secretToken
            eventTypes:
              - repo:refs_changed
      bindings:
        - ref: bitbucket-binding
      template:
        name: bitbucket-template

CEL Interceptors

CEL Interceptors can be used to filter or modify incoming events, using the CEL expression language.

Please read the cel-spec language definition for more details on the expression language syntax.

The cel-trig-with-matches trigger below filters events that don’t have an 'X-GitHub-Event' header matching 'pull_request'.

It also modifies the incoming request, adding an extra key to the JSON body, with a truncated string coming from the hook body.

apiVersion: triggers.tekton.dev/v1alpha1
kind: EventListener
metadata:
  name: cel-listener-interceptor
spec:
  serviceAccountName: tekton-triggers-example-sa
  triggers:
    - name: cel-trig-with-matches
      interceptors:
        - cel:
            filter: "header.match('X-GitHub-Event', 'pull_request')"
            overlays:
            - key: extensions.truncated_sha
              expression: "body.pull_request.head.sha.truncate(7)"
      bindings:
      - ref: pipeline-binding
      template:
        name: pipeline-template
    - name: cel-trig-with-canonical
      interceptors:
        - cel:
            filter: "header.canonical('X-GitHub-Event') == 'push'"
      bindings:
      - ref: pipeline-binding
      template:
        name: pipeline-template

In addition to the standard expressions provided by CEL, Triggers supports some useful functions for dealing with event data CEL expressions.

The body/header of the incoming request will be preserved in this Interceptor’s response.

apiVersion: triggers.tekton.dev/v1alpha1
kind: EventListener
metadata:
  name: cel-listener-interceptor
spec:
  serviceAccountName: tekton-triggers-example-sa
  triggers:
    - name: cel-trig-with-matches
      interceptors:
        - cel:
            filter: "header.match('X-GitHub-Event', 'pull_request')"
            overlays:
            - key: extensions.truncated_sha
              expression: "body.pull_request.head.sha.truncate(7)"
      bindings:
      - ref: pipeline-binding
      template:
        name: pipeline-template
    - name: cel-trig-with-canonical
      interceptors:
        - cel:
            filter: "header.canonical('X-GitHub-Event') == 'push'"
      bindings:
      - ref: pipeline-binding
      template:
        name: pipeline-template

The filter expression must return a true value if this trigger is to be processed, and the overlays applied.

Optionally, no filter expression can be provided, and the overlays will be applied to the incoming body.

apiVersion: triggers.tekton.dev/v1alpha1
kind: EventListener
metadata:
  name: cel-eventlistener-no-filter
spec:
  serviceAccountName: tekton-triggers-example-sa
  triggers:
    - name: cel-trig
      interceptors:
        - cel:
            overlays:
            - key: extensions.truncated_sha
              expression: "body.pull_request.head.sha.truncate(7)"
      bindings:
      - ref: pipeline-binding
      template:
        name: pipeline-template

Overlays

The CEL interceptor supports “overlays”, these are CEL expressions that are applied to the body before it’s returned to the event-listener.

apiVersion: triggers.tekton.dev/v1alpha1
kind: EventListener
metadata:
  name: example-with-multiple-overlays
spec:
  serviceAccountName: tekton-triggers-example-sa
  triggers:
    - name: cel-trig
      interceptors:
        - cel:
            overlays:
            - key: extensions.truncated_sha
              expression: "body.pull_request.head.sha.truncate(7)"
            - key: extensions.branch_name
              expression: "body.ref.split('/')[2]"
      bindings:
      - ref: pipeline-binding
      template:
        name: pipeline-template

In this example, the bindings will see two additional fields:

Assuming that the input body looked something like this:

{
  "ref": "refs/heads/master",
  "pull_request": {
    "head": {
      "sha": "6113728f27ae82c7b1a177c8d03f9e96e0adf246"
    }
  }
}

The output body would look like this:

{
  "ref": "refs/heads/master",
  "pull_request": {
    "head": {
      "sha": "6113728f27ae82c7b1a177c8d03f9e96e0adf246"
    }
  },
  "extensions": {
    "truncated_sha": "6113728",
    "branch_name": "master"
  }
}

The key element of the overlay can create new elements in a body, or, overlay existing elements.

For example, this expression:

- key: body.pull_request.head.short_sha
  expression: "truncate(body.pull_request.head.sha, 7)"

Would see the short_sha being inserted into the existing body:

{
  "ref": "refs/heads/master",
  "pull_request": {
    "head": {
      "sha": "6113728f27ae82c7b1a177c8d03f9e96e0adf246",
      "short_sha": "6113728"
    }
  }
}

It’s even possible to replace existing fields, by providing a key that matches the path to an existing value.

Anything that is applied as an overlay can be extracted using a binding e.g.

apiVersion: triggers.tekton.dev/v1alpha1
kind: TriggerBinding
metadata:
  name: pipeline-binding-with-cel-extensions
spec:
  params:
  - name: gitrevision
    value: $(body.extensions.branch_name)
  - name: branch
    value: $(body.pull_request.head.short_sha)

EventListener Response

The EventListener responds with 201 Created status code when at least one of the trigger is executed successfully. Otherwise, it returns 202 Accepted status code. The EventListener responds with following message after receiving the event:

{"eventListener":"listener","namespace":"default","eventID":"h2bb7"}
  • eventListener - Refers to the EventListener Name.
  • namespace - Refers to the namespace of the EventListener
  • eventID - Refers to the uniqueID that gets assigned to each incoming request

Examples

For complete examples, see the examples folder.

Multi-Tenant Concerns

The EventListener is effectively an additional form of client into Tekton, versus what example usage via kubectl or tkn which you have seen elsewhere. In particular, the HTTP based events bypass the normal Kubernetes authentication path you get via kubeconfig files and the kubectl config family of commands.

As such, there are set of items to consider when deciding how to

  • best expose (each) EventListener in your cluster to the outside world.
  • best control how (each) EventListener and the underlying API Objects described below access, create, and update Tekton related API Objects in your cluster.

Minimally, each EventListener has its ServiceAccountName as noted below and all events coming over the “Sink” result in any Tekton resource interactions being done with the permissions assigned to that ServiceAccount.

However, if you need differing levels of permissions over a set of Tekton resources across the various Triggers and Interceptors, where not all Triggers or Interceptors can manipulate certain Tekton Resources in the same way, a simple, single EventListener will not suffice.

Your options at that point are as follows:

Multiple EventListeners (One EventListener Per Namespace)

You can create multiple EventListener objects, where your set of Triggers and Interceptors are spread out across the EventListeners.

If you create each of those EventListeners in their own namespace, it becomes easy to assign varying permissions to the ServiceAccount of each one to serve your needs. And often times namespace creation is coupled with a default set of ServiceAccounts and Secrets that are also defined. So conceivably some administration steps are taken care of. You just update the permissions of the automatically created ServiceAccounts.

Possible drawbacks: - Namespaces with associated Secrets and ServiceAccounts in an aggregate sense prove to be the most expensive items in Kubernetes underlying etcd store. In larger clusters etcd storage capacity can become a concern. - Multiple EventListeners means multiple HTTP ports that must be exposed to the external entities accessing the “Sink”. If you happen to have a HTTP Firewall between your Cluster and external entities, that means more administrative cost, opening ports in the firewall for each Service, unless you can employ Kubernetes Ingress to serve as a routing abstraction layer for your set of EventListeners.

Multiple EventListeners (Multiple EventListeners per Namespace)

Multiple EventListeners per namespace will most likely mean more ServiceAccount/Secret/RBAC manipulation for the administrator, as some of the built in generation of those artifacts as part of namespace creation are not applicable.

However you will save some on the etcd storage costs by reducing the number of namespaces.

Multiple EventListeners and potential Firewall concerns still apply (again unless you employ Ingress).

ServiceAccount per EventListenerTrigger

Being able to set a ServiceAccount on an EventListenerTrigger allows for finer grained permissions as well.

You still have to create the additional ServiceAccounts.

But staying within 1 namespace and minimizing the number of EventListeners with their associated “Sinks” minimizes concerns around etcd storage and port considerations with Firewalls if Ingress is not utilized.


Except as otherwise noted, the content of this page is licensed under the Creative Commons Attribution 4.0 License, and code samples are licensed under the Apache 2.0 License.


Last modified January 1, 0001