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Writing an Event Filter

Filters are an important part of TriggerMesh's event routing mechanism. They allow for filtering events based on the content of the payload. This content-based event filtering is expressed with Google's Common Expression Language within the TriggerMesh Filter API specification.

Tip

You can verify that the API is available with the following command:

$ kubectl get crd filters.routing.triggermesh.io
NAME                             CREATED AT
filters.routing.triggermesh.io   2021-10-06T09:01:33Z
You can also explore the API specification with:
$ kubectl explain filter

To demonstrate filtering in TriggerMesh we are going to create the event flow depicted in the diagram below. Two sources of kind PingSource will send events on a repeating schedule, and only the events which pass the filter will be displayed on the final event target. The target is the Sockeye application, a microservice which displays the content of a CloudEvent.

Let's create all the required objects:

  • The sockeye target which serves as an event display.
  • Two PingSource to produce events.
  • The Filter to discard unwanted events.

Event display

First we need to have a tool to see our filter results. Create a sockeye service by saving the following YAML manifest in a file called sockeye.yaml and applying it to your Kubernetes cluster:

apiVersion: serving.knative.dev/v1
kind: Service
metadata:
  name: sockeye
spec:
  template:
    spec:
      containers:
        - image: docker.io/n3wscott/sockeye:v0.7.0@sha256:e603d8494eeacce966e57f8f508e4c4f6bebc71d095e3f5a0a1abaf42c5f0e48
kubectl apply -f sockeye.yaml

Open the web interface in a browser at the URL found with the following command:

$ kubectl get ksvc sockeye -o=jsonpath='{.status.url}'

Event producers

Next, create the two PingSources to produce CloudEvents by saving the following YAML manifests in two separate files and applying them to your Kubernetes cluster with kubectl apply:

apiVersion: sources.knative.dev/v1
kind: PingSource
metadata:
  name: ps-filter-demo-1
spec:
  schedule: "*/1 * * * *"
  contentType: "application/json"
  data: '{
   "name": "TriggerMesh",
    "sub": {
        "array": ["hello", "Filter"]
        }
    }'
  sink:
    ref:
      apiVersion: routing.triggermesh.io/v1alpha1
      kind: Filter
      name: filter-demo

The second source uses a different payload to show you how the Filter expression may be used to express complex filtering rules.

apiVersion: sources.knative.dev/v1
kind: PingSource
metadata:
  name: ps-filter-demo-2
spec:
  schedule: "*/1 * * * *"
  contentType: "application/json"
  data: '{
      "answer": 42
    }'
  sink:
    ref:
      apiVersion: routing.triggermesh.io/v1alpha1
      kind: Filter
      name: filter-demo

Filter events

Finally, create the Filter object to filter out events from the first PingSource. Once again save the following YAML manifest in a file and apply it to your Kubernetes cluster with kubectl apply.

apiVersion: routing.triggermesh.io/v1alpha1
kind: Filter
metadata:
  name: filter-demo
spec:
  expression: $sub.array.0.(string) == "hello" && $name.(string) != "TriggerMesh" || $answer.(int64) == 42
  sink:
    ref:
      apiVersion: serving.knative.dev/v1
      kind: Service
      name: sockeye

Verify that your filter is ready with kubectl like so:

$ kubectl get filter
NAME          ADDRESS                                                      READY   REASON
filter-demo   http://filter-adapter.sebgoa.svc.cluster.local/filter-demo   True

Only events from the second source should appear in the sockeye web interface as shown in the screenshot below:

Test your Filter as Code

You can test modifying the filter expression and re-applying it with kubectl. This gives you a declarative event filter which you can manage with your GitOps workflow

More about Filters

Learn more about Filters on the Concepts page.

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