Kubernetes Watch Triggers

A Kubernetes watch trigger invokes a function whenever a watched Kubernetes object changes (added, modified, or deleted). Use it to react to cluster events, for example to run a function each time a Pod is created or a Service is updated.

The kubewatcher component watches the resource you specify and POSTs each event to your function through the router.

How it works

flowchart LR
  subgraph k8s["Kubernetes Cluster"]
    apiserver["API Server"]:::user
    kubewatcher["KubeWatcher"]:::fission
    router["Router"]:::fission
    fnPod["Function Pod"]:::pod
  end

  apiserver -->|"<b>1.</b> object event"| kubewatcher
  kubewatcher -->|"<b>2.</b> POST event"| router
  router -->|"<b>3.</b> forwards request"| fnPod
  classDef user fill:#ffffff,stroke:#94a3b8,color:#1f2a43
  classDef fission fill:#e8f0fe,stroke:#2d70de,color:#1f2a43
  classDef pod fill:#e6f7f1,stroke:#11a37f,color:#1f2a43,stroke-dasharray:5 3

1. You create a watch trigger naming a resource type, a namespace, and (optionally) a label selector.
2. kubewatcher opens a watch on that resource against the Kubernetes API server.
3. For each event, kubewatcher POSTs the serialized object as the request body to your function.
4. The function processes the event and returns a response.

Each request carries the event and object type as HTTP headers so your function can branch on them:

• X-Kubernetes-Event-Type — ADDED, MODIFIED, or DELETED.
• X-Kubernetes-Object-Type — the object kind, for example Pod.

Create a watch trigger

The following creates a watch on Pods in the default namespace and invokes the pod-logger function on every Pod event:

$ fission watch create --name podwatch --function pod-logger --type pod --ns default

You can watch any of the following resource types (case-insensitive) via the --type flag:

• pod
• service
• replicationcontroller
• job

A watch trigger can only invoke a function in the same namespace as the trigger. The function namespace equals the trigger namespace, and cross-namespace watches are rejected.

List your watch triggers to confirm:

$ fission watch list

Related

• Triggers overview
• Functions