Watcher

What is WATCH?

In Kubernetes, WATCH means asking the API Server:

“Keep listening and tell me whenever something changes.”

Instead of repeatedly querying the API Server, WATCH opens a long-lived connection and receives events in real time.

These events describe changes, not full history.

Why WATCH exists

Kubernetes clusters are highly dynamic:

• Pods are created and deleted frequently
• Configurations change
• Nodes join and leave

Polling the API Server again and again would:

• Waste resources
• Add latency
• Miss fast changes

WATCH solves this by making Kubernetes event-driven.

Simple example

When you run:

kubectl get pods -w

• kubectl starts a WATCH request.
  Now, whenever a Pod:
  

  • is created
  • is modified
  • is deleted

  kubectl immediately prints the update.

WATCH events

A WATCH stream sends events of these types:

• ADDED – a new object was created
• MODIFIED – an existing object changed
• DELETED – an object was removed
• ERROR – something went wrong
  

Each event contains:

• The event type
• The latest version of the object

What WATCH does NOT give you

• Objects that already existed before the watch started
• A full snapshot of the current state
• Historical data (If an object already exists and does not change, WATCH will never send it.)

WATCH and resourceVersion

WATCH always starts from a specific resourceVersion.
This tells Kubernetes:- Send me all changes that happen after this point.

Using resourceVersion ensures:

• No missed events
• Correct ordering of updates

WATCH in Kubernetes architecture

Client / Controller
        ↓
     WATCH request
        ↓
     API Server
        ↓
   Event stream

The API Server pushes events as changes occur.

WATCH tells you what changes in the future, but nothing about what already exists.