Blue-Green Deployment: Two Versions, One Traffic Path

Minimal example

We have two Deployment objects.

apiVersion: apps/v1
kind: Deployment
metadata:
  name: api-blue
spec:
  replicas: 3
  selector:
    matchLabels:
      app: api
      version: blue
  template:
    metadata:
      labels:
        app: api
        version: blue
    spec:
      containers:
        - name: api
          image: ghcr.io/example/api:1.9.0

apiVersion: apps/v1
kind: Deployment
metadata:
  name: api-green
spec:
  replicas: 3
  selector:
    matchLabels:
      app: api
      version: green
  template:
    metadata:
      labels:
        app: api
        version: green
    spec:
      containers:
        - name: api
          image: ghcr.io/example/api:2.0.0

Traffic is selected by Service.

apiVersion: v1
kind: Service
metadata:
  name: api
spec:
  selector:
    app: api
    version: blue
  ports:
    - port: 80
      targetPort: 3000

Switching to green means changing the selector:

selector:
  app: api
  version: green

In practice, this can be done through GitOps, a patch, rollout orchestration, or routing above the Service.

The mechanism is the same:

the new version was ready before the switch

What it is really for

Blue-green is useful when we want to separate:

deploying the new version
switching users to it

You can test green before production traffic reaches it.

You can quickly return to blue if the bug is in application code or configuration.

That helps services where cold start is expensive or rollback through another rollout is too slow.

Where the name lies

Blue-green sounds like full rollback.

Often, it is only traffic rollback.

If both versions use the same database, changing a Service will not undo:

schema migrations
new records
changed formats
published events
queued jobs

Blue-green needs the same compatibility discipline as rolling update.

The difference is that we do not mix versions behind one Service for a long time.

But the data is still shared unless we deliberately separate it.

So blue-green is a traffic switching strategy.

It is not magic system rewind.