Rolling Deployment

Also Known As

TL;DR

Explanation

A rolling deployment updates servers sequentially — draining a server of active connections, deploying the new version, verifying health, then moving to the next. At any point during the rollout, both old and new versions serve traffic simultaneously. This means: database migrations must be backward compatible with both versions, API contracts must not have breaking changes, and feature flags may be needed to gate new behaviour. Rolling deployments are simpler than blue/green (no duplicate infrastructure) but slower to roll back — you must roll forward through the remaining servers or trigger a reverse rolling update. Kubernetes performs rolling updates natively via Deployment resources.

Common Misconception

Why It Matters

Common Mistakes

• Database migrations that break the old code version — both versions run simultaneously during a rolling deploy.
• Not handling mixed-version traffic in the application — old and new API contracts must coexist during rollout.
• Rolling too fast — updating all servers before monitoring shows the new version is healthy.
• No rollback plan — if the new version has issues, rolling back means going through the same process in reverse.

Code Examples

# Rolling deploy with breaking DB migration — causes errors:
# Step 1: Run migration that renames column 'user_name' to 'username'
# Step 2: Start deploying new code that uses 'username'
# Step 3: Old code still running on 50% of servers uses 'user_name' — errors!
# Fix: expand-contract — add 'username', dual-write, then remove 'user_name'

# Kubernetes — rolling update (default strategy)
apiVersion: apps/v1
kind: Deployment
spec:
  replicas: 4
  strategy:
    type: RollingUpdate
    rollingUpdate:
      maxUnavailable: 1   # at most 1 pod down at a time
      maxSurge: 1         # at most 1 extra pod above replica count
  template:
    spec:
      containers:
      - name: app
        image: myapp:v2.5.0  # change this to trigger rolling update
        readinessProbe:
          httpGet: { path: /health, port: 9000 }
          # pod only receives traffic once /health returns 200

# kubectl rollout status deployment/myapp
# kubectl rollout undo deployment/myapp   ← instant rollback

References

• ↗ https://kubernetes.io/docs/tutorials/kubernetes-basics/update/update-intro/

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