Blue-Green vs Rolling Deployment: Ensuring Zero Downtime
Introduction
Shipping new versions without downtime is table stakes for modern platforms. Two proven release patterns—blue-green and rolling—minimize risk while keeping services online. They differ mainly in speed, infrastructure needs, and rollback mechanics.
Blue-green deployment
Maintain two identical environments: blue (live) and green (new). Validate green with smoke and integration tests, then switch traffic instantly. If problems appear, switch back just as fast.
- Pros: Instant cutover and rollback; simple blast-radius control.
- Cons: Requires duplicate infrastructure; needs careful data/state planning.
Example: Kubernetes service label switch
# Two Deployments: app-blue and app-green
# Service points at the active color via a selector.
apiVersion: v1
kind: Service
metadata: { name: app-svc }
spec:
selector: { app: myapp, color: blue } # switch to 'green' on cutover
ports: [{ port: 80, targetPort: 8080 }]Rolling deployment
Update a fleet gradually (one or a few instances at a time) while the rest keep serving. Health checks and traffic shaping ensure continuous availability, but rollback is also gradual.
- Pros: Resource-efficient; no duplicate environment; lower instantaneous risk.
- Cons: Slower cutover/rollback; requires careful orchestration of stateful components.
Example: Kubernetes rolling update strategy
apiVersion: apps/v1
kind: Deployment
metadata: { name: myapp }
spec:
replicas: 10
strategy:
type: RollingUpdate
rollingUpdate:
maxUnavailable: 1 # keep capacity
maxSurge: 1 # add one extra during update
template:
spec:
containers:
- name: web
image: registry/myapp:2.0
readinessProbe: { httpGet: { path: /healthz, port: 8080 } }Comparison
| Aspect | Blue-Green | Rolling |
|---|---|---|
| Cutover speed | Instant traffic switch | Gradual, per batch |
| Rollback | Instant (flip back) | Gradual; redeploy prior version |
| Infra cost | Higher (duplicate stack) | Lower (reuse same fleet) |
| State & data | Needs versioned schemas; dual-write or backward-compatible DB | Requires compatibility during overlap window |
| Risk isolation | Excellent (inactive env is safe) | Good (small batches limit blast radius) |
| Traffic management | Single switch (load balancer/DNS) | Per-instance readiness & health checks |
Choosing and operating
- Pick blue-green when you need immediate rollback and can afford duplicate capacity.
- Pick rolling when capacity is tight and you’re comfortable with slower, progressive rollout.
- For databases: use backward-compatible migrations (expand ? deploy ? contract), feature flags, and read-only toggles for risky paths.
- Observability: gate rollout on SLOs; add canaries and error-budget checks.
- Traffic: prefer LB/service switch for blue-green; use readiness probes and maxUnavailable/maxSurge for rolling.
Conclusion
Both patterns achieve zero-downtime releases. Blue-green favors speed and safety at higher cost; rolling favors efficiency with incremental risk control. Choose based on your infrastructure, data model, and risk tolerance.