What are blue-green, canary, and rolling deployment strategies? When would you use each?

Question

At a company running 50+ microservices:

> "Our Friday deployments keep causing Saturday incidents. We need a safer deployment strategy. What are the options and trade-offs?"

Interview OS Community · Accepted Answer

## Deployment Strategies

### 1. Rolling Update (Default in Kubernetes)
```
Old pods gradually replaced by new pods
[v1][v1][v1] → [v1][v1][v2] → [v1][v2][v2] → [v2][v2][v2]

✅ Zero downtime, simple
❌ Old and new versions run simultaneously during rollout
❌ Can't easily rollback if v2 has a data migration issue
```

### 2. Blue-Green Deployment
```
Blue (current): [v1][v1][v1] ← all traffic
Green (new):    [v2][v2][v2] ← no traffic, being tested

Switch traffic: Blue → Green (instant)
If issues: Switch back: Green → Blue (instant rollback)

✅ Instant switch and rollback
✅ Full testing before switch
❌ Requires 2x infrastructure
❌ Database migrations need backward compatibility
```

### 3. Canary Deployment
```
[v1][v1][v1][v2]
 80% traffic ↑↑↑  20% ↑

Monitor metrics for 5-30 minutes:
  - Error rate normal? → Increase v2 to 50% → 100%
  - Error rate spike? → Route all traffic back to v1

✅ Real user testing with blast radius limited
✅ Gradual confidence building
❌ Requires traffic splitting (Istio, load balancer)
❌ More complex setup
```

### 4. Feature Flags (safest)
```typescript
// Deploy v2 code with flag OFF
if (featureFlags.isEnabled("new-checkout", user.id)) {
  return newCheckout(cart);
}
return legacyCheckout(cart);

// Enable for 1% of users → 5% → 25% → 100%
// Kill switch: disable flag = instant "rollback" without redeployment
```

## Decision Matrix

| Strategy | Downtime | Cost | Rollback | Complexity |
|----------|----------|------|----------|------------|
| Rolling | None | 1x | Slow | Low |
| Blue-Green | None | 2x | Instant | Medium |
| Canary | None | 1.2x | Fast | High |
| Feature Flags | None | 1x | Instant | Medium |

**Start with**: Feature flags + rolling updates. Add canary when you have monitoring mature enough to auto-detect issues.

Interview OS

What are blue-green, canary, and rolling deployment strategies? When would you use each?

Question Details

Suggested Solution

Deployment Strategies

1. Rolling Update (Default in Kubernetes)

2. Blue-Green Deployment

3. Canary Deployment

4. Feature Flags (safest)

Decision Matrix

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