Concept

You maintain two identical environments:

• Blue = current production
• Green = new version

You deploy updates to the Green environment and then switch traffic from Blue → Green instantly (usually via load balancer update or DNS switch).

How It Works

1. Deploy new version to Green.
2. Run all tests in isolation.
3. Flip traffic from Blue → Green.
4. Keep Blue as backup for instant rollback.

Pros

✅ Zero downtime deployment

✅ Instant rollback (switch back to Blue)

✅ Perfect for risky features or major changes

✅ Safe testing in a production‑like environment

Cons

❌ Requires double the infrastructure

❌ May increase cost

❌ Database schema changes can be tricky

❌ Not ideal for frequent small updates

Best for

• Critical applications where downtime = loss
• Major version upgrades
• Teams needing strong rollback safety
• Traditional servers, ECS, EKS, EC2, RDS-backed apps

Rolling Deployment

Concept

You update your application gradually, replacing old instances with new ones one batch at a time.

Example:

10 servers → update 2 at a time → until all run the new version.

How It Works

1. Start with a mix of old & new versions.
2. Gradually update instances over time.
3. Traffic is served from both until rollout completes.

Pros

✅ No need for duplicate environments

✅ Lower cost than Blue‑Green

✅ Good for microservices and container workloads

✅ Ideal for frequent, small updates

✅ Supported natively by Kubernetes & AWS ECS

Cons

❌ More complex rollback (need to roll back batches)

❌ Temporary version mismatch (old vs new running together)

❌ Errors may be introduced before detection

❌ Testing is harder because environments are mixed

Best for

• Continuous deployments (CD)
• Microservices / Kubernetes / ECS
• Apps that handle version mismatches
• Frequent incremental updates

When to Choose What?

✔ Choose Blue‑Green if:

• You want maximum safety
• You can afford duplicate infrastructure
• You’re deploying major features or breaking changes
• You need instant rollback

✔ Choose Rolling if:

• You deploy often
• You want a cost-effective strategy
• Your system tolerates mixed versions
• You use Kubernetes, ECS, or microservices

Final Takeaway

Blue‑Green = Safer, costlier, instant rollback

Rolling = Cheaper, gradual, ideal for continuous deployments

Both are powerful — the right strategy depends on your team’s risk tolerance, architecture, and release frequency.