Blue-Green Deployment Calculator
Calculate the doubled resource requirement during a blue-green cutover and the expected rollback time based on DNS/routing TTL.
Inputs
Steady-state CPU cores used by the current (single) environment.
Steady-state memory used by the current (single) environment.
Planned duration both blue and green environments run simultaneously before the old one is decommissioned.
TTL of the DNS record or routing rule that determines how quickly traffic can be redirected during rollback.
Extra buffer time beyond the TTL to account for caching resolvers/clients not honoring TTL exactly.
Peak CPU During Cutover
40.0cores
Peak Memory During Cutover
160.0GiB
Expected Rollback Time
1.50minutes
Extra CPU Needed
20.0cores
Extra Memory Needed
80.0GiB
Step by step
Peak CPU during cutover: 2 × normal
2 × 20
= 40 cores
Peak memory during cutover: 2 × normal
2 × 80
= 160 GiB
Rollback time: DNS TTL + propagation buffer
60s + 30s
= 90s (1.50 min)
How it works
A blue-green deployment keeps both the old (blue) and new (green) environments fully provisioned simultaneously during the cutover window, so peak resource usage doubles: resources = 2 × normal. Rollback speed is bounded by how quickly traffic can be redirected back to the stable environment, which for DNS-based cutovers is governed by the DNS record's TTL plus a buffer for resolvers/clients that cache slightly beyond the nominal TTL: rollback_time = dns_ttl + propagation_buffer.
Formula
total_capacity_during_deploy = blue_instances + green_instances
- blue_instances
- Instances in the current (blue) environment
- green_instances
- Instances in the new (green) environment
Frequently Asked Questions
Why does blue-green require double the resources?
Unlike a rolling update which replaces capacity incrementally, blue-green provisions a complete second environment before cutting over, so both environments must run at full capacity simultaneously during the overlap window to allow instant, safe rollback.
How can I reduce the doubled resource cost?
Shorten the cutover window duration (validate quickly and decommission the old environment sooner), or use a partial blue-green approach where only a subset of capacity is duplicated while traffic gradually shifts — though this starts to resemble a canary deployment.
Why does DNS TTL matter for rollback speed if I'm using a load balancer?
If cutover is done via DNS record changes (e.g. pointing a CNAME to a new load balancer), rollback speed is bounded by DNS TTL and caching resolvers; if cutover is done via a load balancer's own routing rules or service mesh virtual service, rollback can be nearly instant instead, bypassing DNS propagation delay entirely.
What's a safe propagation buffer beyond the nominal TTL?
20-50% of the TTL value is a reasonable buffer to account for resolvers and clients that don't strictly honor TTL, though some misbehaving clients or aggressive caching layers (e.g. some corporate proxies) can hold stale records far longer than the TTL suggests.
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