Mobility scale-up: −30% cloud bill, same SLOs.

An AWS bill that doubles in 18 months without traffic following: that's rarely growth, it's architecture debt being paid in cash.

The context

Urban mobility scale-up, 180 people, Paris and Montreal hubs, 4 million active users. Microservices platform in production for four years, platform team of 12 engineers. The CFO slammed the table: annual cloud bill exceeded 2.1M USD with 22% traffic growth over the same period.

The problem

• AWS spend x2 in 18 months for +22% traffic
• No team budget, no FinOps in place
• Systematic over-provisioning of EKS nodes (14% average CPU)
• MTTR at 47 minutes, noisy alerting, no distributed tracing
• Engineers unable to attribute a cost to a service

The approach

Six weeks of GreenOps audit, then seven months of incremental remediation. No big bang, no re-platforming. We started by measuring, then cut obvious fat, then rethought what needed rethinking.

The four workstreams

• Full observability: Prometheus, Grafana, Tempo, cost attribution per namespace via Kubecost
• Cluster Autoscaler migration to Karpenter: tight packing, spot first, aggressive consolidation
• ARM64 Graviton on 60% of stateless workloads after benchmarks
• Smart scheduling: night batches on interruptible spot, taints/tolerations reset

The stack

• Go 1.22, Kubernetes 1.29 on EKS
• Karpenter 0.34, Graviton2/3 (c7g, m7g)
• Prometheus, Grafana, Tempo, OpenTelemetry SDK
• Kubecost for attribution, Terraform for IaC

The results

1. Cloud bill: -30% at iso-SLO over 9 months (-630k USD/year)
2. MTTR: 47 min to 11 min (divided by 4)
3. Average cluster CPU: 14% to 51%
4. Traffic absorbed: +35% with no added capacity
5. Estimated carbon footprint: -38% (client Scope 3 report)

What we learned

Karpenter is a game changer but demands rigor on pod disruption budgets. ARM64 works on 60% of workloads, not 100%: some third-party C++ binaries resisted us for two months. The real sustainable lever is FinOps embedded in the team: we trained two internal relays so the drop holds after we leave.