Architecture: cost optimisation, blue-green deployment, reduce to 3 projects

- Reduce from 4 to 3 GCP projects (drop sandbox, use staging namespaces)
- Add blue-green deployment strategy via Argo Rollouts
- Add cost optimisation section with monthly estimate (~$175-245)
- Add blue-green flow diagram and cost pie chart to HLD

Co-authored-by: Cursor <cursoragent@cursor.com>

docs/architecture-design-company-inc.md

@@ -10,7 +10,7 @@
 
 This document outlines a robust, scalable, secure, and cost-effective infrastructure design for Company Inc., a startup deploying a web application with a Python/Flask REST API backend, React SPA frontend, and MongoDB database. The design leverages **Google Cloud Platform (GCP)** with **GKE (Google Kubernetes Engine)** as the primary compute platform.
 
-**Key Design Principles:** Security-by-default, scalability from day one, cost optimization for early stage, and GitOps-based operations.
+**Key Design Principles:** Cost awareness from day one, security-by-default, scalability when needed, and GitOps-based operations.
 
 ---
 
@@ -20,20 +20,26 @@ This document outlines a robust, scalable, secure, and cost-effective infrastruc
 
 **Rationale:** GCP offers strong managed Kubernetes (GKE) with autopilot options, excellent MongoDB Atlas integration (or GCP-native DocumentDB alternatives), competitive pricing for startups, and simplified networking. GKE Autopilot reduces operational overhead for a small team with limited Kubernetes expertise.
 
-### 2.2 Multi-Project Structure
+### 2.2 Project Structure (Cost-Optimised)
+
+For a startup, fewer projects mean lower overhead and simpler billing. Start with **3 projects** and add more only when traffic or compliance demands it.
 
 | Project | Purpose | Isolation |
 |---------|---------|-----------|
 | **company-inc-prod** | Production workloads | High; sensitive data |
-| **company-inc-staging** | Staging / pre-production | Medium |
-| **company-inc-shared** | CI/CD, shared tooling, DNS | Low; no PII |
-| **company-inc-sandbox** | Dev experimentation | Lowest |
+| **company-inc-staging** | Staging, QA, and dev experimentation | Medium |
+| **company-inc-shared** | CI/CD, Artifact Registry, DNS | Low; no PII |
+
+**Why not 4+ projects?**
+- A dedicated sandbox project adds billing, IAM, and networking overhead with little benefit at startup scale.
+- Developers can use Kubernetes namespaces within the staging cluster for experimentation.
+- A fourth project can be introduced later when team size or compliance (SOC2, HIPAA) requires it.
 
 **Benefits:**
-- Billing separation per environment
+- Billing separation (prod costs are clearly visible)
 - Blast-radius containment (prod issues do not affect staging)
-- IAM and network isolation
-- Aligns with GCP best practices for multi-tenant or multi-env setups
+- IAM isolation between environments
+- Minimal fixed cost — only 3 projects to manage
 
 ---
 
@@ -96,14 +102,36 @@ flowchart TD
 - **Frontend (React):** Static assets served via CDN or container; 1–2 replicas
 - **Ingress:** GKE Ingress for HTTP(S) routing; consider GKE Gateway API for advanced use
 
-### 4.4 Containerisation and CI/CD
+### 4.4 Blue-Green Deployment
+
+Zero-downtime releases without duplicating infrastructure. Both versions run inside the **same GKE cluster**; the load balancer switches traffic atomically.
+
+```mermaid
+flowchart LR
+    LB[Load Balancer]
+    LB -->|100% traffic| Green[Green — v1.2.0<br/>current stable]
+    LB -.->|0% traffic| Blue[Blue — v1.3.0<br/>new release]
+    Blue -.->|smoke tests pass| LB
+```
+
+| Phase | Action |
+|-------|--------|
+| **Deploy** | New version deployed to the idle slot (blue) |
+| **Test** | Run smoke tests / synthetic checks against blue |
+| **Switch** | Update Service selector or Ingress to point to blue |
+| **Rollback** | Instant — revert selector back to green (old version still running) |
+| **Cleanup** | Scale down old slot after confirmation period |
+
+**Cost impact:** Near-zero — both slots share the same node pool; the idle slot consumes minimal resources until traffic is switched. Argo Rollouts automates the full lifecycle within ArgoCD.
+
+### 4.5 Containerisation and CI/CD
 
 | Aspect | Approach |
 |-------|----------|
 | **Image build** | Dockerfile per service; multi-stage builds; non-root user |
-| **Registry** | Artifact Registry (GCR) in `company-inc-shared` |
-| **CI** | GitHub Actions (or GitLab CI) — build, test, security scan |
-| **CD** | ArgoCD or Flux — GitOps; app of apps pattern |
+| **Registry** | Artifact Registry in `company-inc-shared` |
+| **CI** | GitHub/Gitea Actions — build, test, security scan |
+| **CD** | ArgoCD + Argo Rollouts — GitOps with blue-green strategy |
 | **Secrets** | External Secrets Operator + GCP Secret Manager |
 
 ---
@@ -138,7 +166,29 @@ flowchart TD
 
 ---
 
-## 6. High-Level Architecture Diagram
+## 6. Cost Optimisation Strategy
+
+| Lever | Approach | Estimated Savings |
+|-------|----------|-------------------|
+| **3 projects, not 4** | Drop sandbox; use staging namespaces | ~25% fewer fixed project costs |
+| **GKE Autopilot** | Pay per pod, not per node; no idle nodes | 30–60% vs standard GKE |
+| **Blue-green in-cluster** | No duplicate environments for releases | Near-zero deployment cost |
+| **Spot/preemptible pods** | Use for staging and non-critical workloads | Up to 60–80% off compute |
+| **Committed use discounts** | 1-year CUDs once baseline is established | 20–30% off sustained use |
+| **CDN for frontend** | Offload SPA traffic from GKE | Fewer pod replicas needed |
+| **MongoDB Atlas auto-scale** | Start M10; scale up only when needed | Avoid over-provisioning |
+| **Cloud NAT shared** | Single NAT in shared project | Avoid per-project NAT cost |
+
+**Monthly cost estimate (early stage):**
+- GKE Autopilot (2–3 API pods + 1 SPA): ~$80–150
+- MongoDB Atlas M10: ~$60
+- Load Balancer + Cloud NAT: ~$30
+- Artifact Registry + Secret Manager: ~$5
+- **Total: ~$175–245/month**
+
+---
+
+## 7. High-Level Architecture Diagram
 
 ```mermaid
 flowchart TB
@@ -169,16 +219,17 @@ flowchart TB
 
 ---
 
-## 7. Summary of Recommendations
+## 8. Summary of Recommendations
 
 | Area | Recommendation |
 |------|----------------|
-| **Cloud** | GCP with 4 projects (prod, staging, shared, sandbox) |
+| **Cloud** | GCP with 3 projects (prod, staging, shared) |
 | **Compute** | GKE Autopilot, private nodes, HPA |
+| **Deployments** | Blue-green via Argo Rollouts — zero downtime, instant rollback |
 | **Database** | MongoDB Atlas on GCP with multi-AZ, automated backups |
-| **CI/CD** | GitHub Actions + ArgoCD/Flux |
+| **CI/CD** | GitHub/Gitea Actions + ArgoCD |
 | **Security** | Private VPC, TLS everywhere, Secret Manager, least privilege |
-| **Cost** | Start small; use committed use discounts as usage grows |
+| **Cost** | ~$175–245/month early stage; spot pods, CUDs as traffic grows |
 
 ---