feat: add a devops deployment guide

DEVOPS.md

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+# DevOps & Deployment Guide
+
+This document outlines the infrastructure architecture, networking, security, and CI/CD pipeline for deploying the Spring PetClinic application on AWS Fargate using Terraform and GitHub Actions.
+
+---
+
+## Table of Contents
+
+1. [Architecture Diagram](#architecture-diagram)
+2. [Infrastructure Components](#infrastructure-components)
+3. [Networking & Security](#networking--security)
+4. [CI/CD Pipeline](#cicd-pipeline)
+5. [Deployment Best Practices](#deployment-best-practices)
+6. [Quick Start](#quick-start)
+
+---
+
+## Architecture Diagram
+
+The diagram below provides an end-to-end view of the VPC layout, AWS services, and CI/CD flow:
+
+```mermaid
+flowchart LR
+  subgraph VPC [VPC (e.g. 10.0.0.0/16)]
+    direction TB
+
+    subgraph Public Subnets
+      direction LR
+      PubA[Public Subnet (AZ‑A)]
+      PubB[Public Subnet (AZ‑B)]
+    end
+
+    subgraph Private Subnets
+      direction LR
+      PrivA[Private Subnet (AZ‑A)]
+      PrivB[Private Subnet (AZ‑B)]
+    end
+
+    IGW[Internet Gateway]
+    NATGW[NAT Gateway]
+    ALB[Application Load Balancer]
+    ECSCluster[ECS Cluster]
+    ECSService[ECS Fargate Service]
+    TaskDef[Task Definition]
+    ECR[ECR Repository]
+    RDS[(RDS Database)]
+    ExecRole[(ECS Task Execution Role)]
+    SG_Fargate[(SG: Fargate Tasks)]
+    SG_RDS[(SG: RDS)]
+  end
+
+  Internet -->|80/443| ALB
+  IGW --> ALB
+  IGW --> NATGW
+  ALB -->|Target Group| ECSService
+  PubA --> ALB
+  PubB --> ALB
+  ECSService -->|Runs Task| TaskDef
+  ECSService --> SG_Fargate
+  ECSService --> ExecRole
+  PrivA --> ECSService
+  PrivB --> ECSService
+  NATGW --> PrivA
+  NATGW --> PrivB
+  RDS --> SG_RDS
+  PrivA --> RDS
+  PrivB --> RDS
+  TaskDef -->|Image| ECR
+
+  subgraph CI/CD [GitHub Actions]
+    direction TB
+    Build[Test & Package] --> Docker[Build & Push → ECR]
+    Docker --> Terraform[Terraform Apply Infra]
+    Terraform --> Register[Register Task Def]
+    Register --> Deploy[Create/Update ECS Service]
+    Deploy --> Verify[Wait for Service Stable]
+  end
+
+  CI/CD --- ALB
+  CI/CD --- ECSService
+```
+
+---
+
+## Infrastructure Components
+
+| Component                     | Purpose                                           |
+| ----------------------------- | ------------------------------------------------- |
+| **VPC**                       | Custom VPC with public & private subnets          |
+| **Internet Gateway (IGW)**    | Provides internet ingress/egress                  |
+| **NAT Gateway**               | Enables private subnet egress                     |
+| **Application Load Balancer** | Distributes HTTP/HTTPS traffic to ECS service     |
+| **ECR Repository**            | Stores Docker images for each environment         |
+| **ECS Cluster (Fargate)**     | Serverless container orchestration                |
+| **Task Definition**           | Defines container settings (CPU, memory, ports)   |
+| **ECS Service**               | Manages task count, health, and deployments       |
+| **RDS Database**              | Managed relational storage in private subnets     |
+| **Security Groups**           | Controls traffic between ALB, ECS tasks, and RDS  |
+| **IAM Roles**                 | ECS execution role for ECR pull & CloudWatch logs |
+
+---
+
+## Networking & Security
+
+* **Public Subnets**: Hosts ALB and NAT Gateway; reachable via IGW.
+* **Private Subnets**: Hosts ECS Fargate tasks and RDS; no direct internet access.
+* **Security Groups**:
+
+  * **ALB SG**: Allow inbound 80/443; outbound to ECS SG.
+  * **Fargate SG**: Allow inbound from ALB SG; outbound to RDS SG & NAT Gateway.
+  * **RDS SG**: Allow inbound only from Fargate SG.
+* **Network ACLs**: Optional subnet-level filters.
+* **IAM**: Least-privilege roles for ECS tasks and CI runners.
+
+---
+
+## CI/CD Pipeline
+
+1. **Build & Test**
+
+   * Compile code (Maven/Gradle), run unit and integration tests.
+2. **Docker Build & Push**
+
+   * Build container, tag with commit SHA, push to ECR.
+3. **Terraform Provision**
+
+   * Apply infrastructure: VPC, subnets, ALB, RDS, ECS cluster.
+4. **Register Task Definition**
+
+   * `aws ecs register-task-definition` → exports `TASK_DEF_ARN`.
+5. **Deploy Service**
+
+   * `create-service` if new, else `update-service --force-new-deployment`.
+6. **Verify Deployment**
+
+   * `aws ecs wait services-stable` ensures healthy rollout.
+7. **Cleanup (Optional)**
+
+   * Tear down dev/staging resources via `terraform destroy` and cleanup scripts.
+
+---
+
+## Deployment Best Practices
+
+* **Immutable Infrastructure**: Keep Terraform state remote and versioned.
+* **Zero‑Downtime Deployments**: Use rolling or canary strategies with ALB target groups.
+* **Scaling**: Configure ECS Service Auto Scaling on CPU/memory or custom CloudWatch metrics.
+* **Logging & Monitoring**: Centralize logs in CloudWatch; set alarms for error rates and resource usage.
+* **Secrets Management**: Store DB credentials and API keys in AWS Secrets Manager or Parameter Store.
+* **Network Segmentation**: Strict security group rules and private subnets for sensitive data.
+
+---
+
+## Quick Start
+
+1. Fork or clone the repo.
+2. Add AWS credentials and secrets in GitHub (ECR, RDS, Terraform state).
+3. Update Terraform variables (`environment`, `vpc_id`, `db_username`, `db_password`).
+4. Trigger the **Deploy PetClinic** workflow or push to `dev`/`main`.
+5. Access your app via the ALB DNS name or custom domain.
+
+---
+
+*For full development instructions, refer to [README.md](./README.md).*
+
+---
+
+*This deployment design merges best practices with automated Terraform and GitHub Actions to deliver a robust, secure, and scalable PetClinic setup on AWS Fargate.*