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+# High Availability Architecture
+
+This document outlines the **high availability design** for the application infrastructure.
+
+Goals:
+
+* Primary hosting in **Microsoft Azure**
+* Automatic failover to **on-premise infrastructure**
+* Minimal downtime
+* Disaster recovery backups through **CBM Technology** / **Rader Solutions**
+
+---
+
+# 1. Architecture Overview
+
+```mermaid
+flowchart TB
+
+A[Architecture Overview]
+
+A --> B[Primary Cloud Azure]
+A --> C[Secondary On Prem]
+A --> D[Tertiary CBM Backup]
+
+B --> B1[Azure Application Gateway]
+B --> B2[Main Application Server]
+B --> B3[Primary Database]
+B --> B4[External Access with IP Restrictions]
+
+C --> C1[Replica Application Server]
+C --> C2[Database Replica]
+C --> C3[File Storage Replica]
+C --> C4[Used During Internet Loss]
+
+D --> D1[Daily Snapshots of Azure Primary]
+D --> D2[Disaster Recovery Scenario]
+```
+
+---
+
+# 2. Architecture Flow Diagram
+
+```mermaid
+flowchart LR
+
+PC[User PC]
+
+FW[On Prem Firewall Reverse Proxy]
+
+VPN[Site to Site VPN]
+
+INT[Internet]
+
+AZGW[Azure Application Gateway]
+
+AZAPP[Azure Application Server]
+
+AZDB[(Primary Database Azure)]
+
+OPDB[(Replica Database On Prem)]
+
+OPAPP[On Prem Application Server]
+
+PC --> FW
+FW --> VPN
+VPN --> INT
+INT --> AZGW
+AZGW --> AZAPP
+AZAPP --> AZDB
+
+AZDB --> OPDB
+
+OPDB --> OPAPP
+
+FW --> OPAPP
+```
+
+Normal operation routes traffic to Azure.
+
+If the office internet fails, the firewall redirects users to the on-prem server.
+
+---
+
+# 3. DNS Connection
+
+```mermaid
+flowchart LR
+
+PC[User PC]
+
+GW[Gateway Firewall]
+
+DNS[app.avexmro.com]
+
+AZ[Azure Cloud Server]
+
+OP[On Prem Server]
+
+PC --> GW
+GW --> DNS
+
+DNS -->|WAN UP| AZ
+DNS -->|WAN DOWN| OP
+```
+
+Behavior:
+
+Normal operation
+
+```
+app.avexmro.com → Azure
+```
+
+Internet outage
+
+```
+app.avexmro.com → On Prem
+```
+
+WAN state detection is handled by the gateway firewall.
+
+---
+
+# 4. Database and File Replication
+
+```mermaid
+flowchart LR
+
+AZDB[(Azure Primary Database)]
+
+OPDB[(On Prem Replica Database)]
+
+AZFILES[Azure File Storage]
+
+OPFILES[On Prem File Storage]
+
+AZDB -->|Secure Replication| OPDB
+
+AZFILES -->|Continuous File Sync| OPFILES
+```
+
+Normal state
+
+```
+Azure → On Prem replication
+```
+
+Failover state
+
+```
+On Prem promoted to primary
+Replication paused
+```
+
+Recovery state
+
+```
+On Prem → Azure resync
+Azure resumes primary role
+```
+
+---
+
+# 5. Monitoring Layer
+
+```mermaid
+flowchart TB
+
+MON[Health Monitoring System]
+
+AZAPP[Azure Application Server]
+AZDB[Azure Database]
+
+OPAPP[On Prem Application Server]
+OPDB[On Prem Database]
+
+ALERT[Alerting and Failover Trigger]
+
+MON --> AZAPP
+MON --> AZDB
+MON --> OPAPP
+MON --> OPDB
+
+MON --> ALERT
+```
+
+Monitoring ensures:
+
+* replication health
+* server availability
+* automatic failover triggers
+
+Possible tools:
+
+* Azure Monitor
+* Zabbix
+* Uptime Kuma
+* Prometheus
+
+---
+
+# 6. Failover State Machine
+
+```mermaid
+flowchart TD
+
+A[Normal Operation Azure Primary]
+
+B[Internet Loss Detected]
+
+C[Local Failover Mode On Prem Primary]
+
+D[Internet Restored]
+
+E[Data Resynchronization]
+
+F[Return to Normal Azure Primary]
+
+A --> B
+B --> C
+C --> D
+D --> E
+E --> F
+```
+
+Failover sequence:
+
+1. Azure operates as primary
+2. Internet outage detected
+3. On-prem database promoted
+4. Office operates locally
+5. Internet restored
+6. Data resynchronizes to Azure
+7. Azure resumes primary role
+
+---
+
+# Reliability Model
+
+Expected behavior during failure scenarios:
+
+| Event                  | Result                     |
+| ---------------------- | -------------------------- |
+| Azure VM failure       | Azure HA handles           |
+| Azure maintenance      | No downtime                |
+| Office internet outage | Automatic on-prem failover |
+| Local server failure   | Cloud still available      |
+| Database corruption    | Restore from CBM backup    |
+| Company power outage   | Service interruption       |
+
+---