# 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       |

---