A global e-commerce company is planning to migrate its three-tier web application to Azure. The primary design goals are to achieve high availability with disaster recovery capabilities across two Azure regions (e.g., East US and West Europe), ensure low-latency access for a global user base, and secure the application from common web exploits. The application consists of a web front-end, an application logic tier, and a relational database backend. Propose a comprehensive Azure infrastructure solution that meets these requirements, explaining the role and justification for each selected component.

Recommended High-Availability Multi-Region Architecture

To meet the stringent requirements of a global e-commerce platform, a robust, resilient, and secure architecture is paramount. The proposed solution leverages a combination of Azure's Platform as a Service (PaaS) offerings to maximize availability and performance while minimizing operational overhead. The core design involves deploying active-active application instances across two geographically distant Azure regions, fronted by a global traffic manager, and supported by a geo-replicated database backend.

Global Traffic Management and Security: Azure Front Door

The primary entry point for all user traffic should be Azure Front Door Premium. This service is a modern cloud Content Delivery Network (CDN) that provides global load balancing and web application acceleration. Its selection is justified for several key reasons:

• Global Load Balancing: Front Door operates at Layer 7 and uses priority-based and latency-based routing. It will automatically direct users to the closest and healthiest Azure region (East US or West Europe), ensuring the lowest possible latency and a seamless user experience. This directly addresses the global low-latency requirement.
• Web Application Firewall (WAF): The Premium SKU of Azure Front Door includes a fully managed, integrated Web Application Firewall. This WAF can be configured with managed rule sets to protect the application from common threats like SQL injection, cross-site scripting (XSS), and other OWASP Top 10 vulnerabilities, fulfilling the security requirement.
• SSL Offloading: Front Door can manage SSL certificates and terminate SSL/TLS connections at the edge, which reduces the computational load on the backend application servers and simplifies certificate management.
• Health Probes: It continuously monitors the health of the backend application instances in each region. If an entire region becomes unavailable, Front Door will automatically route all traffic to the remaining healthy region, providing a critical component of the disaster recovery strategy.

Web and Application Tiers: Azure App Service

For hosting the web front-end and application logic, Azure App Service is the ideal choice. As a fully managed PaaS offering, it abstracts away the underlying virtual machines, operating systems, and web servers, allowing developers to focus on the application code.

• Regional Redundancy: An App Service Plan will be deployed in each of the two regions (East US and West Europe). The application code will be deployed to App Service instances in both plans. This active-active deployment is the foundation of the high-availability design.
• Scalability: App Service supports both manual and automatic scaling (autoscale). You can configure rules to scale out (add more instances) based on metrics like CPU utilization or request count to handle traffic spikes during sales events, and scale in to save costs during off-peak hours.
• Secure Connectivity: To secure communication with the database, the App Service can be integrated with an Azure Virtual Network (VNet). Using a Private Endpoint for the database ensures that traffic between the application and the database never traverses the public internet.

Database Tier: Azure SQL Database with Auto-Failover Groups

The relational database backend requires a solution that matches the high-availability and disaster recovery posture of the application tiers. Azure SQL Database is the recommended managed relational database service.

• Geo-Replication and Failover: To meet the multi-region DR requirement, an Auto-Failover Group should be configured. This feature manages the replication and failover of a group of databases to a secondary region. It provides a single read-write listener endpoint and a single read-only listener endpoint that remain unchanged during a failover. The application's connection string points to this listener, so no application changes are needed if a failover occurs.
• High SLA: By selecting the Business Critical service tier, the e-commerce platform benefits from the highest resilience and performance. This tier uses multiple replicas within the primary region for high availability and provides a very low Recovery Point Objective (RPO) and Recovery Time Objective (RTO) for geo-failover events.
• Security: Access to the Azure SQL Database should be restricted using a Private Endpoint. This gives the database a private IP address within your VNet, ensuring that it is only accessible from authorized resources like the integrated App Service, thereby enhancing the overall security posture.