What Feature Does Azure Availability Sets Provide?

In an Azure Availability Set, when you have multiple virtual machines (VMs) deployed within the same set, the primary purpose is to increase high availability and fault tolerance. If one VM in the set experiences an issue or fails, the others can continue to provide the service. However, the behavior of failover and how quickly the service is restored depends on various factors, including the type of application and its specific configuration.

Here’s how it generally works:

1. Distribution Across Fault and Update Domains: VMs within an Availability Set are distributed across different fault domains and update domains. This distribution helps protect against localized hardware failures and maintenance activities.
2. Load Balancing: To achieve high availability for services like a website, it’s recommended to use an Azure Load Balancer in conjunction with the Availability Set. The Load Balancer distributes incoming traffic across the healthy VM instances. If one VM (e.g., server A) fails, the Load Balancer can redirect traffic to the other healthy VMs (e.g., server B). This can help ensure that the service remains available even when one VM goes down.
3. Application Configuration: For your website to provide continuous service in the event of a VM failure, your application needs to be designed with failover in mind. This may involve things like session state management, data replication, and ensuring that the application can continue to function on the remaining VM(s) without interruption.
4. Database and Data Considerations: If your application relies on a database, ensure that the database itself is highly available. Azure provides database services with built-in high availability features.
5. Monitoring and Auto-Scaling: Implement monitoring and auto-scaling to automatically replace failed VMs or add more VMs if necessary to maintain service levels.

So, while an Azure Availability Set and Load Balancer can provide a foundation for high availability, achieving instant failover and uninterrupted service during a VM failure often requires additional considerations in the application design and architecture. It’s not automatic, but it’s a framework that, when used correctly, can help minimize downtime and provide business continuity.