What is SQL Always On vs. SQL Cluster?

SQL Server introduced the Always On feature with SQL Server 2012 (version 11.x), offering high availability and disaster recovery solutions. The Always On feature utilizes technologies such as Windows Server Failover Clustering (WSFC) and SQL Server Failover Cluster Instances (FCI) to ensure continuous operation of database servers in various scenarios. So, what’s the difference between SQL Always On and SQL Cluster?

Difference between SQL Always On and SQL Cluster The fundamental distinction between SQL Always On and SQL Cluster lies in the level of high availability they provide: SQL Always On operates at the database level, whereas SQL Cluster operates at the server level.

SQL Always On is a feature that enables a database group to be shared across multiple servers, each hosting a replica of the database group, known as a replica. Replicas handle database operations, and they come in two types: primary and secondary. The primary replica accepts write operations for the database group and sends data changes to secondary replicas. Secondary replicas, on the other hand, perform only read operations or can be used for backup purposes. If the primary replica fails, one of the secondary replicas automatically or manually becomes the new primary replica, initiating a failover process.

SQL Cluster, on the other hand, involves sharing a server instance across multiple servers, each referred to as a node. A node encompasses all components of the server instance, and while the server instance runs on one node, the other nodes remain inactive. If the node running the server instance fails, one of the other nodes automatically or manually takes over and starts running the server instance, resulting in a failover process.

SQL Always On offers a solution that distributes databases across different locations, catering to disaster recovery scenarios, and is preferred for large-scale setups. On the contrary, SQL Cluster provides local high availability using a shared storage located in a single location, making it suitable for shared-scale architectures.

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