SQL AlwaysOn High Availability step by step Part 1 

AlwaysOn Availability Groups (SQL Server)
The AlwaysOn Availability Groups feature is a high-availability and disaster-recovery solution that provides an enterprise-level alternative to database mirroring. Introduced in SQL Server 2012, AlwaysOn Availability Groups maximizes the availability of a set of user databases for an enterprise. An availability group supports a failover environment for a discrete set of user databases, known as availability databases, that fail over together. An availability group supports a set of read-write primary databases and one to four sets of corresponding secondary databases. Optionally, secondary databases can be made available for read-only access and/or some backup operations.
An availability group fails over at the level of an availability replica. Failovers are not caused by database issues such as a database becoming suspect due to a loss of a data file, deletion of a database, or corruption of a transaction log.
In this Topic:
• Benefits
• Terms and Definitions
• Interoperability and Coexistence with Other Database Engine Features
• Related Tasks
• Related Content
Benefits
AlwaysOn Availability Groups provides a rich set of options that improve database availability and that enable improved resource use. The key components are as follows:
Supports up to five availability replicas. An availability replica is an instantiation of an availability group that is hosted by a specific instance of SQL Server and maintains a local copy of each availability database that belongs to the availability group. Each availability group supports one primary replica and up to four secondary replicas. For more information, see Overview of AlwaysOn Availability Groups (SQL Server). Important
Each availability replica must reside on a different node of a single Windows Server Failover Clustering (WSFC) cluster. For more information about prerequisites, restrictions, and recommendations for availability groups, see Prerequisites, Restrictions, and Recommendations for AlwaysOn Availability Groups (SQL Server).
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