Always on Availability Groups is a solution which improves database availability and supports a failover environment for a discrete set of user databases.
It is introduced in SQL server 2012.
- With the release of Windows Server 2016, Windows Server Failover Clusters will no longer require that all nodes in a cluster reside in the same domain.
- You can now form a WSFC (windows server failover cluster) cluster from machines which are in workgroups. Because of this change, SQL Server 2016 is now able to deploy Always On Availability Groups in environments with:
- All nodes in a single domain
- Nodes in multiple domains with full trust
- Nodes in multiple domains with no trust
- Nodes in no domain at all
- Availability Modes:
- Asynchronous-commit mode: This availability mode is a disaster-recovery solution that works well when the availability replicas are distributed over considerable distances.
- Synchronous-commit mode. This availability mode emphasizes high availability and data protection over performance, at the cost of increased transaction latency. A given availability group can support up to three synchronous-commit availability replicas, including the current primary replica.
- Always On Availability Groups Supports up to nine availability replicas.
- Zero data loss protection in case of synchronous data flow.
- Failover of multiple databases.
- Active use of secondary replica for the purpose of backup and read only operation.
- It supports three major failover forms of availability group
- A listener is assigned to each availability group which is a serve to which client can connect in order to access a database in a primary or secondary replica of AAG.
- It supports automatic page repair in case of a page corruption.
Note: Always on another blog coming soon ……