SQL Server Availability Groups are a high availability and disaster recovery solution that provides an enterprise-level alternative to database mirroring. Introduced in SQL Server 2012, Availability Groups maximize the availability of a set of user databases for an enterprise. An Availability Group supports a failover environment for a set of user databases, known as availability group or AG databases, that fail over together. An Availability Group supports a set of read-write primary databases and one to eight sets of corresponding secondary databases. The number of supported databases varies by which edition of SQL Server you are running – Standard or Enterprise Edition.
High availability Availability Groups can provide automatic failover to a secondary replica if the primary replica fails. This helps to ensure that applications can continue to access databases even if there is a hardware failure or other unplanned outage. This can be used to replicate databases to a remote site, providing protection against site-wide disasters. It can also be used to distribute read-only workloads to secondary replicas, which can help to improve performance and scalability for applications.
Availability Groups can be configured with synchronous or asynchronous replication. So, what is the difference?
Synchronous replication: In synchronous replication, the primary replica waits for an acknowledgment from all secondary replicas before committing a transaction. This ensures that all replicas are always in sync, but it can also impact performance, especially if the secondary replicas are located far away from the primary replica.
Asynchronous replication: In asynchronous replication, the primary replica does not wait for an acknowledgment from the secondary replicas before committing a transaction. This allows for higher performance, but it also means that the secondary replicas may not be completely up-to-date with the primary replica.
When to use synchronous replication:
- When you need the highest possible data consistency
- When you can tolerate some performance impact
- When your secondary replicas are located close to the primary replica
When to use asynchronous replication:
- When you need the highest possible performance
- When you can tolerate some data loss
- When your secondary replicas are located far away from the primary replica
- When you are using Always On availability groups for disaster recovery
In general, synchronous replication is recommended for production environments where data consistency is a top priority. Asynchronous replication is recommended for development and test environments, and for production environments where performance is a top priority and data loss can be tolerated.
To create an Always On Availability Group, you must first create a Windows Server Failover Cluster (WSFC). Once you have created a WSFC, you can use the CREATE AVAILABILITY GROUP using Transact-SQL (T-SQL) statement to create the AG or thru the SSMS GUI. The CREATE AVAILABILITY GROUP statement specifies the name of the AG, the databases to include in the AG, and the replicas for the AG.
Hope this gives you some important information on SQL Server Availability Groups.