Synchronous: Write operations are only completed when the data changes have been successfully applied to all synchronous replicas. This guarantees that all replicas always have exactly the same status and can therefore automatically take over primary operation. Since all replicas must confirm these changes, network latencies must be considered critically. Every millisecond that is lost here is reflected in every write operation. By keeping all data on all replicas synchronized, AlwaysOn Availability Groups offers the possibility of automatic failover. As with the failover cluster instances, operations can be transferred from one replica to another almost immediately.
Asynchronous: Here, applications albania telegram screening are informed of the success of a write operation as soon as this change has been completed on the primary replica; synchronization between the primary replica and other replicas occurs asynchronously. This means that data changes may have taken place on the primary replica that never reach the other replicas due to system failures. Asynchronous synchronization modes therefore rule out automatic failover. Since synchronization takes place asynchronously, network latencies are less of a problem here.
For a detailed description of the failover modes, see the Microsoft Books online documentation [2] .
AlwaysOn Availability Groups also have advantages and disadvantages:
Advantages:
Finally, the other nodes can be used! Most databases read more than they write, so the ability to distribute this read load is a real miracle.
Outsourcing maintenance tasks: With AlwaysOn Availability Groups, you can outsource the often resource-intensive backup from your primary write node without having to slow down normal operations.
Automatic data repair: AlwaysOn Availability Groups is able to detect corrupted data pages (due to hardware failure) within a database and then copies a non-corrupted version from a secondary node.
You can select specific databases instead of an entire instance: Often several databases are running on an instance, but only a few of them are "critical". With AlwaysOn Availability Groups, you can group the critical databases and treat them separately, while the other databases continue to work normally.