SQL Server AlwaysOn Availability Groups is a technology that was initially released by Microsoft with SQL Server 2012.
The main concept is that you have groups of databases that have primary replicas and secondary replicas. These replicas are hosted on different servers and can be synchronized in two modes:
- Asynchronous-Commit Availability Mode
- Synchronous-Commit Availability Mode
With asynchronous-commit mode, the secondary replica is never fully synchronized with the primary replica. The reason for this, is that due to the asynchronous commit nature, any secondary database could lag behind at any point. This means that if you are using the asynchronous-commit mode and perform a forced failover (this is the only form of failover you can perform), then is highly possible to have data loss.
With synchronous commit mode, after being joined to an availability group, a secondary database catches up to the corresponding database and enters the “Synchronized” state. The prerequisite for this of course, is that data synchronization (and thus, data movement) continues and it is not stopped for any reason (i.e. network issues, disk space issues, etc.). If data synchronization continues and synchronous-commit mode is used, then every transaction that is committed on a given primary database has also been committed on the secondary database.
The below diagram illustrates a setup of SQL Server Always On Availability Groups.
All the above make SQL Server Always On a very powerful high-availability solution which must be definitely considered when designing active-active data centers.
For more info, please visit Microsoft Docs.
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