High availability with SQL Server 2012 and 2014

Always Ready

The need for high-availability features with Microsoft's SQL Server is growing fast, especially in the age of the cloud. However, instead of providing the much-needed high-availability features to a wide user base, Microsoft has scrapped database mirroring and restricted its successor, the AlwaysOn high-availability groups introduced with SQL Server 2012 and upgraded in SQL Server 2014, to the Enterprise Edition.

Better Safe than Sorry

High availability (HA) maximizes the accessibility of database servers by mitigating the effect of hardware damage, compensating for the failure of virtualized IaaS services or buffering failure of the network infrastructure. Thanks to high-availability features, downtime can be minimized from the perspective of the end user, and you can largely rule out the loss of transaction data.

High availability of a database, however, is difficult to achieve because you must always ensure the integrity of transactional data. If this integrity is lost, things can quickly become expensive. In the 2012 version, MS SQL Server met these requirements with up to three high-availability features, depending on the configuration:

Database mirroring
AlwaysOn Failover Cluster Instances
AlwaysOn Availability Groups (only available in the Enterprise Edition)

Additionally, features like database replication and log forwarding guarantee data backup in an emergency.

Database Mirroring Discontinued

Synchronous database mirroring in MS SQL Server is a high-availability solution, whereas asynchronous database mirroring (only available in the Enterprise Edition) is a solution for restoring data in an emergency. In any case, the database mirroring environment consists of two instances with local storage: a principal server with the principal database, and a mirror server

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