What are SQL Server Always On Availability Groups?

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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.

What are SQL Server Always On Availability Groups? - Article on TechHowTos.com
SQL Server AlwaysOn Availability Groups – Diagram by MS Data Platform MVP Artemakis Artemiou

 

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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About Artemakis Artemiou 33 Articles
Artemakis Artemiou is a Senior SQL Server and Software Architect, Author, and a former Microsoft Data Platform MVP (2009-2018). He has over 15 years of experience in the IT industry in various roles. Artemakis is the founder of SQLNetHub and TechHowTos.com. Artemakis is the creator of the well-known software tools Snippets Generator and DBA Security Advisor. Also, he is the author of many eBooks on SQL Server. Artemakis currently serves as the President of the Cyprus .NET User Group (CDNUG) and the International .NET Association Country Leader for Cyprus (INETA). Artemakis's official website can be found at aartemiou.com.