In a couple of weeks ago, one of my colleagues discussed with me about some solutions for High Availability and Disaster Recovery in Microsoft SQL Server. However, it seems there were some misunderstanding about Proof of Concepts (POC) among Window Failover Clustering (WSFC), SQL Server Failover Cluster Instances (FCIs) and AlwaysOn Availability Groups.

In this article, I would like to provide basic concepts that can help everyone distinguish what they are and when we should use effectively.

Windows Server Failover Clustering (WSFC)

WSFC cluser is a group of independent servers that work together to increase the availability of applications or services. Each server is called a node in a WSFC cluster and if an application fails or services crashes that were hosted on that node, then they are transferred automatically or manually to another node in the WSFC cluster in process named failover.

Before creating a WSFC cluster, we must enable WSFC on our server that provides infrastructure features that supports high-availability and disaster recovery scenarios of hosted server applications or services such as SQL Server or Microsoft Exchange. This feature is turned off by default when installing Windows Server operation system, to enable this feature click here

SQL Server Failover Cluster Instances (FCIs) or AlwaysOn Failover Cluster Instances

An FCI is high-availability and disaster recovery solution at SQL Server Instance level, it mean SQL Server instance installed across nodes in a WSFC cluster. It leverages WSFC feature to provide local high availability through redundancy at server-instance level.

AlwaysOn Availability Groups

This feature provides high-availability and disaster recovery solution at user-database level. It is similar with the combination between Database Mirroring and WSFC solution. It requires that SQL Server Instance of user-databases resides on WSFC nodes.

Recommended Solution

Both FCI and AlwaysOn Availability Groups require enabled WSFC on nodes in a WSFC cluster, they leverage WSFC functionality. The question when we use FCI and when we use AlwaysOn Availability Groups or when we can combine FCI + AlwaysOn Availability Groups

SQL Server Failover Instances (FCIs) AlwaysOn Availability Groups
Required WSFC Yes Yes
Protection Data Level Instance Database
Storage Type Shared Non-Shared
Storage Solution Direct Attached, SAN, mount point, SMB Depend on node type
Readable Secondaries No Yes
Failed-Over resources Server, Instance and Database Database Only
Applicable fail-over for policy settings WSFC Quorum
FCI-specific
Availability group settings
WSFC Quorum
Availability group settings

The key points are what I highlighted in the table, it is as reference table when we consider to use FCIs or AlwaysOn Availability Groups. For example, my customer needs High Availability and Disaster Recovery solution and they are also able to query data on secondaries so I proposed WSFC + AlwaysOn Availability Groups on EC2 of AWS platform.

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