Relationship Between Always On Availability Groups and Windows Server Failover Clustering

 Relationship Between Always On Availability Groups and Windows Server Failover Clustering

Microsoft SQL Server Always On Availability Groups (AGs) and Windows Server Failover Clustering (WSFC) work together to provide high availability and disaster recovery for mission-critical databases. While AGs handle database-level replication and failover, WSFC provides the underlying cluster framework that manages failover operations at the server level.
WSFC as the Foundation:
Always On AGs rely on WSFC to manage the health monitoring, quorum configuration, and failover coordination of SQL Server instances.
Without WSFC, Always On AGs cannot function because the cluster ensures that a new primary replica is designated in case of failures.
AGs Provide Database-Level Protection:
Unlike Failover Cluster Instances (FCI), which protect the entire SQL Server instance, AGs protect individual databases. Each availability group consists of a primary replica (active) and up to eight secondary replicas (passive or readable).

WSFC’s Role in Failover:
WSFC continuously monitors the health of AG replicas. If the primary replica fails, WSFC triggers an automatic or manual failover to one of the secondary replicas. The cluster’s quorum configuration ensures that failover decisions are made correctly to avoid split-brain scenarios.

Step-by-Step Process of Inner Working of Always On AGs with WSFC

Step 1: Configuring the Windows Server Failover Cluster (WSFC)
Install Failover Clustering Feature on each node
Validate the Cluster Configuration to check network, storage, and system readiness. Create the WSFC using Failover Cluster Manager or PowerShell
Step 2: Enabling Always On Availability Groups on SQL Server Instances
Open SQL Server Configuration Manager → Enable "Always On Availability Groups" feature on all participating instances and Restart the SQL Server service to apply changes.
step 3: Creating an Availability Group
Select the Databases: Ensure the database is in Full Recovery Mode and take a full and transaction log backup.
Define Replicas:
Select a Primary Replica and Secondary Replicas.
Choose between synchronous (high availability) and asynchronous (disaster recovery) commit modes.

Set Up Automatic Failover (Optional):
At least two replicas must be in synchronous mode with automatic failover enabled.
Configure Readable Secondary Replicas (Optional):
Allows read-only workloads on secondary replicas.
Configure Endpoints and Listeners:
Define SQL Server endpoints (TCP 5022 default).
Create an AG Listener (Virtual Network Name with a static IP for client connections).

Step 4: Data Replication & Synchronization
SQL Server replicates transactions from the primary replica to secondary replicas.
Data is committed based on the chosen mode: Synchronous commit or
Asynchronous commit

Step 5: Health Monitoring & Failover
WSFC continuously monitors SQL Server availability.

Step 6: Manual Failover (If Needed)
Failover can also be performed manually using T-SQL or SSMS