Are you preparing for a SQL Server DBA job interview and want to brush up on your knowledge of SQL Server AlwaysOn? In this article, we will explore some common questions and answers related to SQL Server AlwaysOn Availability Groups to help you prepare for your next interview.
What are the differences between a SQL Server Availability Group and Replication?
SQL Server AlwaysOn Availability Group is an advanced feature introduced in SQL Server 2012 to support High Availability (HA) and Disaster Recovery (DR) solutions. On the other hand, Replication is a legacy technology that has been supported in earlier versions of SQL Server.
Here are the main differences between an Availability Group and Replication:
- AlwaysOn Availability Group uses endpoints to transfer data to their replica servers, whereas Replication uses SQL Server Agent Jobs for replicating data from the Publisher to the Distributor and then to the subscribers.
- Availability Groups transfer data directly to their secondary replicas, whereas replication first sends data to the Distributor and then the Subscribers get data from the Publisher.
- In Availability Group configurations, all secondary replicas have the same data as their primary replica. In Replication, you can customize the data and indexes on the subscribers.
- Availability Groups support automatic failover, whereas it’s not possible with Replication for HA or DR purposes.
Can we take a SQL Server Availability Group offline?
Yes, we can take a SQL Server Availability Group offline by running the ALTER statement:
ALTER AVAILABILITY GROUP group_name OFFLINE
What is “Automatic Seeding” in SQL Server AlwaysOn Availability Group?
Automatic Seeding is a term used for the automatic initialization of Availability Groups. This feature was introduced in SQL Server 2016. When you create an Availability Group with automatic seeding, SQL Server automatically creates the secondary replicas for every database in the group. You no longer have to manually backup and restore the secondary replicas.
How many secondary replicas can we configure in a SQL Server AlwaysOn Availability Group?
We can configure up to eight secondary replicas for any Availability Group.
Can we add system databases to an Availability Group?
No, we cannot configure SQL Server Availability Groups for system databases.
Can we configure automatic failover for a SQL Server Availability Group with asynchronous mode of data transfer?
No, we must use synchronous commit data transfer to configure SQL Server Availability Groups with automatic failover.
Can we change failover modes for SQL Server Availability Group replicas?
Yes, we can change failover modes of a SQL Server Availability Group by following these steps:
- In SQL Server Management Studio, navigate to the replica under the Availability Group node and launch the properties window.
- In the Availability Replica Properties dialog box, select the correct Failover mode from the drop-down menu.
- Close the windows to apply this change.
How many types of Availability Groups are there in SQL Server?
There are a few variants of Availability Groups in SQL Server:
- Basic Availability Group: AlwaysOn Basic Availability Groups replace the deprecated Database Mirroring feature and provide a similar level of features. Basic Availability Groups enable a primary database to maintain a single replica.
- Distributed Availability Group: A distributed availability group spans two separate availability groups and can be located in different locations, including cross-domain and cross-platform deployments.
- Traditional Availability Group: A traditional availability group has resources configured in a WSFC cluster.
- Read-Scale Availability Group: SQL Server 2017 introduces Read-Scale Availability Groups without a cluster, allowing for resource conservation for mission-critical workloads.
Do we need to copy SQL Server Agent Jobs and Logins for Availability Group databases?
No, SQL Server Agent Jobs and Logins will not be replicated automatically. We need to manually replicate them to secondary replicas.
What impact will there be on the AlwaysOn Availability Group if we drop and recreate the Windows cluster?
If we drop and re-create the Windows Server Failover Cluster (WSFC), we must disable and re-enable the AlwaysOn Availability Groups feature on each instance of SQL Server that hosted an availability replica on the original WSFC cluster.
Does SQL Server compress data while transferring it to a secondary replica with AlwaysOn Availability Group?
By default, SQL Server compresses data where appropriate while replicating it to a secondary replica with SQL Server AlwaysOn. However, this depends on the failover mode or type of operation chosen in AlwaysOn.
That concludes our discussion on SQL Server AlwaysOn Availability Groups. We hope these questions and answers have helped you prepare for your next SQL Server DBA job interview. Good luck!