Today, many businesses are in the process of looking for ways to leverage the hybrid cloud for high availability (HA) and disaster recovery (DR). The results from this year’s survey by RightScale found that 95 percent of respondents are now using the cloud in some capacity and that hybrid cloud adoption is at 67 percent for all businesses. SQL Server provides the core database support for the mission-critical applications used by many of these businesses, and HA and DR are two of the primary considerations when you’re implementing a business-critical database system. SQL Server has several different HA and DR capabilities but AlwaysOn Availability Groups (AGs) are SQL Server’s primary HA and DR technology.
First introduced with SQL Server 2012, SQL Server’s AlwaysOn AGs provide protection for multiple user databases with up to eight secondary replicas. The primary replica is the source or original host for the protected databases and the secondary replicas serve as a potential failover targets for the AG. SQL Server AlwaysOn AGs support two types of availability modes for the secondary replicas:
- Synchronous-commit mode – Primarily used with other local servers, this availability mode provides HA and automatic failover at the cost of increased transaction latency.
- Asynchronous-commit mode – Primarily used with geographically remote or cloud-based replicas, this availability mode is mainly used for DR. It provides lower latency but does not support automatic failover.
You have the ability to optionally configure one or more secondary replicas to support read-only access to secondary databases enabling reporting or backup to be performed from the databases on those secondary replicas.
AlwaysOn Hybrid Cloud Integration
With the release of SQL Server 2014, Microsoft added hybrid cloud support to AlwaysOn AGs, enabling you to create AlwaysOn AG replicas in Azure. SQL Server’s AlwaysOn AG support enables you to create asynchronous secondary replicas in Azure – essentially enabling Azure to function as low-cost hybrid cloud DR site. To take advantage of this capability, you need an Azure subscription, SQL Server 2014 or higher, and connectivity between your on-premises network and Azure. This type of connection requires an on-premises VPN that has an externally facing public IP address. You can find out more details about linking your on-premises network to Azure at Create a site-to-site connection using the Azure portal.
The secondary AlwaysOn AG replicas run as Azure IaaS VMs. The Azure hybrid cloud integration features are completely integrated into SQL Server and they can be configured using either SQL Server Management Studio (SSMS) or T-SQL. SSMS provides an Add Azure Replica Wizard that guides you through the process of configuring the Azure Replicas. Once the hybrid cloud replicas have been created and the replication process has been established, you can use them for DR and optionally use them to run backups or cloud-based reports. SQL Server’s built-in cloud functionality makes it easy to integrate the hybrid cloud into your HA and DR strategy.