Understanding High Availability in Azure SQL Database
High availability (HA) ensures that your Azure SQL Database remains available for your applications, even in the event of hardware failures, software updates, or unplanned downtime. Azure SQL Database is a managed database service, and its architecture is designed for high availability by default.
Core Concepts
Azure SQL Database uses a combination of technologies to provide automatic failover and minimize downtime:
- Redundancy: Data is replicated across multiple compute and storage nodes within an Azure region.
- Automatic Failover: If a primary instance experiences an issue, Azure automatically detects the problem and redirects connections to a redundant replica.
- Service Level Objectives (SLOs): Different service tiers offer varying levels of HA guarantees.
High Availability Architectures
Azure SQL Database employs different HA architectures depending on the deployment option and service tier:
For Single Databases and Elastic Pools
Azure SQL Database uses a clustered availability group architecture. Each availability group consists of multiple replicas. Your database is housed on a primary replica. There are also secondary replicas that are maintained in sync with the primary.
- Premium and Business Critical Tiers: These tiers utilize a Multi-replica technology (similar to Availability Groups) with automatic failover. The primary replica is placed on a compute node, and three secondary replicas are created for redundancy. One secondary replica is for read-scale, and two are for high availability. Automatic failover is designed to be near-zero downtime.
- General Purpose and Standard Tiers: These tiers use a Page-level replication technology. A database is spread across multiple storage nodes. If a storage node fails, Azure SQL Database automatically redirects the database to another storage node. The compute layer also has redundancy.
For Azure SQL Managed Instance
Azure SQL Managed Instance uses a similar architecture to SQL Server Failover Cluster Instances (FCIs) with Always On Availability Groups, offering high availability within a virtual cluster.
- General Purpose Tier: Uses a remote storage architecture. The compute instance and storage are separate. The compute layer has redundancy, and storage is inherently durable and available.
- Business Critical Tier: Uses local storage and is similar to the single database/elastic pool Premium/Business Critical tiers, with multiple replicas and automatic failover for maximum availability.
Configuring Availability and Durability
While Azure SQL Database is highly available by default, you can influence certain aspects:
- Choosing the Right Service Tier: Select a service tier (e.g., Premium, Business Critical) that aligns with your application's availability requirements.
- Geo-Replication: For disaster recovery and additional availability beyond a single region, consider setting up active geo-replication.
- Failover Groups: These allow you to manage failover of a group of databases to another region.
Monitoring Availability
You can monitor the availability of your Azure SQL Database through Azure Monitor and by querying dynamic management views (DMVs) within the database.
Key Benefits of Azure SQL Database HA
- Automatic Recovery: No manual intervention is required for most failure scenarios.
- Reduced Downtime: Minimize business impact from unexpected outages.
- Scalability: HA is built-in and scales with your database.
- Managed Service: Microsoft handles the complexity of maintaining HA infrastructure.