Business Critical tier - Azure SQL Database


Business Critical tier is called Premium in DTU purchasing model. For a comparison of the vCore-based purchasing model with the DTU-based purchasing model, see Azure SQL Database purchasing models and resources.

Azure SQL Database is based on SQL Server Database Engine architecture that is adjusted for the cloud environment in order to ensure 99.99% availability even in the cases of infrastructure failures. There are three architectural models that are used in Azure SQL Database:

  • General Purpose/Standard
  • Business Critical/Premium
  • Hyperscale

Premium/Business Critical service tier model is based on a cluster of database engine processes. This architectural model relies on a fact that there is always a quorum of available database engine nodes and has minimal performance impact on your workload even during maintenance activities.

Azure upgrades and patches underlying operating system, drivers, and SQL Server Database Engine transparently with the minimal down-time for end users.

Premium availability is enabled in Premium and Business Critical service tiers of Azure SQL Database and it is designed for intensive workloads that cannot tolerate any performance impact due to the ongoing maintenance operations.

In the premium model, Azure SQL database integrates compute and storage on the single node. High availability in this architectural model is achieved by replication of compute (SQL Server Database Engine process) and storage (locally attached SSD) deployed in four node cluster, using technology similar to SQL Server Always On Availability Groups.

Cluster of database engine nodes

Both the SQL database engine process and underlying mdf/ldf files are placed on the same node with locally attached SSD storage providing low latency to your workload. High availability is implemented using technology similar to SQL Server Always On Availability Groups. Every database is a cluster of database nodes with one primary database that is accessible for customer workload, and a three secondary processes containing copies of data. The primary node constantly pushes the changes to secondary nodes in order to ensure that the data is available on secondary replicas if the primary node crashes for any reason. Failover is handled by the SQL Server Database Engine – one secondary replica becomes the primary node and a new secondary replica is created to ensure enough nodes in the cluster. The workload is automatically redirected to the new primary node.

In addition, Business Critical cluster has built-in Read Scale-Out capability that provides free-of charge built-in read-only node that can be used to run read-only queries (for example reports) that should not affect performance of your primary workload.

When to choose this service tier?

Business Critical service tier is designed for the applications that require low-latency responses from the underlying SSD storage (1-2 ms in average), fast recovery if the underlying infrastructure fails, or need to off-load reports, analytics, and read-only queries to the free of charge readable secondary replica of the primary database.

The key reasons why you should choose Business Critical service tier instead of General Purpose tier are:

  • Low IO latency requirements – workload that needs the fast response from the storage layer (1-2 milliseconds in average) should use Business Critical tier.
  • Frequent communication between application and database. Application that cannot leverage application-layer caching or request batching and need to send many SQL queries that must be quickly processed are good candidates for Business Critical tier.
  • Large number of updates – insert, update, and delete operations modify the data pages in memory (dirty page) that must be saved to data files with CHECKPOINT operation. Potential database engine process crash or a failover of the database with a large number of dirty pages might increase recovery time in General Purpose tier. Use Business Critical tier if you have a workload that causes many in-memory changes.
  • Long running transactions that modify data. Transactions that are opened for a longer time prevent truncation of log file that might increase log size and number of Virtual log files (VLF). High number of VLF can slow down recovery of database after failover.
  • Workload with reporting and analytic queries that can be redirected to the free-of-charge secondary read-only replica.
  • Higher resiliency and faster recovery from the failures. In a case of system failure, the database on primary instance will be disabled and one of the secondary replicas will be immediately became new read-write primary database that is ready to process the queries. Database engine doesn't need to analyze and redo transactions from the log file and load all data in the memory buffer.
  • Advanced data corruption protection - Business Critical tier leverages database replicas behind-the-scenes for business continuity purposes, and so the service also then leverages automatic page repair, which is the same technology used for SQL Server database mirroring and availability groups. In the event that a replica cannot read a page due to a data integrity issue, a fresh copy of the page will be retrieved from another replica, replacing the unreadable page without data loss or customer downtime. This functionality is applicable in General Purpose tier if the database has geo-secondary replica.
  • Higher availability - Business Critical tier in Multi-AZ configuration guarantees 99.995% availability, compared to 99.99% of General Purpose tier.
  • Fast geo-recovery - Business Critical tier configured with geo-replication has a guaranteed Recovery point objective (RPO) of 5 sec and Recovery time objective (RTO) of 30 sec for 100% of deployed hours.

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