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Read-only replicas - Taking advantage of free DTUs

A new feature of Azure SQL enables us to simplify how we do our day to day business when we need to have analytics capabilities in near-real time on our databases.

A common scenario is when you have an Azure SQL database that is heavy hit by the clients, and we need reporting or analytics capabilities at the same time. A common solution is to create a read-only replica that it is used for reporting, data aggregation and other daily or weekly small things that you have to do with data.
Even when you have a data warehouse or a reporting layer, you still need for some narrow cases to go directly to the live database for real (near) time analytics.
Another case is when you have many read operations on data that are not changed so often, and you cannot integrate a cache level. Sounds odd, but there are some country regulations that might force you to do that.

Current solution
For all these scenarios usually, it involves creating a replica in the same or another Azure Region that it is used for read-only actions. There is out of the box support for data replication, the impact it is only at cost level, having two instances of the same database. For Elastic Pool case, things are the same, because you need another Elastic Pool that it is used for replication.

New capability
Starting from now, there is full support on Azure SQL to use the replicas that are already created by Azure when Always ON feature is active.
For Premium tier of Azure SQL Database, there are always other replicas that are in sync with the active one in different redundancy zones (SQL classical cluster ring concept). Until now we were not able to access them directly, even if the replicas were created behind the scene to support Always ON feature.
Now we can specify at the moment when we create the Azure SQL Database, or later on, that we want to activate the Read Scale-Out functionality. From that moment on, we can use the replicas for read-only operation.

To be able to connect to the replica and not the main database, you need to use ‘ApplicationIntent’ inside the connection string that can have the following values:
  • ReadOnly – Used when you need only read operations
  • ReadWrite  - Used for Read & Write operations

The advantage
You pay only for one Premium database, but you can take full advantage of the resources that are available on the read-only replicas. No extra charge for it. No code changes are required to integrate this new feature. Only connection string needs to be updated.

To consider

  • There is session level consistency between databases.
  • Small data replication latency can occur.
  • In the case of a connection error, we can be redirected to another replica, and small data inconsistency related to newly written data might appear.

How to enable it?
These can be done from PowerShell using ‘ReadScale’ parameter.

New database

New-AzureRmSqlDatabase -ResourceGroupName <myresourcegroup> -ServerName <myserver> -DatabaseName <mydatabase> -ReadScale Enabled -Edition Premium

Existing one

Set-AzureRmSqlDatabase -ResourceGroupName <myresourcegroup> -ServerName <myserver> -DatabaseName <mydatabase> -ReadScale Enabled

In theory, using this feature you double you DTU that are available for your Azure SQL Database instance. A part of them are available for Read/Write operations and the other part only for Read operations.


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