Azure SQL Database is a fully managed cloud version of the Microsoft SQL Server engine.

But wait! Because Microsoft is driving a cloud-first strategy, the development of the Azure SQL Database is at all times ahead of the development of the Microsoft SQL Server. Only when a feature has established itself in the cloud,  is it also adopted on the on-premises site.

What do I need to know? Regardless of the operation of the actual SQL Database server, each database is provided individually and isolated from the others, and has its own service level with a guaranteed performance level. The service level, and thus also the performance level, can be changed at any time.

Let's return to the terms service level and performance level—both are selection criteria, in the creation process of an Azure SQL Database. However, a service level is only a collection of performance levels, so we should be more concerned with the concept behind the term performance level.

How is a performance level defined? A performance level is defined by the number of permitted Database Transaction Units (DTUs). The second criterion formerly used, the so-called transaction rate (a metric from the Azure SQL Database Benchmark (ASDB) to capture the maximum transaction throughput per hour, minute, or second) will no longer be used.

What is a DTU? A definition of a DTU is a bit difficult, as it can be described as an object, but it is not actually a real object. DTUs are power units to which a certain percentage of the total performance of the database server is allocated in CPU, memory, and read or write rates.

The offered performance levels contain between 5 DTUs (at the basic service level) and 4,000 DTUs (in the premium service level P15). The larger the number of DTUs, the greater the performance of the database.

Another criterion for the selection of a performance level is the maximum size of the database. Also, the maximum size, which can reach the database, depends on the selected performance level.

The maximum size of the database per level is:

• Basic: 2 GB
• Standard (S0 - S12): 250 GB
• Premium (P1 - P6): 500 GB
• Premium (P11 + P15): 4 TB
• Premium RS: 500 GB

Is that all now? No, also the number of simultaneously concurrent requests (maximum concurrent workers), the number of concurrent logins (maximum concurrent logins), and the number of concurrent sessions (maximum sessions) are limited per level.