SQL Azure Architecture

10/2/2010 7:17:53 PM
SQL Azure is a scalable and highly available database utility service in the cloud. Like all other Windows Azure services, it runs in Microsoft data centers around the world. The data center infrastructure provides the SQL Azure service with load balancing, failover and replication capabilities. Figure 1 illustrates the high-level SQL Azure architecture.
Figure 1. SQL Azure Architecture

As shown in Figure 1, the SQL Azure service is composed of four layers: infrastructure, platform, services, and client. All the layers except the client layer run inside a Microsoft data center.

1. Infrastructure Layer

The infrastructure layer is the supporting layer providing administration of hardware and operating systems required by the services layer. This is the core data center layer that is shared across multiple services in a data center.

2. Platform Layer

The platform layer consists of the SQL Server instances and the SQL Azure fabric, and Management services. The SQL Server instances represent the deployed databases, their replicas, and the operating system instances that host the SQL Server instances. The SQL Azure fabric is the underlying framework that automates the deployment, replication, failover, and load balancing of the database servers.

The SQL Azure fabric is responsible for creating three replicas of your database instance and provides automatic failover capabilities to these instances. As shown in Figure 2, if the primary instance of your database experiences a failure, the SQL Azure fabric designates one of the replicas as the primary instance and automatically routes all the communications to the new primary instance. In an effort to maintain three replicas at all times, SQL Azure also creates a new replica of the database.

Figure 2. SQL Azure database replicas

The Management services are responsible for maintaining the health, upgrades, consistency, and provisioning of the hardware and software to support the SQL Azure fabric.

3. Services Layer

The services layer comprises external (customer) facing machines and performs as a gateway to the platform layer. It exposes the tabular data stream (TDS), billing, metering, and account provisioning services to customers.


TDS is the native Microsoft SQL Server protocol that database clients can use to interact with a SQL Server database. You can find the TDS protocol specification at

The services layer exposes the TDS protocol on port 1433 over Secure Sockets Layer (SSL). The services layer is also responsible for routing connections to the primary database instance in the platform layer. This layer maintains runtime information about your database replicas and routes the TDS coming from client applications to the appropriate primary instance. The services layer is also responsible for provisioning your database when you create a database in SQL Azure. The provisioning of databases involves communicating with the SQL Azure fabric in the platform layer to provision appropriate replicas of the database.

The billing and metering service is responsible for monitoring the runtime usage of your database for billing purposes. The billing and metering service tracks the usage of databases at the account level.

4. Client Layer

The client layer is the only layer that runs outside of the Microsoft data center. The client layer doesn't include any SQL Azure–specific components; instead, it uses all the existing features of SQL Server client components like ADO.NET, ODBC, Visual Studio.NET, SQL Server Management Studio, ADO.NET Data Services, and so on. The client API initiates a TDS connection to SQL Azure on port 1433, which is routed by the services layer to the platform layer to the appropriate database instance.

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