Database Reference
In-Depth Information
clients from accessing the database. Each Windows Azure SQL Database replicated to multiple
physical servers for better reliability and security. he logical server replicates all the transactions
conducted by the applications to all the replicas on diferent physical servers. In Figure 2.32, there
is one primary replica and two secondary replicas. Physical SQL Server 1 has the primary replica,
and the other two have the secondary replicas. A transaction made by an application is irst sent
to the primary replica. hen, changes made to the database in the primary replica will be repli-
cated to the secondary replicas when the system is not too busy. In reality, a Windows Azure SQL
Database such as SQL DB 1 in Figure 2.32 can also be distributed to multiple physical servers for
better performance. he front-end users will not see the details of the distribution. All they see is
their own database hosted by the logical server.
he Windows Azure SQL Database plus Windows Azure SQL Database service and front-end
application are the three main components of the Windows Azure SQL Database architecture. Figure
2.33 illustrates the Windows Azure SQL Database architecture, including these three components.
Users on the client side can directly access applications such as programming code, forms,
reports, or web pages. hrough the front-end applications, the users are able to access the Windows
Azure SQL Database services. he applications can be hosted either by the clients' local computers
or by Windows Azure in the Microsoft cloud. Some of the services authenticate the requests and
manage the bills. he other services monitor and replicate the databases.
he actual SQL Server databases are hosted by the computers in a data center. A data center
consists of a large number of computers with the same coniguration and low-to-medium perfor-
mance. In case a computer fails, it can be quickly replaced. At the data center, these computers
form a cluster, which includes up to 1000 computers. For some large jobs that require more than
1000 computers, more clusters can be allocated to meet the requirement. Each of these computers
serves as a data node in the data center. Each data node is installed with an instance of SQL Server.
Each SQL Server instance hosts up to 650 partitions; databases created by users are installed in
one or more partitions, as a primary or secondary replica. All these partitions share one log ile
stored on the data node. Each data node hosts several fabric processes. hese fabric processes can
Windows Azure
SQL Database
ODBC
Application
Client
Programming
languages
Connection service
SQL Server
Forms and reports
Application
Client
Billing service
SQL Azure fabric
Web pages
ASP. NET
Provision service
Management
Development tools
Application
Client
Windows azure SQL
database service
Windows Azure
SQL Database
Front end
applications
Figure 2.33
Windows Azure SQL Database architecture.
