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Windows Azure

Overview of SQL Azure

6/12/2011 4:53:08 PM
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The first question usually asked about SQL Azure is: "Is SQL Azure really SQL Server 2008, or is it something else?" The answer is a little of both. Retail editions of SQL Server 2008 include Web, Workgroup, Standard, Enterprise, and Datacenter. SQL Azure is another edition of SQL Server 2008, and shares many of the same features as the other editions.

Development on SQL Azure is nearly identical to developing on SQL Server, with most commands and objects either fully or partially supported. However, because SQL Azure is a service, there are significant differences in how SQL Azure and on premises SQL Server are managed. At the time of writing, additional components such as SQL Server Reporting Services (SSRS) and SQL Server Analysis Services (SSAS) are not supported on SQL Azure, although on premises installations of SSRS and SSAS can consume data from SQL Azure databases. SQL Azure Reporting was announced at PDC '10, and is expected to be available in 2011.

Replication is not available in SQL Azure (don't panic, our SQL Azure databases are maintained behind the scenes in multiple copies, with automatic failover). Service Broker is another SQL Server feature currently not supported by SQL Azure. Depending on the needs of the application, a queue and worker role may be able to replace the Service Broker. Microsoft promises to introduce additional SQL Azure services soon, so we may one day see these missing features in the cloud.

SQL Azure can be used individually or in conjunction with other Azure services. In either case, local applications can connect directly to SQL Azure. When used in conjunction with Windows Azure services, local applications can also access data stored in SQL Azure via web services.

Perhaps the most common method for accessing data in SQL Azure is to use a Windows Azure Web Role or Worker Role. Using a Web Role is similar to an ASP.NET website backed by SQL Server.

Microsoft touts five key benefits in using SQL Azure:

  • Manageability

  • High availability

  • Scalability

  • A relational data model

  • A familiar development model

We'll use these five benefits as the outline for our overview of SQL Azure.

Manageability

The biggest differences in managing SQL Azure and an on premise SQL Server is that we no longer have to purchase or maintain physical hardware. By design, Microsoft has separated the physical administration tasks from the logical administration tasks, and assumes responsibility for the physical management of SQL Azure. As Microsoft describes it (http://msdn.microsoft.com/en-us/library/ee336241.aspx):

[t]his approach helps SQL Azure provide a large-scale multi-tenant database service that offers enterprise-class availability, scalability, security, and self-healing.

Microsoft has also removed the ability to control physical resources of SQL Azure. For example, there is no option to change either the hard drive or file group in which a database resides. This makes sense, as the file system is not accessible to us. Likewise, the backup and restore options are also not available. Likewise, there is no option to attach a database in SQL Azure.

As Azure has its own methods of load balancing and resource governing, the new Resource Governor is blocked, as are any T-SQL or DDL statements that modify or access the physical resources.

On the downside, a couple of very powerful tools we've come to rely upon are not available. Neither SQL trace flags nor the Database Tuning Advisor are available to us, which will make debugging and performance tuning more complicated.

Managing SQL Azure

SQL Azure databases are managed through the SQL Azure portal, which is part of the same portal as other Azure services, if we're using any. We use the portal to create or delete databases, and to manage database level security, but we use other tools to manage the contents of each database.

Similarities

Each SQL Azure account is provided a single instance of SQL Azure, which can contain multiple databases.

As in SQL Server 2008, we administer our databases, as well as the roles and user accounts. All connections to SQL Azure run through port 1433, same as SQL Server. If an on premise client application needs to connect directly to SQL Azure, firewalls on both ends need to have port 1433 opened. By default, external connections to a SQL Azure instance are blocked for security reasons. In order for our applications or management tools to connect to SQL Azure, we must whitelist certain IP ranges using the SQL Azure portal.

Differences

Most system-stored procedures are not supported (http://msdn.microsoft.com/en-us/library/ee336237.aspx), no system tables are supported, and SQL Azure has limited support for system views (http://msdn.microsoft.com/en-us/library/ee336238.aspx).

Databases are sold in two editions a Web Edition, which has a maximum size of 5 GB, and a Business Edition, which has a maximum size of 50 GB. As we can expect, pricing is a tiered structure based on database size; the current pricing can be found at http://www.microsoft.com/windowsazure/pricing/. Security administration such as creating users and allowing access to databases is handled in the SQL Azure portal. As Microsoft handles the entire infrastructure, patches and service packs are no longer a part of our lives; nor are log files filling up drive space, as logs are not counted as part of the space calculation. However, it is possible a service pack upgrade could break some production code, and this is one of the criticisms levelled at SQL Azure by career database administrators (DBAs). Several noted DBAs discuss their concerns (http://www.sqlmag.com/article/services/considering-sql-azure.aspx), notably:

  • The lack of a backup option prevents restoring the database to a previous time. If someone accidentally deletes a table, there is no way to retrieve it.

  • Some of the more useful system stored procedures are not available, which can make troubleshooting more complicated.

The data are stored on hardware in a network that is physically out of the DBA's control. DBAs are tasked with keeping data safe and secure, and often that involves direct administration of the physical servers and working closely with network managers. With SQL Azure, uptime and security are the responsibility of a faceless group of people, and DBAs are reluctant to make guarantees about anything they cannot control.

Although there is not a true backup option, SQL Azure supports a database copy functionality, not surprisingly named SQL Azure Database Copy. The database can be copied to the same Azure server, or a different Azure server, and the copy can be scheduled. Documentation about the database copy functionality can be found at http://msdn.microsoft.com/en-us/library/ff951624.aspx.

The SQL Azure team addressed the issue of patching SQL Azure in a blog post at http://blogs.msdn.com/b/sqlazure/archive/2010/04/30/10004818.aspx. As SQL Azure keeps three redundant copies of our data on three separate instances, and only a single instance would be patched at one time, failover and redundancy are still in place during an update.

SQL Server Management Studio 2008 R2 supports direct connections to SQL Azure, but because connections to SQL Azure are solely on port 1433, the SQL Browser is not available.

High availability

Another feature of SQL Server missing in SQL Azure is replication. Without the options for backup/restore or replication, how can our data be highly available? Two concepts that we hear most often with regards to SQL Azure are "built-in data protection" and "self healing".

  • Built-in data protection: The built-in data protection involves our data being replicated immediately and automatically across several physical machines. This replication is part of the SQL Azure platform, and we needn't set it up, neither do we have any control over it. Note that this is not the same replication as SQL Server Replication, but is a different mechanism for duplicating our data.

  • Self healing: SQL Azure is self healing in that if a physical machine should become unavailable, there is automatic failover to another machine containing all of our data. As this failover machine is also in the Azure Fabric, the failover process is invisible to clients and no reconfiguration is necessary should a secondary machine come online.

Having high availability does not mean we can be reckless with resource utilization, and we cannot assume a connection will always remain open. In order to be fair to all tenants of SQL Azure, a SQL Azure connection may be closed for a number of reasons, including the following:

  • Excessive resource utilization

  • Long-running queries

  • Long-running transactions

  • Connections being left idle for a longer duration

  • Server failures and the resulting failover

As developers, we cannot assume our connection will always be open, and we'll need to compensate accordingly.

Another way that our data are highly available is due to the scalability aspects of SQL Azure.

Scalability

Should our applications become an overnight sensation, we can add additional storage capacity via the SQL Azure portal. Database capacity can be increased on the fly, up to the subscription limits. For example, a Web Edition database comes in two sizes 1 GB and a 5 GB. For a new site, a database of the size 1 GB is more than sufficient and it also has less cost per month compared to the 5 GB database. As demands grow, we can upgrade to the 5 GB database, by simply using the ALTER DATABASE command to increase the maximum database size.

The load balancing aspects of the Azure Fabric help ensure client requests are answered and met in a timely fashion.

Relational data model

Windows Azure tables and blobs are useful, but they'll only get us so far with a complex data-driven application. For one of these, we need to rely on a relational database provider; SQL Azure is a true RDBMS in the cloud.

Familiar development model

SQL Azure is based on SQL Server need we say more? Actually, yes, a lot more.To start with, SQL Azure supports T-SQL and returns a tabular data stream, the same as SQL Server; so, in many cases, only the connection string needs to be changed (after the database is deployed, of course). We can connect our applications to SQL Azure using familiar drivers, including System.Data.SqlClient, SQL Server 2008 and 2008 R2 ODBC Drivers, and SQL Server 2008 PHP driver (OLEDB is not supported, which is a consideration when using SSIS). Tools we can connect with include SQL Server 2008 R2 Management Studio, SQLCMD, Visual Studio 2010, and a number of third-party tools. To transfer data to SQL Azure, we can use SQL Server Integration Services (SSIS), BCP.exe, System.Data.SqlClient.SqlBulkCopy, or INSERT statements.

If we don't want to utilize direct connections to the database, we can instead use ADO.NET, ASP.NET, or ADO.NET Data Services in our applications to access data in SQL Azure.

Important to realize in this discussion is the term "familiar", rather than "identical". Because SQL Azure is a managed service, some management features have had functionality reduced or have been removed completely. T-SQL is the query language used by SQL Azure, but there are three levels of support for T-SQL commands complete, partial, and unsupported. Further details on T-SQL support can be found at http://msdn.microsoft.com/en-us/library/ee336250.aspx. As we discuss similarities and differences, the discussion of differences is going to seem to outweigh the similarities. We don't need an in-depth discussion on similarities, as these should be familiar concepts and tools. Keep in mind the discussion is comparing SQL Azure and SQL Server 2008.

What's the same in SQL Azure?

One of Microsoft's goals in creating SQL Azure was to provide an environment for experienced SQL Server developers to utilize their skills. Here, we review where SQL Azure is similar to what we already know about it.

Data types

Nearly all data types are supported, including XML and geography. For a table of supported data types, visit http://msdn.microsoft.com/en-us/library/ee336233.aspx. The addition of the geographic and geometric data type around the MIX10 timeline raised hopes that the .NET CLR will be supported, as these data types rely on the CLR.

Database objects

As we'd expect, database tables are the same, including table variables and local temporary tables. Supported table features include:

  • Constants

  • Constraints

  • Triggers

  • Statistics management

  • Index creation and maintenance

SQL Azure also fully supports views, stored procedures, and user-defined functions.

Fully supported T-SQL commands

For a complete overview of supported T-SQL commands, refer to the MSDN documentation at http://msdn.microsoft.com/en-us/library/ee336270.aspx. Some of the most common T-SQL commands supported by SQL Azure are mentioned next, but this is not the complete list.

  • Data Definition Language (DDL) commands:

    • Alter Role/Schema/View

    • Create Role/Schema/Statistics/View

    • Drop Login/Role/Procedure/Schema/Statistics/Synonym/Type/User/View

    • DBCC SHOW_STATISTICS

    • UPDATE STATISTICS

  • Data Manipulation Language (DML) commands:

Select clause/@local_variable From
Begin_Transaction Group By
Begin…End Having
Cast Order By
Convert Top
Ceiling Try...Catch
Coalesce Where
Delete Commit/Rollback/Save Transaction
Declare Cursor While
Delete  
Truncate Table  
If...Else  

  • Data Control Language (DCL) commands

    • Deny Object/Schema Permissions

    • Grant Object/Schema Permissions

    • Revoke Object/Schema Permissions

Partially supported T-SQL commands

Partial support for T-SQL commands indicates that the SQL Azure syntax does not support all the arguments or options that the SQL Server 2008 syntax does. For instance, CREATE TABLE in SQL Server 2008 has a parameter to choose the filegroup (usually ON PRIMARY); because filegroups are not selectable in SQL Azure, the filegroup argument is not supported. There are additional options not supported by the SQL Azure version of CREATE TABLE, and the official MSDN documentation should be consulted if there are questions about the support for a particular command.

The following table summarizes some of the more common T-SQL commands with partial support in SQL Azure. For the complete list of partially supported T-SQL commands, read the MSDN article at http://msdn.microsoft.com/en-us/library/ee336267.aspx.

Create/Alter Function Grant/Deny/Revoke Database Permissions
Create/Alter/Drop Index Execute
Create/Alter/Drop Table Create/Alter User
Create/Alter/Drop Trigger Alter Login
Create/Alter View Enable/Disable Trigger

SQL Server built-in functions

As with T-SQL commands, the intrinsic functions of SQL Server 2008 have varying degrees of support in SQL Azure. For full details regarding SQL Azure support of SQL Server 2008 intrinsic functions, consult the official MSDN documentation at http://msdn.microsoft.com/en-us/library/ee336248.aspx.

Function type Support Examples of supported statements
Aggregate Full AVG, COUNT, MAX, MIN, SUM
Ranking Full DENSE_RANK, NTILE, RANK, ROW_NUMBER
Configuration Partial @@LOCK_TIMEOUT, @@SERVERNAME, @@SPID
Cursor Full @@CURSOR_ROWS, @@FETCH_STATUS, CURSOR_STATUS
Date and Time Full DATEADD, DATEDIFF, DATEPART, GETDATE, DAY, MONTH, YEAR
Mathematical Full ABS, CEILING, FLOOR, LOG, ROUND, SQUARE
Metadata Partial COL_LENGTH, COL_NAME, INDEX_COL, OBJECT_NAME
Security Partial CURRENT_USER, SESSION_USER, USER_NAME
String Full CHAR, LEFT, LEN, LTRIM, RIGHT, RTRIM
System Partial APP_NAME, CASE, CAST, CONVERT, COALESCE, @@IDENTITIY, ISDATE, ISNULL, @@ROWCOUNT
Text/Image Partial PATINDEX
ODBC String Full BIT_LENGTH, CONCAT
ODBC Numeric Full TRUNCATE
ODBC Date/Time Full CURRENT_DATE, CURRENT_TIME, DAYNAME, HOUR, MINUTE, QUARTER

Multiple active result sets

First introduced with ADO.NET 2.0 and SQL Server 2005, multiple active result sets (MARS) is the ability for multiple commands to be executed against a single connection, and to maintain multiple open recordsets. MARS can improve application performance by not limiting applications to a single command or result set.

What's different in SQL Azure?

It might seem there are more differences than similarities between SQL Server 2008 and SQL Azure, but features that are the same don't merit much discussion. In many cases, differences between SQL Azure and SQL Server 2008 are due to SQL Azure being a managed service, and much of the administration has been abstracted away from us.

One of the first differences is that we cannot choose the file placement of the database files (neither data nor log). We have no way to manage filegroups, so those options are also not available to us.

Another difference at the server level is how we set collation. Collation cannot be set at the server or database level; instead, collation can only be set at the column or expression level. The default collation for SQL Azure is SQL_LATIN1_GENERAL_CP1_CI_AS, which is a fairly general collation for US-based applications. The following is the deciphered collation:

  • LATIN1_GENERAL = US English

  • CP1 = code page 1252

  • CI = case-insensitive

  • AS = accent-sensitive

If a feature was deprecated in SQL Server 2008, SQL Azure does not support that feature. One such feature is SQL Server trace flags, which were used for debugging performance issues. SQL Azure does not support SQL trace flags.

In a major departure from SQL Server 2008, SQL Azure does not support the Common Language Runtime (CLR). We also can't access server configuration options, as some don't exist and others are the responsibility of Microsoft. Additionally, SQL Azure does not support any of the SQL Server 2008 system tables.

At the database level, SQL Azure does not support database mirroring. There is no need for this, as our data are replicated across multiple physical servers.

Number of databases

When we establish a SQL Azure account, a SQL Azure instance is provisioned for us. Each SQL Azure instance can contain up to 150 databases, including the master database. Additional databases will require a separate SQL Azure instance.

Database objects

There is one difference regarding tables. Database tables must have a clustered index created before we can insert data. It is possible to create a table without a clustered index, but no inserts can be made until such an index is added. Global temporary tables are not supported in SQL Azure.

Service Broker, SQL Browser, and DTC

The SQL Server Service Broker handles request queuing and asynchronous messaging in a local installation of SQL Server. In SQL Azure, some of this functionality is redundant with the Azure Fabric, and hence there is no Service Broker in SQL Azure.

The SQL Browser is also not available in SQL Azure. The only port we can access SQL Azure through is 1433, and the SQL Browser relies on dynamic ports.

Finally, SQL Azure does not support either distributed queries or transactions, and there is no Distributed Transaction Coordinator (DTC). All transactions must be local.

T-SQL commands

SQL Azure does not support the USE command for changing databases. If we're running a long set of commands and need to switch databases, we can't in SQL Azure. Instead, we must create a connection to each database, and execute the commands against the desired connection.

Also, 4-identifier referencing (<database_name>.<schema>.<table_name>.<column>) is not supported.

The majority of the unsupported T-SQL commands are system administration commands, which don't apply to us. Notably, most of the DBCC commands are not supported, nor are most of the ALTER commands related to databases and servers. The table that follows is a partial list of some common T-SQL commands that are unsupported; for the complete list of unsupported commands, consult the MSDN documentation at http://msdn.microsoft.com/en-us/library/ee336253.aspx.

ALTER DATABASE GRANT/REVOKE/DENY Server Permissions
BACKUP KILL
BULK INSERT OPENROWSET
CREATE/ALTER/DROP LOGIN RESTORE
DBCC CHECKDB SELECT INTO Clause
DBCC DBREINDEX SET ANSI_NULLS
DBCC INDEXDEFRAG SET ANSI PADDING_OFF
DBCC SHRINKDATABASE WRITETEXT

System functions

A number of system functions are also not supported by SQL Azure as they could compromise information abstracted from us. Again, the following table is a partial list; the complete list of unsupported system functions is in the MSDN documentation at http://msdn.microsoft.com/en-us/library/ee336253.aspx.

fn_get_audit_file sys.login_token
fn_get_sql sys.user_token
sys.fn_validate_plan_guide sys.numbered_procedure_parameters

Data synchronization

We do not have the option of replication or transaction log shipping in SQL Azure, but there is a data synchronization service currently in CTP. Utilizing the Microsoft Sync Framework, we can perform one-way or bidirectional synchronization between a number of SQL Azure databases, set up in a hub-and-spoke arrangement (rather than a true replication). When we set up synchronization, we choose a hub database to be our master database, and then pair with our member databases. Synchronization can be the entire database, or limited to a selected group of tables. Synchronization can be on demand or scheduled. Foreign key constraints are not enforced in the member databases so that data can be inserted in any order. If a foreign key relationship is necessary for our application, the member databases would not be suitable for using as a back-end database.

On the first synchronization, the database schema will be created for us in the member databases, and data will be completely synchronized. After the initial synchronization, only modified data will be synchronized. Unfortunately, schema changes will not be synchronized after the initial synchronization we'll have to remove the synchronization, modify our database, then modify the hub database, re-establish the synchronization pairings, and start the process all over again. Member databases will be reallocated as if we were performing an initial synchronization.

An introduction to the data sync service can be found at http://blogs.msdn.com/b/sqlazure/archive/2010/07/06/10035099.aspx. Because this service is in CTP at the time of writing, we recommend reviewing additional information that may have been published later for the most up-to-date information.

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