The Same, Only Different: Comparing Windows Azure SQL Database and On-Premise SQL Server

One of the most attractive aspects of Windows Azure SQL Database is that it shares virtually the same code-base and exposes the very same tabular data stream (TDS) as on-premise SQL Server. Thus, to a great extent, the same tools and applications that work with SQL Server work just the same and just as well with SQL Database. Notice that I said to a great extent, because despite their commonality, there are quite a few SQL Server features that SQL Database does not support. In this blog post, I discuss how and why these two platforms differ from one another, and explain the SQL Database constraints that you need to be aware of if you have previous experience with SQL Server. Where possible, I also suggest workarounds.

SQL Server and SQL Database differ in several ways; most notably, in terms of size limitations, feature support, and T-SQL compatibility. In many cases, these constraints are simply the price you pay for enjoying a hassle-free, self-managing, self-healing, always-available database in the cloud. That is, Microsoft cannot responsibly support features that impair its ability to ensure high-availability, and must be able to quickly replicate and relocate a SQL Database. This is why SQL Database places limits on database size, and doesn’t support certain specialized features such as FILESTREAM, for example.

Another common reason why a particular feature or T-SQL syntax might not be supported in SQL Database is that it’s simply not applicable. With SQL Database, administrative responsibilities are split between Microsoft and yourself. Microsoft handles all of the physical administration (such as disk drives and servers), while you manage only the logical administration (such as database design and security). This is why any and all T-SQL syntax that relates to physical resources (such as pathnames) is not supported in SQL Database. For example, you don’t control the location for primary and log filegroups. This is why you can’t include an ON PRIMARY clause with a CREATE DATABASE statement, and indeed, why SQL Database does not permit a filegroup reference in any T-SQL statement. Plainly stated, everything pertaining to physical resources (that is, infrastructure) is abstracted away from you with SQL Database

Yet still, in some cases, a certain SQL Server feature or behavior may be unsupported merely because Microsoft has just not gotten around to properly testing and porting it to SQL Database. Windows Azure is constantly evolving, so you need to keep watch for updates and announcements. This blog post is a great starting point, but the best way to stay current is by reviewing the Guidelines and Limitations section of the SQL Database documentation on the MSDN web site.

Size Limitations

With the exception of the free, lightweight Express edition of SQL Server, there is no practical upper limit on database size in any edition of SQL Server. A SQL Server database can grow as large as 524,272 terabytes (for SQL Server Express edition, the limit is 10 gigabytes).

In contrast, SQL Database has very particular size limitations. You can set the maximum size by choosing between the Web and Business editions. With a Web edition database, you can set the maximum database size to either 1 or 5 gigabytes. With a Business edition database, the maximum database size can range from 10 to 150 gigabytes. Other than the available choices for maximum database size, there is no difference in functionality between the two editions. The absolute largest supported database size is 150 gigabytes, although horizontal partitioning strategies (called sharding) can be leveraged for scenarios that require databases larger than 150 gigabytes.

Connection Limitations

SQL Database is far less flexible than SQL Server when it comes to establishing and maintaining connections. Keep the following in mind when you connect to SQL Database:

  • SQL Server supports a variety of client protocols, such as TCP/IP, Shared Memory, and Named Pipes. Conversely, SQL Database allows connections only over TCP/IP.
  • SQL Database does not support Windows authentication. Every connection string sent to SQL Database must always include a login username and password.
  • SQL Database often requires that @<server> is appended to the login username in connection strings. SQL Server has no such requirement.
  • SQL Database communicates only through port 1433, and does not support static or dynamic port allocation like SQL Server does.
  • SQL Database does fully support Multiple Active Result Sets (MARS), which allows multiple pending requests on a single connection.
  • Due to the unpredictable nature of the internet, SQL Database connections can drop unexpectedly, and you need to account for this condition in your applications. Fortunately, the latest version of the Entity Framework (EF6) addresses this issue with the new Connection Resiliency feature, which automatically handles the retry logic for dropped connections.

Unsupported Features

Listed below are SQL Server capabilities that are not supported in SQL Database, and I suggest workarounds where possible. Again, because this content is subject to change, I recommend that you check the MSDN web site for the very latest information.

  • Agent Service You cannot use the SQL Server Agent service to schedule and run jobs on SQL Database.
  • Audit The SQL Server auditing feature records server and database events to either the Windows event log or the file system, and is not supported in SQL Database.
  • Backup/Restore Conventional backups with the BACKUP and RESTORE commands are not supported with SQL Database. Although the BACPAC feature can be used to import and export databases (effectively backing them up and restoring them), this approach does not provide transactional consistency for changes made during the export operation. To ensure transactional consistency, you can either set the database as read-only before exporting it to a BACPAC, or you can use the Database Copy feature to create a copy of the database with transactional consistency, and then export that copy to a BACPAC file.
  • Browser Service SQL Database listens only on port 1433. Therefore, the SQL Server Browser Service which listens on various other ports is naturally unsupported.
  • Change Data Capture (CDC) This SQL Server feature monitors changes to a database, and captures all activity to change tables. CDC relies on a SQL Server Agent job to function, and is unsupported in SQL Database.
  • Common Language Runtime (CLR) The SQL Server CLR features (often referred to simply as SQL CLR) allow you to write stored procedures, triggers, functions, and user-defined types in any .NET language (such as C# or VB), as an alternative to using traditional T-SQL. In SQL Database, only T-SQL can be used; SQL CLR is not supported.
  • Compression SQL Database does not support the data compression features found in SQL Server, which allow you to compress tables and indexes.
  • Database object naming convention In SQL Server, multipart names can be used to reference a database object in another schema (with the two-part name syntax schema.object), in another database (with the three-part name syntax database.schema.object), and (if you configure a linked server) on another server (with the four-part name syntax server.database.schema.object). In SQL Database, two-part names can also be used to reference objects in different schemas. However, three-part names are limited to reference only temporary objects in tempdb (that is, where the database name is tempdb and the object name starts with a # symbol); you cannot access other databases on the server. And you cannot reference other servers at all, so four-part names can never be used.
  • Extended Events In SQL Server, you can create extended event sessions help to troubleshooting a variety of problems, such as excessive CPU usage, memory pressure, and deadlocks. This feature is not supported in SQL Database.
  • Extended Stored Procedures You cannot execute your own extended stored procedures (these are typically custom coded procedures written in C or C++) with SQL Database. Only conventional T-SQL stored procedures are supported.
  • File Streaming SQL Server native file streaming features, including FILESTREAM and FileTable, are not supported in SQL Database. You can instead consider using Windows Azure Blob Storage containers for unstructured data files, but it will be your job at the application level to establish and maintain references between SQL Database and the files in blob storage, though note that there will be no transactional integrity between them using this approach.
  • Full-Text Searching (FTS) The FTS service in SQL Server that enables proximity searching and querying of unstructured documents is not supported in SQL Database.
  • Mirroring SQL Database does not support database mirroring, which is generally a non-issue because Microsoft is ensuring data redundancy with SQL Database so you don’t need to worry about disaster recovery. This does also mean that you cannot use SQL Database as a location for mirroring a principal SQL Server database running on-premises. However, if you wish to consider the cloud for this purpose, it is possible to host SQL Server inside a Windows Azure virtual machine (VM) against which you can mirror an on-premise principal database. This solution requires that you also implement a virtual private network (VPN) connection between your local network and the Windows Azure VM, although it will work even without the VPN if you use server certificates.
  • Partitioning SQL Server allows you to partition tables and indexes horizontally (by groups of rows) across multiple filegroups within a database, which greatly improves the performance of very large databases. SQL Database has a maximum database size of 150 GB and gives you no control over filegroups, and thus does not support table and index partitioning.
  • Replication SQL Server offers robust replication features for distributing and synchronizing data, including merge replication, snapshot replication, and transactional replication. None of these features are supported by SQL Database, however SQL Data Sync can be used to effectively implement merge replication between a SQL Database and any number of other SQL Databases on Windows Azure and on-premise SQL Server databases.
  • Resource Governor The Resource Governor feature in SQL Server lets you manage workloads and resources by specifying limits on the amount of CPU and memory that can be used to satisfy client requests. These are hardware concepts that do not apply to SQL Database, and so the Resource Governor is naturally unsupported.
  • Service Broker SQL Server Service Broker provides messaging and queuing features, and is not supported in SQL Database.
  • System stored procedures SQL Database supports only a few of the system stored procedures provided by SQL Server. The unsupported ones are typically related to those SQL Server features and behaviors not supported by SQL Database. At the same time, SQL Database provides a few new system stored procedures not found in SQL Server that are specific to SQL Database (for example, sp_set_firewall_rule).
  • Tables without a clustered index Every table in a SQL Database must define a clustered index. By default, SQL Database will create a clustered index over the table’s primary key column, but it won’t do so if you don’t define a primary key. Interestingly enough, SQL Database will actually let you create a table with no clustered index, but it will not allow any rows to be inserted until and unless a clustered index is defined for the table. This limitation does not exist in SQL Server.
  • Transparent Data Encryption (TDE) You cannot use TDE to encrypt a SQL Database like you can with SQL Server.
  • USE In SQL Database, the USE statement can only refer to the current database; it cannot be used to switch between databases as it can with SQL Server. Each SQL Database connection is tied to a single database, so to change databases, you must connect directly to the database.
  • XSD and XML indexing SQL Database fully supports the xml data type, as well as most of the rich XML support that SQL Server provides, including XML Query (XQuery), XML Path (XPath), and the FOR XML clause. However XML schema definitions (XSD) and XML indexes are not supported in SQL Database.
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