Choose the appropriate data storage model based on technical requirements
May include but is not limited to: SQL Azure, Cloud drive, performance, scalability, accessibility from other applications and platforms, Windows Azure storage services: blobs, tables and queues
SQL Azure in itself is big enough to fill a book (In fact, it does fill a book. Most if this section is based on the book Pro SQL Azure by Scott Klein and Herve Roggero) so this section is just a quick introduction. SQL Azure is a transactional database based on SQL Server 2008. It supports the T-SQL language and a limited set of functions from SQL Server. It also supports ADO.NET and ODBC data access. You can even use your favorite SSMS to connect and manage SQL Azure databases, but there’s an online solution, too.
You should be aware that SQL Azure runs in a multitenant environment. This means that you have restrictions on query time, CPU, etc. So if you have a long running query, massive CPU usage, or something similar that might affect another users’ databases on the server, your database connection can be (and will be) throttled (terminated).
Despite this fact you should be aware that using SQL Azure you pay for storage (GBs of database size)*, so you should perform some CPU intensive tasks within SQL Azure instead of your application. The benefit is that CPU usage in SQL Azure is free, while you have to pay for it on an hourly base in your app hosted in the cloud.
Scalability in SQL Azure is revolving around sharding. The design guidelines are explained here. Sharding is a kind of horizontal partitioning; you store rows separately instead of columns. I’ll explain the concept in another blog post later.
Last but not least, have a look at the (most important) limitations of SQL Azure:
- No support for backing up/restoring databases (there are workarounds, of course)
- No USE statement, and you cannot use database names (this ends cross-database queries)
- No Windows Authentication
- Setting server level collation is disabled
- No heap tables, clustered indexes are a must
- Maximal database size is 150 GB
- No SQL Server Agent
- Idle connection are terminated (after 30 minutes)
For the full list of limitations, see http://msdn.microsoft.com/en-us/library/ee336245.aspx.
Windows Azure Storage Services
Blobs – as their name shows are large binary objects stored in the cloud. At the time of this writing, their size maxes out at 200 GB in the case of a block blob and 1 TB when using page blobs. Usually you would store images or video/audio in blobs. Video usage is especially useful, because block blobs support streaming.
I introduced two different kinds of blobs, block and page blobs. Let’s elaborate further on them. If you need further info, refer to MSDN.
A block blob is built from blocks, which can have the maximum size of 4 MBs (the largest block supported in one operation). You are free to modify, delete and insert block of a block blob, commit or discard your changes as needed. The maximum size of a block blob is 200 GB, and it can contain a total of 50.000 blocks.
Page blobs are optimized for random access. They can be 1 TB large, and they are built from 512 byte pages. You cannot “version” your pages, so updates of one or more pages are immediately in effect.
A special subtype of page blobs is Azure Drive (or Cloud drive). This is a VHD mounted as a local drive letter. It was mostly used before the other APIs were available.
Windows Azure provides a queue-based messaging service that you can use for communication between Azure roles (more on them later). Your messages can be 64 KB in size, and generally they are FIFO, but no guarantee exists that they will be treated in this fashion. You can of course process messages bigger than 64 KBs, by using blobs.
Tables allow you to store entities of 1 MB up to 100 TB. An entity can have 255 “columns” with different data types. Unlike SQL Azure, there is no relational support, so you can’t have foreign keys, joins, etc. The best usage of these tables is for example a leader board for a game. Small in size, not complex, no relationships required.
Table entities have three reserved properties which define a key for the entity: a partition key for the table itself, a row key for the entity within the table, and a timestamp.
Tables come with a fairly limited type set, these are byte, bool, DateTime, double, Guid, int, long and string. For more info on tables, refer to MSDN.
There is more info about the various storage options in Windows Azure in this Technet article.
*The full truth is that you pay for two things: storage and bandwidth, however, bandwidth within an SQL Azure database and an application running inside Windows Azure is free.