Comment: I do not see Windows Azure SQL Database as a feasible solution for a firm that expects its business to scale. The reason is simple: You can not use a component in your system that its replacement will require a long downtime (yes, we are talking about hours if you will have a significant database size). The only way to migrate from Windows Azure SQL Database is to export its data and import it on a regular instance, and it's not acceptable when you have a significant traffic.
High Availability
The requirement for high availability is common: you don't want downtime, as downtime mean less business and it hurts your business image.
The Azure Catch
Azure SQL Server VM is just like having a SQL Server on a regular VM.
VM maintenance includes two layers: 1) maintaining the VM (installing patches, hardening...) and 2) doing the same to the host underneath.
A common large size private and public cloud operation usually include an auto fail over, so when a host is having maintenance or unfortunately fails, the system automatically migrate the running VMs to another host(s) w/o stopping them. You could find this behavior in VMWare VMotion and at Amazon EC2 that runs over XEN.
Well... this is not the case at Microsoft Azure. When Microsoft updates its hosts, don't expect your instances to be available (and yes the downtime may take dozens of minutes and it is not controlled by you). This is an acceptable practice when dealing with web and application servers (place several instances behind a LB and use a queue mechanism to deal with it). However, it is not good one when you deal with databases it's can be a major issue.
The Solution: Have a Master-Master Configuration
As you may understood, a master-slave solution is not acceptable in this case, and therefore, you will need to avoid Log Shipping (although it can be used various other scenarios).
Therefore, we were left with two solutions:
Mirroring:
This was a solution for HA architectures.
However "This feature will be removed in a future version of Microsoft SQL Server. Avoid using this feature in new development work, and plan to modify applications that currently use this feature. Use AlwaysOn Availability Groups instead."
http://technet.microsoft.com/en-us/library/ms189852.aspx
AlwaysOn Availability Groups
This solution is described by MS as the "enterprise-level alternative to database mirroring. Introduced in SQL Server 2012".
However, "The non-RFC-compliant DHCP service in Windows Azure can cause the creation of certain WSFC cluster configurations to fail, due to the cluster network name being assigned a duplicate IP address (the same IP address as one of the cluster nodes). This is an issue when you implement AlwaysOn Availability Groups, which depends on the WSFC feature."
http://technet.microsoft.com/en-us/library/hh510230.aspx
The Second Catch
According to our analysis it seems that both Mirroring (end of life) and AlwaysOn (severe bugs due to DHCP) are not recommended, so we actually left w/o good HA solution, and therefore with no good MS data store solution for the Azure environment.
We tried to get answers from Microsoft stuff, but we did not get good ones.
Bottom Line
When evaluating Azure as a cloud platform for your needs, you should consider your data solution and how it fits your needs. In this case you may need to consider some open source solutions such as MySQL, Cassandra and MongoDB on a Linux VM, instead of going the MS default stack.
Keep Performing,
Moshe Kaplan
