Why are there no out-of-the-box high-availability solutions for SQL Server in the cloud?

When selecting any cloud service, you would like to get some guarantees about the reliability (uptime) and durability (what is the chance that your data is lost) of the service. Now, let’s see what we can find on cloud relational database services, either as a PaaS (Platform as a Service), or installed as part of an IaaS (Infrastructure as a Service, i.e. virtual machines) configuration.

  • Windows Azure SQL Database gives an uptime SLA of 99.9%. The database is replicated treefold (within the same datacenter), but Microsoft gives no SLA for durability of the data. It indicates that to secure your data against widespread datacenter failure, you will need to export your complete database yourself (as a SQL script, or a BACPAC file).
  • Amazon RDS (Relation Database Service)  has no SLA at all for uptime or durability. In the past, RDS has turned out to be the most unreliable of Amazon’s services, because it is build on top of the EC2 services (virtual machines) and the EBS service (virtual drives). RDS uses SQL Server logfiles to be able to restore a complete server instance (not separate databases) to a maximum time of 5 minutes ago. It also maintains daily snapshots of the instance, which can be retained for a maximum of 35 days. RDS also offers live replication and failover to another datacenter (Multi-AZ), but this is not available for SQL Server.
  • SQL server running on virtual machines of Amazon (EC2) or Microsoft (Azure Virtual Machines) have no SLA. There is only an SLA for the underlying virtual machines from both Amazon and Microsoft for 99,95% uptime.

So, from this I have to conclude there’s really a lot of uncertainty about the reliability and durability of any database service in the cloud. It also means that for SaaS applications where you can not afford to a have a few hours (or more) downtime you have to look into separate high-availability solutions. What is available in this area, specifically for SQL Server?

  • Windows Azure SQL Database has no built-in high-availability solution. You can copy a database to a new database in the same datacenter, but to copy it to another datacenter you have to export/import all data to a new database. Failover to a database copy will have to be managed manually.
  • Amazon RDS has a Multi-Availability-Zone option (Multi-AZ), where a database replica is maintained in another datacenter in the same region. This option, however, is only available for Oracle and MySQL, not for SQL Server. For SQL server your only option is to make instance snapshots, but you can not copy RDS snapshots to another datacenter.
  • On Amazon or Microsoft Azure virtual machines, you can implement any of the standard high-availability solutions of SQL Server (SQL Server 2012 AlwaysOn, database mirroring, log shipping). But keep in mind the following:
    • SQL Server 2012 AlwaysOn, and asynchronous database mirroring,  are only available in the Enterprise Edition of SQL Server. You will have to buy your own license for this, which is extremely price ($7000 per core).

    All of these high-availability solutions that you implement yourself on top of cloud virtual machines require a lot of expertise, ongoing management, and more than double the cost of the solution.

Conclusion: In general, for SaaS suppliers durability of data is much more important than uptime. In most cases 99,9% uptime would be sufficient (less than 1 hour downtime each month), but data durability has to be “Nine Nines” (99,9999999%), because any permanent data loss is unacceptable. Currently, none of the cloud relational database suppliers can offer this reliabiliy “out of the box” for SQL Server. Amazon RDS’s Multi-AZ solution comes closest (at almost double the price), but it is not available for SQL Server. Suggested manual solutions (based on export/import of data) are in many cases completely unrealistic for large amounts of data, because it could take days to complete an import of terabytes of data.

With all this,  are cloud relational database services currently an option for business-critical applications? I think they are, but the situation with regards to high-availability needs to improve rapidly, so that high-availability is simply an option to check. In the mean time, especially when you need to keep costs in check, we will need to rely on old-fashioned methods like log-shipping and data-export to safeguard our own data.

Amazon EC2 monthly price tables for SaaS-relevant configurations, in euros

EC2 is Amazon’s cloud service for flexible use of virtual machines. One of the big advantages of this service is that Amazon’s hourly prices also include the cost for the Windows and/or SQL Server license. Amazon has just announced lower prices for Windows EC2 instances.

It’s not always easy to calculate total monthly costs for various configurations, so in this post I will try to give an indication of montly costs for configurations that would be relevant for SaaS applications with 1,000 – 50,000 customers.
Note: In the monthly prices below I have incorporated the upfront costs for 1- and 3-year contracts in the monthly prices (spread out over the full term).
All prices are calculated for the EU region (Ireland).

Windows web server. No additional storage.

Configuration Resources On-demand (hourly) p/m 1-year term p/m 3-year term p/m
M1 large instance 2 cores, 7.6 GB memory €200 €131 €99
M1 extra large instance 4 cores, 15 GB memory €400 €262 €198
M3 double extra large instance 8 “second-generation” cores, 30 GB memory €859 €554 €419

Traffic out (to the internet) is an additional €92 per TB.

Load-balanced high-activity Windows web server.

Amazon Elastic Load Balancer
3 M3 double extra large EC2 instance on 3-year terms
5 TB of internet traffic per month
Total cost per month = €1763

Database server, EC2 instance with Windows server 2008/2012 and SQL Server Web Edition 2008/2012.

Prices below include 1TB of database storage on EBS (Elastic Block Storage, Amazon’s virtual drives). For instances without guaranteed I/O I have included 500 million I/O requests per months. Upfront costs have been spread over the months of the term.

Configuration Resources Guaranteed I/O On-demand (hourly) p/m 1-year term p/m 3-year term p/m
High-memory extra-large instance 2 cores, 17 GB memory (no guaranteed I/O) €463 €292 €249
High-memory double extra-large instance 4 cores, 34 GB memory 500 Mbps €798 €467 €384
High-memory quadruple extra-large instance 8 cores, 68 GB memory 1000 Mbps €1491 €829 €666

Note that you would also need additional storage for database backups, which could add €100-500 per month, depending on backup methods and backup retention.

Some important gotchas when starting with Amazon RDS SQL Server

RDS is the relational database service of Amazon Web Services. It is a ready-to-use cloud service: No OS or RAID to set up; you just specify the type of RDBMS you want, the memory/storage sizes, and a few minutes later you have a database instance up and running. The instance has its own (very long) DNS name, and you can access this database server from any place in the world using the standard SQL Server tools, if you give the client IP-addresses access through the “Security Group” linked to the database instance. Currently RDS is offered for Microsoft SQL Server, Oracle, and MySQL.

My company (Yuki) is currently using RDS for some smaller projects, and I’m investigating if/how our main database infrastructure could be moved to RDS, so that we can achieve improved scalability, world-wide reach, and lower maintenance costs on our database servers (which are by far the biggest bottlenecks for the web applications of Yuki). In this process I have discovered some important gotchas when using RDS SQL Server, that are not that well advertised, but can be big stumbling blocks:

  1. The sysadmin server role is not available. That’s right, you specificy a master user/password when creating the RDS instance, but this user is not in the sysadmin role; this user does have specific rights to create users and databases and such. Amazon has, of course, done this to lock down the instance. However, this can be a big problem when installing third-party software (such as Microsoft SharePoint) that requires the user to have sysadmin rights on the SQL Server.
  2. The server time is fixed to UTC. The date/time returned by the SQL Server function GETDATE is always in UTC, with no option to change this. This can give a lot of problems if you have columns with GETDATE defaults, or queries that compare date/time values in the database to the current date/time. For us, this currently is a big problem, which would require quite extensive changes in our software.
  3. No SQL Server backup or restore. Because you have no access to the file system (and the backup/restore rights are currently locked down in RDS), you can not move your data to RDS by restoring a backup. You have to use BCP or other export/import mechanisms. It also means, that you can only use the backup/restore that Amazon offers for the complete instance, meaning that you can not backup or restore individual databases. This point could easily be the biggest hurdle for many companies to move to RDS SQL Server.
  4. No storage scaling for SQL Server. Both Oracle and MySQL RDS instance can be scaled to larger storage without downtime, but for SQL Server you are stuck with the storage size you specify when you create the instance. This is a huge issue, since you have to allocate (and pay for!) the storage right at the start, when you have no idea what your requirements will be in a year’s time. This greatly undermines the whole scalability story of AWS.
  5. No failover for SQL Server. Again, both Oracle and MySQL can be installed with “Multi-AZ Deployment”, meaning there is automatic replication and failover to a server in another datacenter in the same Amazon region. No such option for SQL Server, meaning your only option in failure situations is to manually restore a backup that Amazon made of your instance.

All in all, still quite a few shortcomings, and some of which can be hurdles for deployment that can not be overcome. Personally, I love the service, as setting up the hardware and software for a reliable database server is a really complex task, which not very many people have any serious knowledge of. Let’s hope that Amazon keeps up its quick pace of innovation to improve the above points for RDS SQL Server.

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