When adopting cloud computing services, reliability availability and durability are key concerns. It is important to determine what level the cloud provider is offering for the above parameters. Lets take Amazon S3 as an example. S3 is described by Amazon as a highly durable storage infrastructure designed for mission critical and primary data storage. Amazon then highlight the following facts:
The above, quoted by Amazon, give us a feeling that durability is incredibly high. However, the wording is clever. It states that S3 is designed to provide 99.999999999% durability and 99.99% availability of objects over a given year. This is potentially very different to what is actually provided. Reading the Service Level Agreement highlights that the S3 SLA offers no guarantee of durability, just availability. Further, this availability is not at 99.99% but 99.9%. I quote the SLA below.
AWS will use commercially reasonable efforts to make Amazon S3 available with a Monthly Uptime Percentage (defined below) of at least 99.9% during any monthly billing cycle (the “Service Commitment”)
If the availability falls between >=99% and < 99.9% customers will receive 10% of the monthly charge in credit. Availability < 99% and customers receive a credit of 25% of the monthly charge.
The reason I write about this is not as a criticism of Amazon – I think their services are first class and use them widely. I write to highlight the fact that when considering moving to the cloud, the SLA's need to be read in detail so that there is no misunderstanding between what is actually being provided and what you think is being provided.