How Spot Instances Work
Spot Instances perform exactly like other EC2 instances while running. However, they can be interrupted by Amazon EC2 when EC2 needs the capacity back.
When EC2 interrupts your Spot Instance, it either terminates, stops, or hibernates the instance, depending on the interruption behavior that you choose.
If EC2 interrupts your Spot Instance in the first hour, before a full hour of running time, you're not charged for the partial hour used. However, if you stop or terminate your Spot Instance, you pay for any partial hour used (as you do for On-Demand or Reserved Instances). For information about how you're billed for interrupted Spot Instances running on different operating systems, see Billing for interrupted Spot Instances in the EC2 User Guide.
The Spot price for each instance type in each Availability Zone is determined by long-term trends in supply and demand for EC2 spare capacity. You pay the Spot price that's in effect, billed to the nearest second.
You can optionally specify a maximum price for your Spot Instance. If you don't specify a maximum price, the maximum price defaults to the On-Demand price. Note that you never pay more than the Spot price that is in effect when your Spot Instance is running. We recommend that you do not specify a maximum price, but rather let the maximum price default to the On-Demand price. A high maximum price does not increase your chances of launching a Spot Instance and does not decrease your chances of having your Spot Instance interrupted (because EC2 can still interrupt your Spot Instance when it needs the capacity back).
The Spot price for an instance type in an Availability Zone might change at any time, but in general, it does not change frequently. AWS publishes the current Spot price and the historical prices for Spot Instances through the DescribeSpotPriceHistory API, as well as in the AWS Management Console, which reflects the data from the API. This can help you assess the levels and timing of fluctuations in the Spot price over time.
