DrAFTS -- Durability Agreements From Time Series for AWS Spot Instances

DrAFTS is an acronym for Durability Agreements From Time Series. Using historical pricing data from AWS and Quantile Bounds Estimation from Time Series (QBETS) -- a non-parametric time series forecasting methodology for predicting bounds on future values -- this site provides predicted bid prices (in US dollars) and time durations (in hours) with probabilistic guarantees. Each prediction is of the form where For example, a prediction of the form should be interpreted to mean that at Wed Feb 25 05:30:42 2015 a bid of $1.1041 an c4.4xlarge instance type would be above the "spot price" (with probability 0.96) at which AWS can terminate the instance for at least 1.72 hours (with probability 0.95).

NEW: Minimum Bid Calculator

DrAFTS computes the minimum value that can be specified as the "AWS Maximum Bid" such that the instance will start and run for the predicted duration with probability that is greater than or equal to 0.95. Any bid above the DrAFTS bid will also start and run with probability greater than or equal to 0.95 but for (potentially) a longer duration. The Bid Calculator graph (accessed by clicking on "What Should I Bid") shows the relationship between maximum AWS bid and the duration that is statistically guaranteed with probability at least 0.95. Using the graph it is possible to select the bid value to specify that assures a specific execution duration with probability 0.95 or greater.

Because Amazon obscures the global Availability Zone identities, the minimum bid calculator cannot determine what Availability Zone you should use -- only that the bid that it computes will be correct for which is ever zone is currently least expensive. To use the bid, request an instance in the Availability Zone with the lowest current price.

The site recomputes its predictions periodically. Note that the the historical pricing data provided by AWS is not necessarily complete. The algorithm attempts to compensate when it can, but some times are not represented or have insufficient data necessary to make a reliable prediction. Note also that AWS does not necessarily terminate a spot instance when the spot price exceeds the bid price -- it is just entitled to do so. For more information on spot instances, please see the AWS documentation for spot instances.

Using the Predictions as the basis for SLAs

The predictions are intended to provide a statistical "Service Level Agreement" guarantee associated with each spot instance type in each availability zone. That is, bidding the predicted price at the time stamp shown will "guarantee" that the spot price does not exceed the bid for at least the duration shown with a probability that is the product (assuming independence) of the two probabilities shown in each entry. Thus they do not reflect the instantaneous spot price or the duration it persists but rather a prediction of the upper bound on the spot price and the duration that the upper bound will persist.

Minimum Predicted Bids by Region

These predictions represent the minimum bid that guarantees the listed duration with probability at least 0.95.

Predicted Bid Prices by Instance Type for each Region

Predicted Bid Prices by Region for each Instance Type

If you are running an instance in a launch group and there is an active instance in that group, AWS will attempt to co-locate the new instance in the same availability zone with the running instance if the bid price is high enough for that availability zone. These prices reflect the bid price necessary to achieve an SLA per availability zone.

HOWEVER, AWS does not use the same mapping of availability zone names to physical resources for each user. Thus these predictions reflect what a "random" user sees in terms of availability zone names. If you wish to use them, you'll need to try and figure out how they map to the availability zone names AWS shows to your specific user ID.

This work is part of the Aristotle Federated Cloud Project sponsored by NSF.