Amazon Web Services (AWS) Spot Fleets are collections of AWS spot instances, virtual servers from Amazon’s pool of spare capacity, offered at discounts of up to 90%. Spot instances need to be carefully managed, because they are terminated by Amazon at short notice when the market price goes about your bidding price. Applications can make requests for Spot Fleets via the Spot Fleet application programming interface (API) or the command line interface (CLI). Because spot instances pricing often changes, EC2 constantly attempts to balance the capacity according to predefined values.