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Spot instances are Amazon's excess compute capacity that is not currently in use. Customers can place bids to use this extra capacity at tremendous discounts up to 90% off the On-Demand rate. Spot instances can be reclaimed by Amazon at any moment leaving your application vulnerable to downtime. Instance Orchestrator runs in the background automatically switching back and forth between Spot and On-demand instances. This ensures that applications are always available and maximizes savings by taking advantage of Spot prices as frequently as possible.