Distributed applications assume that datacenter servers are available unless they fail. Transient servers, which are available temporarily for an uncertain amount of time, are cheaper and more energy efficient than stable servers, which have continuous availability. This article looks at an important characteristic of transient servers: advance warning of unavailability. The Yank system defines a bounded-time virtual machine migration mechanism that leverages an advance warning of a few seconds to provide high availability cheaply and efficiently at large scales.