  We design and implement Megaphone, a data migration mechanism for stateful
distributed dataflow engines with latency objectives. When compared to existing
migration mechanisms, Megaphone has the following differentiating
characteristics: (i) migrations can be subdivided to a configurable granularity
to avoid latency spikes, and (ii) migrations can be prepared ahead of time to
avoid runtime coordination. Megaphone is implemented as a library on an
unmodified timely dataflow implementation, and provides an operator interface
compatible with its existing APIs. We evaluate Megaphone on established
benchmarks with varying amounts of state and observe that compared to na\"ive
approaches Megaphone reduces service latencies during reconfiguration by orders
of magnitude without significantly increasing steady-state overhead.
