Runtime migration has been widely adopted to achieve several tasks such as load balancing, performance optimization, and fault-tolerance. However, existing migration techniques do not work for event detectors in distributed publish/subscribe systems that are used to analyze sensor data. Since low-latency time-constraints are no longer valid they reorder streams incorrectly and cause erroneous event detector states. This paper presents a safe runtime migration of stateful event detectors that respects low-latency time-constraints and seamlessly orders input events correctly on the migrated host. Event streams are only forwarded until timing delays are properly calibrated, the migrated event detector immediately stops processing after its state is transferred, and the processing overhead is negligible. On a Realtime Locating System (RTLS) we show that we can efficiently migrate event detectors at runtime between servers where other techniques would fail.
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