DescriptionWe describe a system being deployed at the National Radio Astronomy Observatory's Karl G. Jansky Very Large Array (VLA) to commensally identify and record millisecond timescale astrophysical transient events in real time. This system distributes a high time resolution data stream to a dedicated fast transient detection system while allowing processing of a primary observation to continue with the typical (lower) time resolution data. This form of dual time resolution, commensal observing is enabled by the vys protocol, implemented with existing VLA computing infrastructure. The fast transient detection system performs real-time analysis in situ to detect events of interest and record relatively short duration data "cut-outs" of those events. By selectively recording high time resolution data, provided by vys at rates of up to 1.4 GB/s, realfast will reduce the recorded data volume by an estimated factor of up to 1000. This makes it possible to search for transients commensally in a high data rate stream over the thousands of hours needed to find the rarest events.