This API is also used for resimulating. Because of this, the input data is const and its lifetime is maintained by the internal thread. It is expected that callbacks are "pure" in the sense that they rely only on the input data and affect the simulation in a repeatable and deterministic way. This means that if the same inputs are passed into the callback, we expect the exact same output and that any simulation changes are the same. We rely on this to cache results and skip callbacks when possible during a resim. See functions for more details.