The Temporal/Cadence/SWF operate as libraries any application can include to be able to implement workflow and activity logic. So hooking into low-level VM event loop was not an option. So they rely purely on event sourcing to reexecute the program code from the beginning assuming that the workflow code is deterministic. The library provides various API wrappers to execute multithreaded code deterministically using cooperative multithreading. In the future, WebAssembly can be used as a container as determinism is one of its core features.