At Estuary, we have an in-house Rust crate [1] for building scale-out durable actors / FSMs in Postgres. It powers all async activity in our control plane -- slews of fine-grain scheduled actions, complex change propagation through data-flow topologies, reliable alert and email delivery, and more -- at hundreds to thousands of state transitions per second (today). It's been a wonderful pattern to build on, and is all of three source files.

Here's a an example computing a Fibonacci sequence (very inefficiently, with lots of spawned sub-tasks and message passing) [2]

[1] https://github.com/estuary/flow/tree/master/crates/automatio... [2] https://github.com/estuary/flow/blob/master/crates/automatio...