Temporal – Workflow as Code (fork of Uber's Cadence, by the same team)

I began using this product a month ago, and I find it truly fascinating.

You can write long-running business logic as naive Java/go functions with 0 error handling and they just work. You can sleep for months, do transparent retries, and convert nearly any custom workflow engine to take advantage of it. You can mix and match Java and go as well.

I believe they're planning to launch a production version in late-June, and Python and C# clients.

The downside (which it shares with Cadence) is the very limited documentation, but the development team is very active on Slack and SO.