There many workflow system out there but I was wondering which one of the open source workflow management system is the closest to the AWS Flow Framework (with Amazon SWF like capability build in)?
AWS Flow Framework itself is open source under Apache 2.0 license. Its source code is available on github: https://github.com/aws/aws-swf-flow-library.
Update: If you need an open source solution that includes the service and is much more features rich check out Temporal.