Default implementation of FlowExecution that uses a stack-based data structure to manage flow sessions . This class is closely coupled with FlowSessionImpl and FlowControlContextImpl. The three classes work together to form a complete flow execution implementation based on a finite state machine.

This implementation of FlowExecution is serializable so it can be safely stored in an HTTP session or other persistent store such as a file, database, or client-side form field.

Note: this implementation synchronizes the signalEvent Flow execution entry point. It is locked on a per client basis for this flow execution. Synchronization prevents a client from being able to signal other events before previously signaled ones have processed in-full, preventing possible race conditions.