Can Serializable Scope protect concurrent access within a <flow> ?

[Ricky Ho <riho@cisco.com>]

I believe the intention of "serializable scope" is to provide some 
mechanism to protect shared data under concurrent access.  This mechanism 
relies on a clear demarcation point for the "critical section" (typically 
there won't be new thread of execution created within the scope).  I guess 
the intention is for protection concurrent execution of message handlers.

However, it is not very clear to me if the same mechanism can be used 
within a <flow> to protect shared data under concurrent access.  (In a DAG 
model, the start and end of a new thread of execution is not easily 
identifiable and so specifying the scope is hard).

Therefore, is there any other mechanism to protect shared data within a 
<flow> against concurrent access ?

Rgds, Ricky