[GitHub] tardieu commented on issue #3202: Support action continuations in the controller

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tardieu commented on issue #3202: Support action continuations in the controller
URL: https://github.com/apache/incubator-openwhisk/pull/3202#issuecomment-363658304
 
 
   I agree that the code with a future is simpler than the code with a promise so I am ok with the change. But, FWIW, there was a decent reason to use the more complex concept that I try to summarize below (before I forget).
   
   The future-based implementation is now similar to the sequence implementation. It creates one pending future (ie pending continuation) for each level of nesting (sequence inside a sequence, composition inside a composition). In contrast the promise-based implementation only requires one pending continuation irrespective of nesting. It used instead a call stack (now removed caller field in session object) and unwinds the stack upon termination of a nested composition. Concretely, the callee resumes the execution of the caller explicitly with the promise implementation, whereas with the future implementation all the stack levels simultaneously exist as pending tasks in the scheduler. In short, I believe the future implementation is by nature more expensive. But since we cap the recursion depth to a low value, it is most likely irrelevant in practice.

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