Future object deletion in async code

[Kaustubh Deorukhkar]

Hello,

I am trying out the cpp driver and had a question related to future object in async code handling.

I believe the future object created during any cassandra should be deleted(free) only after callback is called? Am not aware of internals how the driver handles it, but a logical guess.

Here is the sample am referring to:
Object created and free'd immediately here
https://github.com/datastax/cpp-driver/blob/1.0/examples/callbacks/callbacks.c#L80-L82

Callback called uses future object here:
https://github.com/datastax/cpp-driver/blob/1.0/examples/callbacks/callbacks.c#L96

Can someone please advice if this is correct in this drivers context?

Thanks,
Kaustubh

[Michael Penick]

The future can be safely freed immediately after setting the callback, but the application cannot safely use that future anymore except for inside the registered callback using the "future" parameter. The driver keeps a reference to the future object and that reference is used to extend the lifetime of that future for duration of the request and the register callback. The future is immediately destroyed after the callback.

Mike

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[Kaustubh Deorukhkar]

Hi Mike, Thanks for the info! Its a nice pattern!
Also Is it possible to keep the reference active after the callback, lets say the callback posts the future object to a queue and application can later free it after accessing the data?

Cheers,
Kaustubh

[Michael Penick]

In that case then don't immediately free the future, push the future into the queue from the callback, then the queue consumer is responsible for freeing the future after it has been dequeued. 

Something like this (pseudocode):

SomeQueueType* queue;

void on_execute(CassFuture* future, void* data) {

   enqueue(queue, future);

}

void consume_queue() {

   CassFuture* future = dequeue(queue);

   /* Do something with the future */

   cass_future_free(future); 

}

int main() {
  /* Connect session */

 /* ...*/  

  CassFuture* future = cass_session_execute(session, ...);
  cass_future_set_callback(future, on_execute, session);

  /* Don't free immediately */

/* ... */

}

Mike

[Kaustubh Deorukhkar]

Great! Thanks for your help!!