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Futures

Futures are returned from any driver call that would result in blocking the client application/thread. This allows the client application to continue processing and/or also submit multiple queries in succession. Although the driver has an asynchronous design it can be used synchronously by immediately attempting to get result or explicitly waiting on the future.

Waiting for Results

Futures results can be waited on indefinitely by either calling the cass_future_wait() method or by attempting to get the result with cass_future_get_result(). They can also be waited on for an explicit amount of time or periodically polled without waiting to execute application code.

Synchronously Waiting on the Future

CassFuture* future = NULL;

/* Do some operation to get a future */

/* Block until a result or error is set */
cass_future_wait(future);

cass_future_free(future);

Synchronously Waiting for the Result

void wait_for_future(CassSession* session, CassStatement* statement) {
  CassFuture* future = cass_session_execute(session, statement);

  /* Blocks and gets a result */
  const CassResult* result = cass_future_get_result(future);

  /* If there was an error then the result won't be available */
  if (result == NULL) {
    /* The error code and message will be set instead */
    CassError error_code = cass_future_error_code(future);
    const char* error_message;
    size_t error_message_length;
    cass_future_error_message(future, &error_message, &error_message_length);

    /* Handle error */

    cass_future_free(future);
    return;
  }

  /* The future can be freed immediately after getting the result object */
  cass_future_free(future);

  /* Use the result object */

  cass_result_free(result);
}

Timed Wait

CassFuture* future = NULL;

/* Do some operation to get a future */

cass_duration_t microseconds = 30 * 1000000; /* 30 seconds */

/* Block for a fixed amount of time */
if (cass_future_wait_timed(future, microseconds)) {
  /* A result or error was set during the wait call */
} else {
  /* The operation hasn't completed yet */
}

cass_future_free(future);

Polling

CassFuture* future = NULL;

/* Do some operation to get a future */

/* Poll to see if the future is ready */
while (!cass_future_ready(future)) {
  /* Run other application logic or wait*/
}

/* A result or error was set */

cass_future_free(future);

Callbacks

A callback can be set on a future to notify the client application when a request has completed. Using a future callback is the lowest latency method of notification when waiting for several asynchronous operations.

Important: The driver may run the callback on thread that’s different from the application’s calling thread. Any data accessed in the callback must be immutable or synchronized with a mutex, semaphore, etc.

void on_result(CassFuture* future, void* data) {
  /* This result will now return immediately */
  CassError rc = cass_future_error_code(future);
  printf("%s\n", cass_error_desc(rc));
}

int main() {
  CassFuture* future = NULL;

  /* Do some operation to get a future */

  /* Set a callback instead of waiting for the result to be returned */
  cass_future_set_callback(future, on_result, NULL);

  /* The application's reference to the future can be freed immediately */
  cass_future_free(future);

  /* Run other application logic */

}