cassandra.concurrent - Utilities for Concurrent Statement Execution




(session, statements_and_parameters, concurrency=100, raise_on_first_error=True, results_generator=False)

Executes a sequence of (statement, parameters) tuples concurrently. Each parameters item must be a sequence or None.

The concurrency parameter controls how many statements will be executed concurrently. When Cluster.protocol_version is set to 1 or 2, it is recommended that this be kept below 100 times the number of core connections per host times the number of connected hosts (see Cluster.set_core_connections_per_host()). If that amount is exceeded, the event loop thread may attempt to block on new connection creation, substantially impacting throughput. If protocol_version is 3 or higher, you can safely experiment with higher levels of concurrency.

If raise_on_first_error is left as True, execution will stop after the first failed statement and the corresponding exception will be raised.

results_generator controls how the results are returned.

  • If False, the results are returned only after all requests have completed.

  • If True, a generator expression is returned. Using a generator results in a constrained memory footprint when the results set will be large – results are yielded as they return instead of materializing the entire list at once. The trade for lower memory footprint is marginal CPU overhead (more thread coordination and sorting out-of-order results on-the-fly).

A sequence of ExecutionResult(success, result_or_exc) namedtuples is returned in the same order that the statements were passed in. If success is False, there was an error executing the statement, and result_or_exc will be an Exception. If success is True, result_or_exc will be the query result.

Example usage:

select_statement = session.prepare("SELECT * FROM users WHERE id=?")

statements_and_params = []
for user_id in user_ids:
    params = (user_id, )
    statements_and_params.append((select_statement, params))

results = execute_concurrent(
    session, statements_and_params, raise_on_first_error=False)

for (success, result) in results:
    if not success:
        handle_error(result)  # result will be an Exception
        process_user(result[0])  # result will be a list of rows

Note: in the case that generators are used, it is important to ensure the consumers do not block or attempt further synchronous requests, because no further IO will be processed until the consumer returns. This may also produce a deadlock in the IO event thread.



(session, statement, parameters, *args, **kwargs)

Like execute_concurrent(), but takes a single statement and a sequence of parameters. Each item in parameters should be a sequence or None.

Example usage:

statement = session.prepare("INSERT INTO mytable (a, b) VALUES (1, ?)")
parameters = [(x,) for x in range(1000)]
execute_concurrent_with_args(session, statement, parameters, concurrency=50)