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


(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)