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Cassandra multiprocessing can't pickle _thread.lock objects


I tried to use Cassandra and multiprocessing to insert rows (dummy data)
concurrently based on the examples in

http://www.datastax.com/dev/blog/datastax-python-driver-multiprocessing-example-for-improved-bulk-data-throughput

This is my code

class QueryManager(object):

concurrency = 100  # chosen to match the default in execute_concurrent_with_args
def __init__(self, session, process_count=None):
    self.pool = Pool(processes=process_count, initializer=self._setup,
initargs=(session,))
@classmethoddef _setup(cls, session):
    cls.session = session
    cls.prepared = cls.session.prepare("""
INSERT INTO test_table (key1, key2, key3, key4, key5) VALUES (?, ?, ?, ?, ?)
""")
def close_pool(self):
    self.pool.close()
    self.pool.join()
def get_results(self, params):
    results = self.pool.map(_multiprocess_write,
(params[n:n+self.concurrency] for n in range(0, len(params),
self.concurrency)))
    return list(itertools.chain(*results))
@classmethoddef _results_from_concurrent(cls, params):
    return [results[1] for results in
execute_concurrent_with_args(cls.session, cls.prepared, params)]

def _multiprocess_write(params):
    return QueryManager._results_from_concurrent(params)

if __name__ == '__main__':

    processes = 2

    # connect cluster
    cluster = Cluster(contact_points=['127.0.0.1'], port=9042)
    session = cluster.connect()

    # database name is a concatenation of client_id and system_id
    keyspace_name = 'unit_test_0'

    # drop keyspace if it already exists in a cluster
    try:
        session.execute("DROP KEYSPACE IF EXISTS " + keyspace_name)
    except:
        pass

    create_keyspace_query = "CREATE KEYSPACE " + keyspace_name \
                        + " WITH replication = {'class':
'SimpleStrategy',    'replication_factor': '1'};"
    session.execute(create_keyspace_query)

    # use a session's keyspace
    session.set_keyspace(keyspace_name)

    # drop table if it already exists in the keyspace
    try:
        session.execute("DROP TABLE IF EXISTS " + "test_table")
    except:
        pass

    # create a table for invoices in the keyspace
    create_test_table = "CREATE TABLE test_table("

    keys = "key1 text,\n" \
           "key2 text,\n" \
           "key3 text,\n" \
           "key4 text,\n" \
           "key5 text,\n"

    create_invoice_table_query += keys
    create_invoice_table_query += "PRIMARY KEY (key1))"
    session.execute(create_test_table)

    qm = QueryManager(session, processes)

    params = list()
    for row in range(100000):
        key = 'test' + str(row)
        params.append([key, 'test', 'test', 'test', 'test'])

    start = time.time()
    rows = qm.get_results(params)
    delta = time.time() - start
    log.info(fm('Cassandra inserts 100k dummy rows for ', delta, ' secs'))

when I executed the code, I got the following error

TypeError: can't pickle _thread.lock objects

which pointed at

self.pool = Pool(processes=process_count, initializer=self._setup,
initargs=(session,))


I am wondering how to resolve the issue.

cheers