Python3: cassandra.cluster.NoHostAvailable: (“Unable to connect to any servers using keyspace 'test'”, ['127.0.0.1']) when using execute_async future

[dev loper]

Hi All, 

I am trying to fetch data from Cassandra from a specific table and trying to insert it into another table in Cassandra after making some changes. Both the tables are located in keyspace "test". When I am trying to get the data from the first table everything works fine and it is able to fetch the data. However, in the future handler which handles the output of the first query, I am trying to insert the data into another table under the same Cassandra instance and it is gettingting failed. I am getting an error from the application stating "cassandra.cluster.NoHostAvailable: ("Unable to connect to any servers using keyspace 'test'", ['127.0.0.1'])" . I am not sure where I am going wrong. I have posted the question in stackoverflow 
https://stackoverflow.com/questions/65686641/python3-cassandra-cluster-nohostavailable-unable-to-connect-to-any-servers-u
Even after two bounties I have not received any answer so far. The problem is simple enough and it can be reproduced with any table and it is not dataset dependent as well. I kindly request you to help me and guide me where I am going wrong.  


import threading
from threading import Event
from cassandra.query import SimpleStatement
from cassandra.cluster import Cluster


hosts=['127.0.0.1']
keyspace="test"
thread_local = threading.local()
cluster_ = Cluster(hosts)
def get_session():
    if hasattr(thread_local, "cassandra_session"):
        print("got session from threadlocal")
        return thread_local.cassandra_session
    print(" Connecting to Cassandra Host " + str(hosts))
    session_ = cluster_.connect(keyspace)
    print(" Connecting and creating session to Cassandra KeySpace " + keyspace)
    thread_local.cassandra_session = session_
    return session_


class PagedResultHandler(object):

    def __init__(self, future):
        self.error = None
        self.finished_event = Event()
        self.future = future
        self.future.add_callbacks(
            callback=self.handle_page,
            errback=self.handle_error)

    def handle_page(self, rows):
        for row in rows:
            process_row(row)

        if self.future.has_more_pages:
            self.future.start_fetching_next_page()
        else:
            self.finished_event.set()

    def handle_error(self, exc):
        self.error = exc
        self.finished_event.set()

def process_row(row):
    print(row)
    session_ = get_session()
    stmt = session_.prepare(
        "INSERT INTO test.data(customer,snr,rttt, event_time) VALUES (?,?,?,?)")
    results = session_.execute(stmt,
                               [row.customer, row.snr, row.rttt,row.created_time])
    print("Done")

session = get_session()
query = "select * from test.data_log"
statement = SimpleStatement(query, fetch_size=1000)
future = session.execute_async(statement)
handler = PagedResultHandler(future)
handler.finished_event.wait()
if handler.error:
    raise handler.error
cluster_.shutdown()



However, when I try to execute the python file the application is throwing an error "cassandra.cluster.NoHostAvailable: ("Unable to connect to any servers using keyspace 'test'", ['127.0.0.1'])" from getSession() call from "process_row" method. Clearly, the first call to Cassandra is getting succeeded without any issues. There is no connectivity issue and the Cassandra instance is running fine locally. I am able to query the data using cqlsh. If I call the process_row method outside the future handler everything is working fine, I am not sure what needs to be done to make it happen from the Future Handler.

Connecting to Cassandra Host ['127.0.0.1']
Connecting and creating session to Cassandra KeySpace test
Row(customer='abcd', snr=100, rttt=121, created_time=datetime.datetime(2020, 8, 8, 2, 26, 51))
 Connecting to Cassandra Host ['127.0.0.1']
Traceback (most recent call last):
  File "test/check.py", , in <module>
    raise handler.error
  File "cassandra/cluster.py", line 4579, in cassandra.cluster.ResponseFuture._set_result
  File "cassandra/cluster.py", line 4777, in cassandra.cluster.ResponseFuture._set_final_result
  File "test/check.py"",  in handle_page
    process_row(row)
  File "test/check.py"",  in process_row
    session_ = get_session()
  File "/test/check.py"", in get_session
    session_ = cluster_.connect(keyspace)
  File "cassandra/cluster.py", line 1715, in cassandra.cluster.Cluster.connect
  File "cassandra/cluster.py", line 1772, in cassandra.cluster.Cluster._new_session
  File "cassandra/cluster.py", line 2553, in cassandra.cluster.Session.__init__
cassandra.cluster.NoHostAvailable: ("Unable to connect to any servers using keyspace 'test'", ['127.0.0.1'])

Process finished with exit code 1



Thanks
Dev Loper