Cassandra and ScyllaDB LWT error handling dtest example

lwt_error_handling.py

from dtest import Tester, debug
from cassandra import ConsistencyLevel

class LWTExample(Tester):
    def test_lwt(self):
        cluster = self.cluster
        cluster.populate(4)
        cluster.start(wait_other_notice=True, wait_for_binary_proto=True)
        node = cluster.nodelist()[0]
        session = self.patient_cql_connection(node)
        session.execute("CREATE KEYSPACE ks WITH replication={'class':'SimpleStrategy', 'replication_factor': 3}")
        session.execute("CREATE TABLE ks.users (id INT PRIMARY KEY, name TEXT)")

        # The IF NOT EXIST part makes the query LWT
        # query runs in 2 rounds
        #       1. to check the condition
        #       2. tries to apply the change with Paxos
        insert_stmt = session.prepare("INSERT INTO ks.users (id, name) VALUES (1, 'foo') IF NOT EXISTS")
        #https://docs.datastax.com/en/developer/python-driver/3.24/api/cassandra/query/#cassandra.query.Statement.consistency_level
        insert_stmt.consistency_level = ConsistencyLevel.QUORUM        # First read round CL
        # https://docs.datastax.com/en/developer/python-driver/3.24/api/cassandra/query/#cassandra.query.Statement.serial_consistency_level
        # https://docs.datastax.com/en/ddac/doc/datastax_enterprise/dbInternals/dbIntConfigSerialConsistency.html
        insert_stmt.serial_consistency_level = ConsistencyLevel.SERIAL # Second Paxos round CL
        try:
            res = session.execute(insert_stmt)
        except Exception as exc:
            debug(f"LWT INSERT query error {exc}")
        else:
            if not res[0].applied:
                debug(f"LWT INSERT query failed (user exists)")
            else:
                debug("Success")