Lesson Learned #512: Handling Connection Reuse in ODBC After a Critical Error

Working on several connectivity cases with ODBC across different programming languages, I’ve noticed a behavior that I would like to share, as it can lead to incorrect conclusions about whether a connection has actually been closed. 

In one of these cases, I found that once a connection is established, if a query execution results in a code error, timeout, or any other critical issue, reusing the same connection to execute another query may produce Connection closed messages.

This behavior occurs because, when a critical error happens, ODBC drivers set an internal flag indicating that the connection is no longer in a 'clean' state. This flag prevents further commands from being executed on the connection until it is fully closed and reopened. 

In these situations, the best approach is to open a new connection to avoid further issues. 

def TestConnectionErrorReuse():
    try:
        thread_id = threading.get_ident()
        conn, db_name = ConnectToTheDB(querytimeout=10)
        
        if conn is None:
            logging.info(f'(TestConnectionErrorReuse) - Thread: {thread_id} - Failed to establish initial connection. Exiting...')
            return

        cursor = conn.cursor()
        
        # Step 1: Execute a query that will cause an error (division by zero)
        try:
            logging.info(f'(TestConnectionErrorReuse) - Thread: {thread_id} - Executing a query that will cause an error (SELECT 1/0)...')
            cursor.execute("SELECT 1/0")  # This should trigger a division by zero error
        except pyodbc.Error as e:
            logging.error(f'(TestConnectionErrorReuse) - Thread: {thread_id} - Error during query execution: {e}')

        # Step 2: Attempt to reuse the connection by running another query
        try:
            logging.info(f'(TestConnectionErrorReuse) - Thread: {thread_id} - Attempting to execute a follow-up query after error...')
            cursor.execute("SELECT 1")  # Simple query to test reuse of connection
            row = cursor.fetchone()
            print(f"(TestConnectionErrorReuse) - Thread: {thread_id} - Follow-up query result: {row[0]}")
        except pyodbc.Error as e:
            logging.error(f'(TestConnectionErrorReuse) - Thread: {thread_id} - Error reusing connection after previous failure: {e}')
        
    except Exception as e:
        logging.error(f'(TestConnectionErrorReuse) - Thread: {thread_id} - Unexpected error occurred: {e}')
    
    finally:
        if conn:
            conn.close()
            logging.info(f'(TestConnectionErrorReuse) - Thread: {thread_id} - Connection closed.')