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.')
