Lesson Learned #515:Recommendations for Troubleshooting - Login failed for user 'XXX'. (18456)

During a recent support case, a customer encountered the error: pyodbc.InterfaceError: ('28000', "[28000] [Microsoft][ODBC Driver 18 for SQL Server][SQL Server]Login failed for user 'XXX'. (18456) (SQLDriverConnect); ") using a Python code. Following, I would like to share my lessons learned to fix this issue. 

The error code 18456 is typically caused by login issues, such as incorrect or missing credentials, rather than connectivity or networking problems. In our investigation, we identified the root cause and suggested recommendations to prevent and resolve similar issues.

Root Cause

The application was configured to retrieve the database server and host name from environment variables. However:

• Missing Environment Variables: One or more variables were not set.
• Default Value Misconfiguration: The code defaulted to a hardcoded value when variables were missing. For example, the server defaulted to "localhost", which was not the intended database server.

As a result, the application attempted to connect to an unintended server with incorrect or missing credentials, leading to the Login failed error.

Recommendations

1. Validate Environment Variables

Always validate critical environment variables like server, username, and password. If a required variable is missing or empty, the application should raise an explicit error or log a clear warning.

2. Avoid Misleading Defaults

Use placeholder values, such as "NOT_SET", as defaults for critical variables. This approach ensures that misconfigurations are immediately visible and do not silently fail.

3. Log Connection Details

Log critical details like the server and database being accessed. Ensure this information is included in application logs to make troubleshooting easier. Avoid logging sensitive information such as passwords.

Python Solution

I was thinking how to improve the Python code, implementing a validation of environment variables, handle errors and log critical connection details:

import os

def get_env_variable(var_name, default_value=None, allow_empty=False):
    """
    Retrieves and validates an environment variable.
    
    :param var_name: The name of the environment variable.
    :param default_value: The default value if the variable is missing.
    :param allow_empty: If False, raises an error for empty variables.
    :return: The value of the environment variable or default_value.

    Example: server = get_env_variable("DB_SERVER", default_value="NOT_SET")
    """
    value = os.getenv(var_name, default_value)
    if value is None or (not allow_empty and (value.strip() == "" or value.strip() == "NOT_SET" or default_value is None)):
        raise ValueError(f"Environment variable '{var_name}' is required but not set.")
    return value