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LOG_LEVELS_EXPLAINED.md

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Log Levels explained

AI generated.

Levels

INFO:

  • Purpose: Give general information about the application’s normal operations.
  • Use Case:
    • Startup messages.
    • Major lifecycle events (e.g., server listening, database connections established).
    • High-level application state changes.
  • Example:
logger.info('Server is running on http://localhost:3000');

FATAL:

  • Purpose: Log unrecoverable errors causing the application to shut down.
  • Use Case:
    • Catastrophic failures, such as critical configuration errors or inability to bind to a network port.
  • Example:
logger.fatal('Unable to initialize the application!');
process.exit(1);

ERROR:

  • Purpose: Capture runtime errors that need attention but are not necessarily fatal.
  • Use Case:
    • Errors in processing requests, failed database queries, or third-party service failures.
    • Errors not caused by the user.
  • Example:
logger.error('Database query failed', error.stack);

WARN:

  • Purpose: Highlights issues that could potentially become errors.
  • Use Case:
    • Errors caused by bad API usage like invalid user input or deprecated functionality
    • Near-capacity resource usage.
  • Example:
logger.warn('High memory usage detected: 85% of heap used');

DEBUG:

  • Purpose: Provides detailed context for debugging purposes.
  • Use Case:
    • Tracing application logic, function calls, or API requests.
  • Example:
logger.debug(`Fetching user details for ID: ${userId}`);

TRACE (= VERBOSE):

  • Purpose: Offers very detailed and low-level information, often more extensive than debug.

  • Use Case:

    • Tracing application internals, such as library-specific operations.
    • Annotating each step in an algorithm or a query.
    • Useful when trying to trace obscure issues that require full insight into the system's behavior.

  • Example:

logger.verbose('Starting authentication process for user: johndoe');
logger.verbose('Extracted JWT from Authorization header: eyJhbGciOiJIUz...');
logger.verbose('Decoded JWT payload: { "userId": "12345", "roles": ["admin"] }');
logger.verbose('Checking user existence in database for userId: 12345');
logger.verbose('Authentication successful: user johndoe granted access');