Understanding Log Levels: DEBUG, INFO, WARN, ERROR, and FATAL

Log levels indicate how important a log message is, allowing users to filter key information about system status and reduce noise and alert fatigue.

The article introduces several common log level types to help you master log monitoring.

1. DEBUG (Debug)

DEBUG logs output detailed debugging information, useful during development and troubleshooting to trace code execution paths, variable values, and loop iterations.

Applicable scenarios: Recording detailed program execution, such as function calls, variable values, and iteration counts, to locate and resolve issues.

Example: Logging function input parameters and return values, loop iteration counts, and branch execution details.

2. INFO (Information)

INFO is the standard level for normal operational messages, such as program start-up, configuration details, and completion of key actions, providing insight into the program’s state.

Applicable scenarios: Recording normal operations, important events, and status information to understand program behavior.

Example: Logging program start and shutdown events, configuration file loading, user login and logout.

3. WARN (Warning)

WARN logs non‑fatal warnings that indicate potential problems while allowing the program to continue running.

Applicable scenarios: Notifying developers or users of issues that may need attention but do not stop execution.

Example: Missing configuration items, excessive resource usage, network timeout.

4. ERROR (Error)

ERROR logs error conditions that the program encounters but can still continue processing.

Applicable scenarios: Recording errors that do not cause the program to crash.

Example: Database connection failures, file read/write errors, HTTP request returning error status codes.

5. FATAL (Fatal)

FATAL logs critical errors that cause the system to crash or become unrecoverable, requiring immediate shutdown and emergency handling.

Applicable scenarios: Recording severe errors that force the program to stop immediately.

Example: Critical component crashes, loss of essential database data, severe security vulnerabilities.

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