Log Levels Explained: TRACE, DEBUG, INFO, WARN, ERROR, and FATAL

Log levels indicate the severity of an event. Understanding them is essential for reading logs effectively, configuring applications, and responding to incidents. This guide explains each standard level and when it applies.

The standard log levels

Most logging frameworks use these levels, from least to most severe:

TRACE

The most detailed level. TRACE messages capture the finest-grained information — function entry and exit, variable values at specific points, and step-by-step execution flow.

Use TRACE when debugging complex logic that cannot be reproduced in a development environment. TRACE is almost always disabled in production due to volume.

DEBUG

Detailed information useful for debugging. DEBUG messages are less granular than TRACE but still more verbose than normal operation. They might include database query details, request parameters, or intermediate computation results.

Use DEBUG during development and troubleshooting. Like TRACE, DEBUG is typically disabled in production.

INFO

Normal operational messages. INFO confirms that things are working as expected. Application startup, successful connections, configuration loaded, and request completion are typical INFO events.

INFO is the default level for most production systems. It provides enough context to understand system behavior without overwhelming detail.

WARN

Warning messages indicate something unexpected happened, but the application continues to operate. Deprecated configuration options, retry attempts, and degraded performance are common WARN events.

WARN messages deserve attention but do not require immediate action. They often indicate issues that could become problems if left unaddressed.

ERROR

Error messages indicate a failure occurred. The application may continue running, but a specific operation failed. Database connection failures, request timeouts, and unhandled exceptions are typical ERROR events.

ERROR messages require investigation. They indicate that something the application expected to succeed did not.

FATAL

The most severe level. FATAL messages indicate the application is about to stop running. Unrecoverable errors, critical resource exhaustion, and unhandled fatal exceptions are FATAL events.

FATAL messages require immediate attention. The application has terminated or is about to.

Choosing the right level

Filtering by level in Filelume

When you open a log file in the Log Viewer, you can filter by level to focus on what matters:

• Show only ERROR and FATAL to find failures quickly.
• Show WARN and above to see potential problems.
• Show INFO and above for a complete operational picture.
• Show DEBUG only when troubleshooting specific issues.

Common variations

Not all systems use the same level names. Some frameworks use:

• CRITICAL instead of or alongside FATAL
• NOTICE between INFO and WARN (syslog tradition)
• VERBOSE instead of TRACE

Filelume recognizes common variations and maps them to the standard levels for consistent filtering.