Logging levels in Log4j for Mule Apps

Log levels are a fundamental concept in logging frameworks like Log4j. They determine the severity or importance of the log messages and help control which messages are captured and stored. Each log level represents a different kind of event, ranging from detailed debugging information to critical system errors.
In MuleSoft we use the following Log levels:

• Trace
• Debug
• Info
• Warn
• Error
• Fatal

It’s essential to strike a balance between the verbosity of the logs and performance, as excessive logging can impact the application’s performance.
Let’s see them in detail to understand what information each level contains and when we should use each:

Log Levels

TRACE

The TRACE level is the most detailed level, often used for fine-grained, step-by-step tracing of code execution. With TRACE level we can:

• Track the flow of execution in complex methods.
• Log the value of variables at various points.
• Diagnose issues that are not obvious at higher log levels.

DEBUG: 

DEBUG is less detailed than TRACE but still provides significant insights into the internal workings of an application. It's generally used to log information that might be helpful for debugging during development or troubleshooting. With DEBUG level we can:

• Log configuration settings.
• Detai method entries and exits.
• Output values of variables or states.

INFO: 

INFO level is used to log informational messages that highlight the progress of the application at a coarse-grained level. It provides a general overview of the application’s functioning and is typically enabled in production environments. With INFO level we can::

• Log application start and stop events.
• Highlight major milestones or significant operations.
• Provide general information about system status.

WARN: 

WARN level indicates potentially harmful situations or events that are not necessarily errors but could lead to problems. It alerts us to issues that may require attention but are not critical. With WARN level we can identify:

• Deprecated API usage.
• Configuration issues that could lead to problems.
• Recoverable errors or unexpected conditions.

ERROR: 

ERROR level logs error events that might still allow the application to continue running but represent significant problems. It is used for logging runtime errors, exceptions, or other conditions that are serious but not fatal to the application. For example, with ERROR level we would be logging:

• Exceptions that are caught but not fatal.
• Major issues in processing user requests.
• Failures in critical operations (like database access).

FATAL

FATAL level is used for the most severe error events that lead to the application’s premature termination or render the application unusable. These are critical conditions where immediate attention is required. With FATAL level we can log:

• Unrecoverable system failures.
• Fatal exceptions that cause the application to abort.
• Critical resource failures (like the inability to start a required service).

How to Define Log Levels in Log4j

Log levels are associated to Loggers in Log4j. Loggers are the main components in the Log4j framework responsible for capturing log messages. A logger is associated with a specific part of the application, usually identified by a name (often the name of the class or package where the logger is used).

Have a look at this post if you want to dive deeper into Loggers

MuleSoft defines different loggers for various components like connectors, transports, and core services. Each logger can be configured with different log levels (DEBUG, INFO, WARN, ERROR) and outputs (console, files).

The configuration of the logger is specified in the Loggers section of the log4j2.xml file. Each logger will set the log level with its own level tag (see example below). The root logger (mandatory in the log4j2.xml) sets the log level for the whole application.

MuleSoft loggers inherit configurations from parent loggers unless explicitly overridden. This hierarchical structure allows fine-grained control over logging. For example, you might set the root logger to INFO but enable DEBUG for a specific connector or component to troubleshoot an issue without flooding logs with too much data.

• For example:

<Loggers>
        <!-- HTTP logger -->
        <AsyncLogger name="org.mule.service.http.impl.service.HttpMessageLogger" level="DEBUG" />
       
        <AsyncRoot level="INFO">
            <AppenderRef ref="file"/>
        </AsyncRoot>
    </Loggers>

With this configuration, we are setting the log level to INFO for all the application with the AsyncRoot logger and we are specifying the DEBUG level only for the http connector.

Best Practices for Using Log Levels

1. Understand the Log Levels 

The first thing we need to do when working with Log Levels is to fully understand the purpose of each log level. As a summary of what we’ve seen, remember:

• TRACE: Very detailed logging, including highly granular details. Rarely used in production due to its verbosity.
• DEBUG: Detailed information typically of interest only when diagnosing problems. Useful in development and staging environments.
• INFO: Informational messages that highlight the progress of the application. Generally used in production.
• WARN: Potentially harmful situations that are not errors but could lead to issues. Use in production to catch non-critical issues.
• ERROR: Error events that might still allow the application to continue running. Essential for production environments.
• FATAL: Severe error events that lead to premature application termination. Critical to monitor in all environments.

2. Set Appropriate Log Levels for Different Environments

• Development Environment:
• Use DEBUG or TRACE to capture detailed logs that help in understanding the flow and identifying issues early.
• Enable TRACE and DEBUG levels only during development or troubleshooting to understand better your flows and identify issues early in dev.
• Staging/Testing Environment:
• INFO and DEBUG levels are ideal to ensure everything is functioning as expected without overwhelming log files.
• Production Environment:
• Stick to INFO, WARN, ERROR, and FATAL levels to capture essential information while minimizing log overhead. Avoid DEBUG and TRACE unless diagnosing a specific issue.

3. Log Only What’s Necessary

• Avoid logging sensitive information such as passwords, API keys, and personal data, especially at higher log levels like DEBUG or TRACE.
• Focus on logging messages that provide value for monitoring and troubleshooting. Excessive logging can clutter log files and obscure important information.

4. Monitor and Alert on Critical Logs

• Set up monitoring and alerting on ERROR and FATAL logs. This ensures that critical issues are detected and addressed promptly.
• Use WARN logs to alert on potential issues that may not require immediate action but could lead to problems if ignored.

5. Test Logging Configuration

• Regularly test your logging configuration in different environments to ensure it’s capturing the right level of detail without impacting performance.
• Adjust log levels as necessary based on the environment and the nature of the application.

6. Audit Logs Periodically

• Regularly audit your logs to ensure that they are providing useful information and that the log levels are appropriately set.
• Adjust the log levels and log messages based on the findings from these audits to continuously improve your logging strategy.

Log levels in Log4j are a powerful tool for controlling the verbosity and focus of logging within an application. By understanding and strategically using these log levels, you can ensure that your application logs provide the right level of detail needed for development, troubleshooting, and monitoring, without overwhelming your logging system or impacting performance.