When using HAProxy, which is a popular open source software load balancer that provides a high availability load balancing and proxying solution for TCP and HTTP-based applications, it is critical to have detailed logging to diagnose client-server interactions, server health states, and backend server response times. However, some users have reported issues regarding the Core log Lua not logging to the default HAProxy log file under certain configurations. We will explore what this entails and how to address such cases effectively.

Understanding HAProxy Logging

HAProxy allows extensive logging capabilities that help in debugging and monitoring purposes. The logs can be directed to different output destinations which typically include:

• STDOUT, often redirected to a file or a logging daemon on Unix-based systems.
• Syslog server, handling both local and remote log management.

The default logging configuration in HAProxy does an excellent job for standard operations. However, specialized scripts like those implemented with Lua in HAProxy can have some unique logging requirements and behaviors.

Lua Scripting in HAProxy

Introduced in HAProxy version 1.6, Lua scripting capabilities allow users to extend load balancer functionalities. This is accomplished by scripting custom actions in the Lua programming language. Lua scripts in HAProxy can interact with the core via the HAProxy Lua API, influencing decisions or generating complex behaviors.

The Issue: Lua Logging Not Appearing in Default Logs

The core logs for Lua in HAProxy are meant to provide insights into the operation of Lua scripts—debugging outcomes, errors, or custom logs created by the Lua script itself. Unfortunately, users sometimes observe that Lua-generated logs do not appear in the default HAProxy log files. This discrepancy generally arises from how the Lua logs are handled differently within the HAProxy architecture.

Causes and Solutions

1. Lua Print Statements Go to STDERR: Lua's print() statements write to the standard error (STDERR), not the standard output (STDOUT). Since the default HAProxy logs collect messages from STDOUT and typically ignore STDERR where Lua scripts send their logs, these messages will not appear in the expected log files.
   Solution: Redirect STDERR to STDOUT in your HAProxy service configuration. This can typically be added in the service’s start-up script or by adjusting the logging parameters in HAProxy’s global settings.
2. Different Syslog Channel: If HAProxy is configured to use Syslog for logging, Lua logs might be directed to a different Syslog channel or facility, which could cause them to be filtered out or ignored based on the Syslog daemon’s configuration.
   Solution: Ensure that the Syslog’s configuration accepts and redirects logs from the HAProxy service and its Lua scripts to the appropriate files. Adjustments may need to be made both in HAProxy configuration and the Syslog daemon’s configuration.

Technical Example

core.log(core.info, "This will log to the core.")
core.alert("This is an alert - critical info.")

In the Lua snippet above, logs are generated at different severity levels, but without proper STDERR redirection, these might not appear in the default log file.

Summary Table

Conclusion

By understanding the intricacies of HAProxy’s logging mechanisms and Lua script interactions, administrators can better configure their systems to ensure that all critical information, including that generated by Lua scripts, is logged accurately. Proper logging is essential for effective monitoring, troubleshooting, and maintaining the reliability of services provided by HAProxy.