Logback appender instrumentation breaks default appender behavior

[vghero]

Describe the bug

Hey folks.
We're using opentelemetry java agent 2.20.1 with logback 1.5.19 in a Spring Boot 3.1.9 application under JDK 21, but using the agent plain without Spring auto-configuration using -javaagent and env/system-properties.

It seems the default instrumentation of Logback appenders changes the behavior of how appenders can work with the ILoggingEvent that is passed via doAppend.

We have a custom appender in place, that does PII redaction of log statements, by inspecting the arguments provided by the event. If it detects any PII related data in such, it will replace the content of the argument with "redacted". This worked fine for now. But when we enabled the OTEL agent, our appender gets still called and it also redacts the arguments, but still the log statement contains the un-redacted log message. It seems that the modification to the arguments array is not respected anymore but ignored.

For now we disabled the appender instrumentation using OTEL_INSTRUMENTATION_LOGBACK_APPENDER_ENABLED=false and everything is back to normal. But I'm curious if this is a bug that needs fixing or expected behavior :). Thanks!

Steps to reproduce

Create a custom appender, register it and within the doAppend(), modify the arguments of the ILoggingEvent parameter with some values.

Expected behavior

The changed arguments should appear in the final log message.

Actual behavior

The origin arguments appear in the final log message.

Javaagent or library instrumentation version

2.20.1

Environment

JDK: openjdk version "21.0.8" 2025-07-15 LTS
OS: Debian trixie/sid

Additional context

No response

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[laurit]

Without a minimal sample application it is hard to say why this happens. My guess would be that since we call loggingEvent.getFormattedMessage() it could be that the formatted message is constructed and cached before you have the change to modify the arguments (see https://github.com/qos-ch/logback/blob/0e3ab572919177605327c3e52b8b3cf8d8c7c200/logback-classic/src/main/java/ch/qos/logback/classic/spi/LoggingEventVO.java#L116). Though could also be something completely different. Currently our instrumentation runs before the appenders, I guess we could try moving it to run after the appenders.

[github-actions]

This issue has been labeled as stale due to lack of activity and needing author feedback. It will be automatically closed if there is no further activity over the next 7 days.