SLF4J Error message Log on first execution.

[feinstein]

The first time my lambda code runs, it leave a SLF4J error message on CloudWatch logs:

You can see on the above image that I run my lambda code several times and the SLF4J error only appears on the first execution.

In order to be sure this was not a problem from my code, I downloaded the Jersey sample and uploaded it to my lambda instance. On the first execution there's also an error message, but from Log4J this time:

I am not using SLF4J on my project, I am using pure log4j2 with the AWS Appender. But my code is very simple, just an echo so I can test the whole architecture and even before I placed log4j2 the SLF4J error was still there.

In order to reproduce this issue you have to upload a new jar since this only happens on the first execution (cold start) of a lambda function.

If you want I can send you my test code so you can see the SLF4J error message yourself.

[sapessi]

Hey @feinstein, are you including Lambda's log4j appender in your dependencies?

[feinstein]

Hi @sapessi, I am, but in any case the second screen shot is the sample code from the library, pure, I didn't modify it, just compiled and uploaded it.

[sapessi]

ok. Let me look into it. The library uses SLF4J as an interface and trusts that an appender is included (the Lambda log4j appender should work). I'll run some tests to figure out what is going on. We'll use this issue to track it.

[feinstein]

Great, if you need my project's code, please let me know.

Also, IMHO I believe libraries should default to the no-op SLF4J Appender when none is present, with no error messages, since most applications want libraries to be transparent and not pollute the logs with cryptic error messages.

[feinstein]

I am not very much experienced on SLF4J, I did a quick search and this website has the error message and the explanation.

https://www.slf4j.org/manual.html

So maybe there's a configuration documentation missing, saying that I need to provide a SLF4J binding.

The error message from the sample I believe is due to the lack of Log4j2 Appender configuration, so it will default to the console.

[sapessi]

Hey @feinstein, getting back to this - could you share your pom.xml file?

[sapessi]

The issue happens because there is no dependency in the project that defines an appender for slf4j. I have intentionally not included an appender in the library because I want each developer to use their preferred one. Having said that, I have found a pretty awesome library that defines an appender for slf4j specifically for Lambda - it also includes the request id in the log lines: https://blog.symphonia.io/a-love-letter-to-lambda-logging-974b0eb49273

I have updated all of the samples to include this library.

[feinstein]

@sapessi Thank you for the recommendation! I will read more about it ASAP!

[feinstein]

@sapessi take a look at this issue that I opened, if you want to log Exceptions, I would advise you to refrain from adopting their library for now