[META 321] Make log_level configuration option dynamic

[basepi]

Implementing elastic/apm#332
#134 is a prerequisite

Note that the implementation of this spec also includes adding the option to Kibana.

[felixbarny]

The PHP agent supports a NOTICE log level which is not one of the commonly agreed log levels. Is there a specific reason why NOTICE is required for PHP or could it be removed?

[SergeyKleyman]

My thought process on deciding the levels was as follows:

1. I started with the PHP standard PSR-3 which is AFAICS is direct mapping of syslog severity levels.
2. I merged EMERGENCY, ALERT and CRITICAL into CRITICAL because I didn't see the syslog's distinction between them as relevant to the agent.
3. I added TRACE as the most verbose level (and also the level under which the agent can log something that can be security sensitive - the agent should make best effort not to do that under less verbose levels).

So to answer your question

Is there a specific reason why NOTICE is required for PHP or could it be removed?

No, there is no specific reason - it was just inherited from syslog so it can be removed. I do see some benefits to having it - to log something that might be an issue but the code logging it doesn't have enough information to definitely say one way or the other. The way I see it, INFO is purely informational (not for drawing attention to a potential problem) and WARNING is for drawing attention to a potential problem that doesn't raise to the level of ERROR (for example WARNING for problems that might affect performance while ERROR issues might affect correctness). So NOTICE between WARNING and INFO is a "nice to have" but it's not a "must".

Do you think PHP having NOTICE level might be an issue when getting log_level from central configuration (assuming that central configuration won't have it) or is your concern more about general consistency between the agents?

[felixbarny]

My only concern would just be the general cross-agent consistency. Does the PHP agent currently use the notice log level?

[SergeyKleyman]

Yes, the agent uses NOTICE in a few place (https://github.com/elastic/apm-agent-php/search?q=ifNoticeLevelEnabled - I see that the most of it is in component tests code) but it can be easily find-and-replace-ed by let's say WARNING.

[SergeyKleyman]

And it's going to be a breaking change for logging configuration but the agent is not GA yet so if we are going to remove NOTICE it's better to do it before GA.

[felixbarny]

@SergeyKleyman, could you link the PR that fixed this?

[SergeyKleyman]

@felixbarny I am sorry - I was too quick in closing the issue. The PR (#635) still has some issues. You are right about missing link between PR and the issue - I will make sure to use them in the future.