[tor-bugs] #20540 [Metrics]: define log-levels for all java metrics-products

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#20540: define log-levels for all java metrics-products
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 Reporter:  iwakeh       |          Owner:
     Type:  enhancement  |         Status:  needs_information
 Priority:  Medium       |      Milestone:
Component:  Metrics      |        Version:
 Severity:  Normal       |     Resolution:
 Keywords:               |  Actual Points:
Parent ID:               |         Points:
 Reviewer:               |        Sponsor:
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Comment (by karsten):

 Replying to [comment:1 iwakeh]:
 > A summary of what is in place or known already:
 >
 > * java projects should use slf4j (our default implementation is logback,
 but that is less important)

 Yep.

 > * errors are possibly mailed by logging frameworks (in their default
 settings). So error level should be important (an 'action item') for
 operation.
 > * info-level should give an indication that all things are running as
 expected.

 Agreed about those two, but we'll have to decide when to use the other
 three levels in slf4j.  Here's a suggestion:

  - trace: I suggest disregarding this level, because the slf4j developers
 themselves only added it to "bow to popular demand".  It seems we can
 easily get away by just using debug instead.
  - debug: Let's use this for detailed messages to debug a problem, under
 the assumption that these logs are typically disabled in production and
 only enabled when debugging a problem.
  - info: It seems useful to log whenever a process or major step starts or
 ends, but under the assumption that these logs will only be written to
 files and not mailed out to operators.  It might be a good requirement to
 write info-level logs in a way that operators can understand them without
 having to read any code.  One example where info might be too high is
 where metrics-lib informs us which `DescriptorCollector` implementation
 it's serving us, because that's something the operator wouldn't normally
 care about.
  - warn: We could use warn to inform the operator of a problem that we
 were able to recover from but that they should be aware of.  The warning
 should be written in a way that the operator understands, and it should be
 something that the operator can do something against.  Stated differently,
 we should expect to be contacted by operators who are unclear what to do
 about a given warning.  And if they cannot do anything against it, maybe
 it should be an info message rather than a warning.  We might want to
 recommend that operators include warnings in any automated notifications
 they receive from their service instance.
  - error: We should use error for problems that we cannot recover from.
 Otherwise they're similar to warnings.

 > * maybe, log statistics to separate log files (as in Onionoo).

 Are there log domains of some sort?  It seems that we should leave the
 decision of log files to the operator, but could say that these log
 messages go into a "statistics" log domain that the operator may log to
 the same or a different file.  By the way, we'd probably want to log these
 messages on info level.

 > * what else?

 Maybe one thing:

  * Log levels used by metrics-lib, where a problem with parsing a
 descriptor can have different consequences depending on the application.
 In other words, if we log a warning or even error, we should give the
 application the opportunity to tone down that warning, or ignore it,
 because it doesn't care as much.  What we could do is use a log domain
 "parsing", or we could let applications define logging by logger name and
 tone down `org.torproject.descriptor.*` loggers.

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Ticket URL: <https://trac.torproject.org/projects/tor/ticket/20540#comment:3>
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