ERL-1075: Confusion as to what the default level is for the primary configuration in logger

[OTP-Maintainer]

Original reporter: adnilsson
Affected version: OTP-22.1
Component: kernel
Migrated from: https://bugs.erlang.org/browse/ERL-1075

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h3.  Explanation
The 22.1 version of the Reference Manual for the type {{primary_config()}} in logger in the kernel application [says|http://erlang.org/doc/man/logger.html#type-primary_config]:
{quote}
The following default values apply:
* level => info
{quote}

Much to my surprise, then, I noticed that my {{?LOG_INFO}} messages were never displayed.
Investigating the primary configuration revealed that the logging level was actually set to {{notice}} and not {{info}} as I had inferred from the docs:
{noformat}
Erlang/OTP 22 [erts-10.5.2] [source] [64-bit] [smp:4:4] [ds:4:4:10] [async-threads:1] [hipe]

Eshell V10.5.2  (abort with ^G)
1> logger:i(primary).
Primary configuration: 
    Level: notice
    Filter Default: log
    Filters: 
        (none)
ok
{noformat}

On the other hand, the kernel User's Guide says in [section 2.6|http://erlang.org/doc/apps/kernel/logger_chapter.html#configuration] under *Primary Logger Configuration* that {{level}} "Defaults to {{notice}}".

At this point I felt I had two conflicting pieces of  information, but looking closer I saw that the documentation is not incorrect per se. {{logger:internal_init_logger/0}} sets the level of the primary configuration by calling another internal function {{get_logger_level}}:

{code:erlang}
get_logger_level() ->
    case application:get_env(kernel,logger_level,info) of
        Level when ?IS_LEVEL_ALL(Level) ->
            Level;
        Level ->
            throw({logger_level, Level})
    end.
{code}

Clearly,  the call to {{application:get_env/3}} suggests that the default should become {{info}} as I never configured anything else.  But we saw from the output of {{logger:i}} that the primary logging level will be {{notice}}. The only explanation is that the {{logger_level}} parameter is actually set by the kernel application. [Lo and behold|https://github.com/erlang/otp/blob/6611181ae71422a1c66798718b37474641a090a9/lib/kernel/src/kernel.app.src#L148]:  in kernel/src/kernel.app.src you will find {{\{env, \[\{logger_level, notice\} ,  _ \]\}}}. 

I think the issue here is that the documentation is talking about different defaults. On one hand there is the kernel application default setting ({{notice}}), and on the other there is the default for when nothing is said ({{info}}). In the end, only one truly matters for the typical Erlang user.

h3. Improvement suggestion
I see no point in having two separate defaults that causes unexpected behaviour with respect to the Reference Manual. Why not prevent this headache from happening again by removing the {{logger_level}} parameter in kernel.app.src and make {{notice}} the default value in {{logger:get_logger_level/0}}?