[erlang-questions] Extending error_logger

Martin Bjorklund mbj@REDACTED
Wed Aug 15 09:46:29 CEST 2007

"denis" <dloutrein.lists@REDACTED> wrote:
> Hi all,
> In my server, which is an OTP application using supervisor, gen_servers, .,
> I use error_logger to log my information.
> I would like to modify error_logger to add one kind of message: debug, link
> error, debug, info.
> Is there a clean way to add it to error_logger ? I dug the source, but not
> find an easy way.
> For now, the only solution I found is to implement my own logger
> implementing all I need, and trap the events sent to error_logger by OTP
> framework. 
> If I'm not mistaken, I can do it by writing my logger as event handler and
> register it with error_logger: add_report_handler.
> Am I one the good track ? Maybe there is a simpler method to do it ?

You can use error_logger:error_report/1,2 for this, and make sure your
handler formats these reports.

> Note that I also want to store logs as text file. I think that write a new
> error handler should do the job, like log_mf_h.

IMO, the log_mf_h/rb approach in OTP was a mistake.  The last 10 years
or so we have always used text based error logs in all our projects.
I think the standard log_mf_h should be replaced with disk_log_h which
is in Jungerl.

Anyway, since OTP R10B-9, you can use the disk_log_h and logger
modules in jungerl w/o having to patch OTP.  So, go to jungerl and
grab the files lib/msc/src/disk_log_h.erl and logger.erl.  (I just
checked in my updated versions of these modules) Then, create the
following module:

-export([start_errlog/0, test/0]).

start_errlog() ->
    Opts = [{name, logger},
            {file, "./elog"},
            {type, wrap},
            {format, external},
            {force_size, true},
            {size, {1024*1024, 5}}], % 5 files
      {disk_log_h, logger},
      disk_log_h:init(fun logger:form_no_progress/1, Opts)).

test() ->
    error_logger:error_msg("testing ~p\n", [self()]).

Start erlang:

erl -pa jungerl/lib/msc/ebin/ -boot start_sasl

1> aa:start_errlog().
2> exit(whereis(overload), foo).
3> aa:test().

You should have some nicely formatted errors in elog.1.   You can of
course you halt logs and everything else supported by disk_log.

Disclaimer: I haven't tested with R11.


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