[erlang-questions] Mnesia doesn't report insertion errors when underlying DETS file is full?

[Ulf Wiger]

> On 23 May 2015, at 10:38, Éric Pailleau <eric.pailleau@REDACTED> wrote:
> 
> Just wanted to know if mnesia subscribe on system events can raise a
> 
> {mnesia_error, Format, Args}
> 
> In such case ?

It *almost* does, if you set mnesia’s debug level to ‘verbose’. ;-)

The following call is made from mnesia_tm if do_update_op/3 crashes:

            verbose("do_update in ~w failed: ~p -> {'EXIT', ~p}~n",
                    [Tid, Op, Reason]),

…which, in its turn ends up calling the following in mnesia_lib:

important(Format, Args) ->
    save({Format, Args}),
    report_system_event({mnesia_info, Format, Args}).

The only problem is that the dets operation in question doesn’t crash, but instead returns {error, Reason}, so that particular issue won’t be reported. The value *is* captured by mnesia_tm:do_update/4, and it will return the last non-ok value from the commit. However, this return value is discarded, except in a few cases where it’s captured as debug info.

Of course, even if it were reported, it would be of the category “oops - your database was just corrupted!”, which is good to know, but not so easy to handle.

BR,
Ulf W

Ulf Wiger, Co-founder & Developer Advocate, Feuerlabs Inc.
http://feuerlabs.com