Debugging bugs that don't happen every time is painful. It needs both technique and luck. When dealing with these, a mistyped continue command is irreversible and will take us a whole afternoon just to reproduce the issue again.

mruby-rr comes to rescue! It's a Time Traveling Debugger for mruby that based on Mozilla's rr. mruby-rr supports record and replay of mruby program execution. We can record the tough bug using mruby-rr for just once. Afterwards we can playback the execution as many times as we want. mruby-rr can also do time traveling operations like reverse-next and evaluating expressions.