sub backend_call {
    my $self = shift;
    my $method = shift;

    my $todo = $self->{"on_$method"} || $self->can("do_$method");
    if (!$todo) {
        my $sub = [caller(1)]->[3];
        $sub =~ s/.*:://;
        croak join "", (ref $self || $self),"->",$sub,
            ": no backend found for $method";
    };

    return $todo->($self, @_);
};