How to save intermediate iterations during SPMD in MATLAB?

I am experimenting with MATLAB SPDM. However, I have the following problem to solve:

I am running a quite long algorithm and I would like to save the progress along the way in case the power gets cut, someone unplugs the power plug or memory error.
The loop has 144 iterations that take each around 30 minutes to complete => 72h. A lot of problems can occur in that interval. Of course, I have the distributed computing toolbox on my machine. The computer has 4 physical cores. I run MATLAB R2016a.
I do not really want to use a parfor loop because I concatenate results and have dependency across iterations. I think SPMD is the best choice for what I want to do.

I'll try to describe what I want as best as I can: I want to be able to save at a set iteration of the loop the results so far, and I want to save the results by worker.

Below is a Minimum (non)-Working Example. The last four lines should be put in a different .m file. This function, called within a parfor loop, allows to save intermediate iterations. It is working properly in other routines that I use. The error is at line 45 (output_save). Somehow, I would like to "pull" the composite object into a "regular" object (cell/structure).

My hunch is that I do not quite understand how Composite objects work and especially how they can be saved into "regular" objects (cells, structures, etc).

% SPMD MWE

% Clear necessary things
clear output output2 output_temp iter kk


% Useful thing that will be used later on
Rorder=perms(1:4);

% Stem of the file to save the data to
stem='MWE_MATLAB_spmd';

% Create empty cells where the results of the kk loop will be stored
output1{1,1}=[];
output2{1,2}=[];

% Start the parpool
poolobj=gcp;

% Define which worker/lab will do which iteration
iterperworker=ceil(size(Rorder,1)/poolobj.NumWorkers);
for i=1:poolobj.NumWorkers
    if i<poolobj.NumWorkers
        itertodo{1,i}=1+(iterperworker)*(i-1):iterperworker*i;
    else
        itertodo{1,i}=1+(iterperworker)*(i-1):size(Rorder,1);
    end
end

%Start the spmd
% try
    spmd
        iter=1;
        for kk=itertodo{1,labindex}
            % Print which iteration is done at the moment
            fprintf('\n');
            fprintf('Ordering %d/%d \r',kk,size(Rorder,1));

            for j=1:size(Rorder,2)
            output_temp(1,j)=Rorder(kk,j).^j; % just to populate a structure
            end
            output.output1{1,1}=cat(2,output.output1{1,1},output_temp);  % Concatenate the results
            output.output2{1,2}=cat(2,output.output1{1,2},0.5*output_temp);  % Concatenate the results

            labindex_save=labindex;

            if mod(iter,2)==0
                output2.output=output; % manually put output in a structure
                dosave(stem,labindex_save,output2); % Calls the function that allows me to save in parallel computing
                end
                iter=iter+1;
            end
        end
    % catch me
    % end


    % Function to paste in another m-file
    % function dosave(stem,i,vars)
    %     save(sprintf([stem '%d.mat'],i),'-struct','vars')
    % end

Solution

A Composite is created only outside an spmd block. In particular, variables that you define inside an spmd block exist as a Composite outside that block. When the same variable is used back inside an spmd block, it is transformed back into the original value. Like so:

spmd
    x = labindex;
end
isa(x, 'Composite') % true
spmd
    isa(x, 'Composite') % false
    isequal(x, labindex) % true
end

So, you should not be transforming output using {:} indexing - it is not a Composite. I think you should simply be able to use

dosave(stem, labindex, output);