Listing 4. Job control stack pop function implemented in MATLAB. Queries
job control table for next available data file to process. If data is
available, the path to it is returned. If none is available, returns an
empty string causing further processing to end.

function stJobEntry = pop_job_table(taskid, entryid)

% determine which node we are on for table update (for performance
% tracking)
iNodeID = str2num(getenv('SLURM_PROCID'));

sMYSQL = 'mysql -h myhost -u myuser -p<password> myDB';

% select one entry from the job management table that hasn't been
% done yet. Immediately update with this processor's node_id to try
% to lock the row out from further requests.
sPOPSQL = sprintf(['echo "set @B=%d; set @C=%d;' ...
                   'select @A:=entry_id as entry_id, datapath from ' ...
                   'JobManagement where task_id = @B and entry_id = @C' ...
                   'and node_id ' ...
                   'is null limit 1 for update;' ...
                   'update JobManagement set node_id = %d,' ...
                   'node_start = now() where task_id = @B and ' ...
                   'entry_id = @A;' ...
                   'set @A=0;"'], taskid, entryid, iNodeID);

% execute the table pop and get a day that is not being processed
[status, cmdout] = system([sPOPSQL ' | ' sMYSQL ' | head -2 | tail ' ...
                    '-1']);

stJobEntry = struct('entry',0, 'datapath','');

if length(cmdout) > 0
    % parse cmdout to entry_id, datapath
    iTokens = str2num(cmdout);
    stJobEntry.entry = iTokens(1);
    stJobEntry.datapath = iTokens(2);
end