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Send and Recieve data between client and workers

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Hello everybody,
I am trying to update text fields of a GUI using multiple workers with parfevalOnAll and parallel.pool.DataQueue. I cannot use labRecieve or labSend because spmd wouldn't work for my application. To recieve the calculated data from the workers and update the text field I am using this code on the client:
parpool(4)
pool = gcp();
fig = uifigure();
grid = uigridlayout(fig, [2, 2]);
btn = uibutton(grid,'Text','SendtoWorker');
textField = uitextarea(uipanel(grid));
Q1 = parallel.pool.DataQueue();
listener1 = afterEach(Q1, @(data) UpdateTextField(textField, data));
fut1 = parfevalOnAll(@worker1, 0, Q1);
btn.ButtonPushedFcn = @(~,~) SendDataToworker(); %Send data to worker on Q2 when button is pushed
function UpdateTextField(textField, data)
str = num2str(data);
textField.Value = str;
end
function SendDataToworker() %Send data1 to worker on Q2
Q2 = parallel.pool.DataQueue();
data1 = 0;
send(Q2,data1)
end
On the workers I have defined the dataQueue Q2 and a listener2 to recieve the variables from client as such:
function worker1()
global acquiredData;
acquiredData = 1; % initial value for the data
Q2 = parallel.pool.DataQueue(); % Queue to recieve data from client
listener2 = afterEach(Q2, @(data) calculateData(data));
function calculateData(data)
acquiredData = data;
send(Q1,acquiredData + 1) %sending calculated value back to client
end
end
Although the Help Section mentions that parallel.pool.DataQueue should work in reverse direction I am not seeing the results on the GUI. I also don't get any errors during execution. Thank you for your help.
  5 Comments
Mohammad Sami
Mohammad Sami on 10 Apr 2021
Ok after some experimentation this seems to work. If the queue are created in the main thread they can only be used to receive data from the worker but cannot be used to send data. Hence we need to create queues in the main thread and the worker and exchange them to have full bi directional comms.
I have also replaced the nested function to a while loop with an exit condition. You can change the exit condition to suit your purpose. Or perhaps you can send an exit command on q2 e.t.c.
For the main thread
q1 = parallel.pool.DataQueue; % this is to retrieve the data
afterEach(q1,@disp);
% this is used to retrive the pollable data q from the worker
q11 = parallel.pool.PollableDataQueue;
p = gcp;
parfeval(@worker1,0,q1,q11);
q2 = poll(q11,10); % retrieve the pollable data queue from the worker
for i = 1:10
send(q2,i); % use the retrieved q to send data / comms to worker
end
For the worker
function worker1(q1,q11)
nodatacounter = 0;
% create the q and send it back to main thread so that we can use it to rcv
% data from the main thread
q2 = parallel.pool.PollableDataQueue;
send(q11,q2);
while nodatacounter < 10 % kill the worker if no comms for 10 iterations
[data,datarcvd] = poll(q2,10); % 10 second timeout
if datarcvd
send(q1,data);
nodatacounter = 0;
else
nodatacounter = nodatacounter + 1;
end
end
end

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Accepted Answer

Mohammad Sami
Mohammad Sami on 11 Apr 2021
Moved to answer.
Ok after some experimentation this seems to work. If the queue are created in the main thread they can only be used to receive data from the worker but cannot be used to send data. Hence we need to create queues in the main thread and the worker and exchange them to have full bi directional comms.
I have also replaced the nested function to a while loop with an exit condition. You can change the exit condition to suit your purpose. Or perhaps you can send an exit command on q2 e.t.c.
For the main thread
q1 = parallel.pool.DataQueue; % this is to retrieve the data
afterEach(q1,@disp);
% this is used to retrive the pollable data q from the worker
q11 = parallel.pool.PollableDataQueue;
p = gcp;
parfeval(@worker1,0,q1,q11);
q2 = poll(q11,10); % retrieve the pollable data queue from the worker
for i = 1:10
send(q2,i); % use the retrieved q to send data / comms to worker
end
For the worker
function worker1(q1,q11)
nodatacounter = 0;
% create the q and send it back to main thread so that we can use it to rcv
% data from the main thread
q2 = parallel.pool.PollableDataQueue;
send(q11,q2);
while nodatacounter < 10 % kill the worker if no comms for 10 iterations
[data,datarcvd] = poll(q2,10); % 10 second timeout
if datarcvd
send(q1,data);
nodatacounter = 0;
else
nodatacounter = nodatacounter + 1;
end
end
end
  1 Comment
Yusuf Salikoglu
Yusuf Salikoglu on 13 Apr 2021
Mohammed thank you very much, it worked hassle-free!

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