Parallelization of MATLink

Question

I am attempting to parallelize the construction of a table using MATLink and ParallelTable.

Let's consider a sample MATLAB function defined by

    function [out1,out2] = TestFunction(in1,in2,in3)
    %TestFunction Example function
    out1=in1;
    out2=in2+in3;
    end

Consider the non-parallel construction of the table

    Needs["MATLink`"];
    OpenMATLAB[];
    TestFunction = MFunction["TestFunction", "OutputArguments" -> 2];
    tab = Table[
       {out1, out2} = TestFunction[i, 2 i, 3 i];
       {out1, out2}
       , {i, 1, 3}];

Which yields the output

   {{1.,5.},{2.,10.},{3.,15.}}

My attempt to parallelize the construction is as follows:

First, launch kernels and load MATLink:

   LaunchKernels[];
   ParallelNeeds["MATLink`"];
   ParallelEvaluate[
      OpenMATLAB[];
      TestFunction = MFunction["TestFunction", "OutputArguments" -> 2];
    ];

Which yields the following two errors for each of the kernels

   Global`OpenMATLAB::shdw :  Symbol OpenMATLAB appears in multiple contexts   {Global`,MATLink`}; definitions in context Global` may shadow or be shadowed by other definitions.
   Global`MFunction::shdw :  Symbol MFunction appears in multiple contexts {Global`,MATLink`}; definitions in context Global` may shadow or be shadowed by other definitions.

and MATLAB is not opened on any kernals. As a result, the parallel table construction

   tab = ParallelTable[
      {out1, out2} = TestFunction[i, 2 i, 3 i];
      {out1, out2}
      , {i, 1, 3}];

yields the error

Set::shape :  Lists {out1,out2} and Global`MFunction[TestFunction,OutputArguments->2][1,2,3] are not the same shape.

for each kernel. No output is generated.

What is the correct way to use MATLink on multiple kernels?

Additional details:

The output of FindFile["Matlink"]` is

   In[1]:=FindFile["MATLink`"]
   Out[1]=C:\Users\VHYDAN001\AppData\Roaming\Mathematica\Applications\MATLink\Kernel\init.m

The output of ParallelEvaluate[FindFile["MATLINK"]` is

   In[2]:=ParallelEvaluate[FindFile["MATLink`"]]
   Out[2]={C:\Users\VHYDAN001\AppData\Roaming\Mathematica\Applications\MATLink\Kernel\init.m,
   C:\Users\VHYDAN001\AppData\Roaming\Mathematica\Applications\MATLink\Kernel\init.m,
   C:\Users\VHYDAN001\AppData\Roaming\Mathematica\Applications\MATLink\Kernel\init.m,
   C:\Users\VHYDAN001\AppData\Roaming\Mathematica\Applications\MATLink\Kernel\init.m}

The output of $Path is

   In[3]:= $Path
   Out[3]= {C:\Program Files\Wolfram Research\Mathematica\11.1\AddOns\Applications,
   C:\Program Files\Wolfram Research\Mathematica\11.1\SystemFiles\Links,
   C:\Users\VHYDAN001\AppData\Roaming\Mathematica\Kernel,
   C:\Users\VHYDAN001\AppData\Roaming\Mathematica\Autoload,
   C:\Users\VHYDAN001\AppData\Roaming\Mathematica\Applications,
   C:\ProgramData\Mathematica\Kernel,
   C:\ProgramData\Mathematica\Autoload,
   C:\ProgramData\Mathematica\Applications,
   .,
   C:\Users\VHYDAN001,
   C:\Program Files\Wolfram Research\Mathematica\11.1\AddOns\Packages,
   C:\Program Files\Wolfram Research\Mathematica\11.1\SystemFiles\Autoload,
   C:\Program Files\Wolfram Research\Mathematica\11.1\AddOns\Autoload,
   C:\Program Files\Wolfram Research\Mathematica\11.1\AddOns\Applications,
   C:\Program Files\Wolfram Research\Mathematica\11.1\AddOns\ExtraPackages,
   C:\Program Files\Wolfram Research\Mathematica\11.1\SystemFiles\Kernel\Packages,
   C:\Program Files\Wolfram Research\Mathematica\11.1\Documentation\English\System,
   C:\Program Files\Wolfram Research\Mathematica\11.1\SystemFiles\Data\ICC}

The output of First@ParallelEvaluate[$Path] is

   In[4]:= First@ParallelEvaluate[$Path]
   Out[4]= {C:\Program Files\Wolfram Research\Mathematica\11.1\SystemFiles\Links,
   C:\Users\VHYDAN001\AppData\Roaming\Mathematica\Kernel,
   C:\Users\VHYDAN001\AppData\Roaming\Mathematica\Autoload,
   C:\Users\VHYDAN001\AppData\Roaming\Mathematica\Applications,
   C:\ProgramData\Mathematica\Kernel,
   C:\ProgramData\Mathematica\Autoload,
   C:\ProgramData\Mathematica\Applications,
   .,
   C:\Users\VHYDAN001,
   C:\Program Files\Wolfram Research\Mathematica\11.1\AddOns\Packages,
   C:\Program Files\Wolfram Research\Mathematica\11.1\SystemFiles\Autoload,
   C:\Program Files\Wolfram Research\Mathematica\11.1\AddOns\Autoload,
   C:\Program Files\Wolfram Research\Mathematica\11.1\AddOns\Applications,
   C:\Program Files\Wolfram Research\Mathematica\11.1\AddOns\ExtraPackages,
   C:\Program Files\Wolfram Research\Mathematica\11.1\SystemFiles\Kernel\Packages,
   C:\Program Files\Wolfram Research\Mathematica\11.1\Documentation\English\System,
   C:\Program Files\Wolfram Research\Mathematica\11.1\SystemFiles\Data\ICC}

Answer 1

The package was loaded on the subkernels, but not the main kernel. ParallelNeeds only loads it on the subkernels. You must also use Needs.

What happened is that since the package wasn't loaded on the main kernel, the symbol OpenMATLAB was interpreted by the main kernel as Global`OpenMATLAB, not has MATLink`OpenMATLAB. It sent the Global` version to the subkernels, which then issued a warning about multiple symbols with the same name in different contexts.

Please try like this:

LaunchKernels[];
Needs["MATLink`"];
ParallelNeeds["MATLink`"];   
ParallelEvaluate[
   OpenMATLAB[];
   TestFunction = MFunction["TestFunction", "OutputArguments" -> 2];
];