Parallel kernel and front end

Question

I have a piece of code which looks like this:

ParallelTable[

  Monitor[
    Table[
    <code here>
    ,{i,i1,i2];
  ,i];

]

so basically a Table which I parallelize. Each element of this parallelized table being itself a Table which I want to monitor.

I receive this error message:

FrontEndObject::notavail: A front end is not available; certain operations require a front end.

This error originates from the Monitor[]. I suspect that because my first Table is parallelized, somehow the dynamic cell into which Monitor[] will write to is not connected to any Front End or something...

How would I solve this issue?

Answer 1

As pointed in the comments by @Nasser, Monitor needs a front end, so it should be outside ParallelTable. It also needs a variable that acts as a "counter" and that is shared among all the kernels participating in the calculation, for that you can use SetSharedVariable. Therefore, a solution to your question would be:

SetSharedVariable[iter]
iter = 0
Monitor[
  ParallelTable[ 
   iter += 1;
   FactorInteger[999999999]
   , {1000}
   ], ProgressIndicator[iter, {0, 1000}]];

Its is true that its questionable if the cost of keeping the iteration counter updated makes parallelism worthwhile, as pointed by @mfvonh. This should be evaluated case by case, and its not impossible that its advantageous. I have found this trick useful at least once.