Variable not available in kernel despite `DistributeDefinitions`

Question

I am following this answer to evaluate certain cells in a separate kernel. I have created a separate kernel called K2 and have the following MWE:

myvar = 1;
DistributeDefinitions[myvar]

{myvar}

Which - I believe - makes the definition of myvar available in all kernels.

Now I am running e.g.

CellPrint@Cell[BoxData[MakeBoxes[Defer[1 + 1]]], "Input", Evaluator -> "K2"]

which produces a cell

1+1

that can be evaluated (in kernel K2) and yields 2 (unsurprisingly). So far, so good.

Now, if I run

CellPrint@Cell[BoxData[MakeBoxes[Defer[myvar]]], "Input", Evaluator -> "K2"]

a cell

myvar

is created. But evaluating it still yields only myvar rather than 1. (The same holds if I omit the Defer).

I believe I am misunderstanding the concept of separate kernel evaluation? Ultimately, I want to send not only a variable, but a custom function to K2.

Answer 1

You are confusing parallel kernels (subkernels) with the ability to have multiple evaluators set up in the front end.

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DistributeDefinitions is part of the parallel tools. Mathematica can run code in parallel by launching several "slave kernels" or subkernels and sending jobs to each. The subkernels are controlled from by the main kernel.

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We can also create several kernel setups through the Evaluation -> Kernel Configuration Options menu item. Then we can assign a different evaluator to each notebook or each cell within a single notebook. This is what the Evaluator option does.

This is a Front End feature and has absolutely no relationship to the parallel tools. The Front End communicates directly with each evaluator you set up. They are all "main kernels", to use the parallel tools terminology.

There is no ready made mechanism to communicate between these. You can try to set up some sort of manual communication using MathLink. Whether this is a reasonable idea at all depends on what you are trying to achieve. In most cases I can imagine it isn't a good idea to do this.