Extend docs on compositional coupling with deadlock example

[uekerman]

I promised in precice/precice#1907 to add a docs section on deadlocks in compositional coupling. I tried to add the simplest possible example.

The mermaid graph will probably not work on the website. Would this be easy to add? If not, I would add the comment in a comment and add the svg instead.

graph LR;
    A -- first to second --> B;
    B -- first to second --> C;
    C -- first to second --> A;

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@fsimonis @carme-hp Enough information covered and would you prefer to see more examples and problems?

@orlandoisepic Could be a potential case for the checker a well.

[carme-hp]

I think that the explanation for the deadlock and its solution is clear:)

[carme-hp]

Suggested change

	For this example, all three participants are executed in parallel to one another, whereas `A` exchanges data with `B` and `C`, but not the latter two with each other.
	For this example, all three participants are executed in parallel to one another, whereas `A` exchanges data with `B` and `C`, but `B` and `C` do not exchange data with each other.

I know it says the same, but I think it's better to make it even more clear.

Maybe it will be nice to add a comment that "first" and "second" doesn't matter?

[fsimonis]

The mermaid graph will probably not work on the website. Would this be easy to add? If not, I would add the comment in a comment and add the svg instead.

Not difficult. But wouldn't it be neater to see an actual config visualization? You mentioned that you have a minimal example, right?

[orlandoisepic]

This error can only occur with serial coupling schemes? And is it possible to occur with two or more participants?

[uekerman]

@fsimonis

But wouldn't it be neater to see an actual config visualization? You mentioned that you have a minimal example, right?

https://github.com/precice/precice/blob/develop/tests/serial/circular/Explicit.xml and swap participants in line 65. In the config visualization the coupling scheme is separated from the exchanges, right?

[uekerman]

@orlandoisepic

This error can only occur with serial coupling schemes?

yes

And is it possible to occur with two or more participants?

You need at least three participant to get such a circular dependency. For two participants, you can only define one coupling scheme.