A very clear explanation of why writing parallel apps is a challenge with procedural languages, versus declarative approaches. Although Java is procedural, our Java implementation of dataflow is avoids this pitfall.

"The ability to assign a memory location with variable data is the cornerstone of computer programming. It turns out the ability to re-assign that same memory location is perhaps one of the biggest consternations in the parallel programming world. At the time, multiple assignment seemed like (and was) a good idea. Coupled with a looping control structure, massive calculations could be performed. For instance, one could write a program to sum the numbers from 1 to 100 (and beyond with simple variable change):"

Let SUM = 0
FOR I = 1 to 100
LET SUM = SUM + I
NEXT

"While incredibly useful, multiple assignment creates “state” within the program because the value of variables can change as program runs. The typical programming language allows you to manage the state of program variables. i.e. the memory location that holds the variable SUM changes over time. As any programmer will tell you, program state is easily managed on a single Von Neuwman CPU through the plethora of programming languages. (Some may argue this point, however.) On a large number of CPUs, managing coupled program states becomes extremely difficult. In a parallel environment, it is up to the programmer to manage the state of I and SUM. In a sequential single CPU environment, the programmer can assume I = 100 after the loop completes. In a parallel environment, where the loop may have been broken into parts, I may equal 10 on one CPU, and 20 on another. Of course, the example is simple and obvious. Creating large complex parallel applications does not present such luxuries, however."

"The simple powerful idea of multiple assignment has come back to bite us in the asymptote."