How to articulate the difference between asynchronous and parallel programming?
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Question
Many platforms promote asynchrony and parallelism as means for improving responsiveness. I understand the difference generally, but often find it difficult to articulate in my own mind, as well as for others.
I am a workaday programmer and use async & callbacks fairly often. Parallelism feels exotic.
But I feel like they are easily conflated, especially at the language design level. Would love a clear description of how they relate (or don't), and the classes of programs where each is best applied.
Answer
It is a question of order of execution.
If A is asynchronous with B, then I cannot predict beforehand when subparts of A will happen with respect to subparts of B.
If A is parallel with B, then things in A are happening at the same time as things in B. However, an order of execution may still be defined.
Perhaps the difficulty is that the word asynchronous is equivocal.
I execute an asynchronous task when I tell my butler to run to the store for more wine and cheese, and then forget about him and work on my novel until he knocks on the study door again. Parallelism is happening here, but the butler and I are engaged in fundamentally different tasks and of different social classes, so we don't apply that label here.
My team of maids is working in parallel when each of them is washing a different window.
My race car support team is asynchronously parallel in that each team works on a different tire and they don't need to communicate with each other or manage shared resources while they do their job.
My football (aka soccer) team does parallel work as each player independently processes information about the field and moves about on it, but they are not fully asynchronous because they must communicate and respond to the communication of others.
My marching band is also parallel as each player reads music and controls their instrument, but they are highly synchronous: they play and march in time to each other.
A cammed gatling gun could be considered parallel, but everything is 100% synchronous, so it is as though one process is moving forward.