当一个经理有几个员工，并且可以给他们每个人一个单独的任务时，并行性就会发生。工人们做他们的工作，并向经理提供结果。如果任务不能完全分离，例如任务之间的结果有一定的依赖性，或者需要在没有其他推理的情况下使用相同的资源，那么并行度就受到这些约束，不能完全实现。

Concurrency happens when a manager has several tasks but only less workers, hence some workers are given more than one task. Any worker given multiple tasks, divides each original given task to several steps and does the steps interleaved, each task result will be given back to manager as soon as every steps of it finished. Manager receive a task result while other tasks started and progressed several steps but have not finished yet. If any worker with multiple task decides not to start a single step of a given task before finishing every steps of an already started task, this is called sequentiality.

Asynchrony is any of the two above mixed or separated, seen from the manager's point of view. When the manager assigns the tasks to either few or enough workers he shall not be awaited stalled until any results are given back. He can do his personal jobs or whatever, while jobs are progressing. Usually workers do not decide how tasks should be divided into steps. An inversion of control means manager decides over steps and gives single steps to workers. So when he receives a step result from a worker, give him another step, maybe from another task. The whom under control is responsible for composing back step results into task results as well. So Asynchronicity comes with responsibility for control and probably coordination. If any worker is urged to work sequentially, from manager's point of view he is a synchronous worker.

Summary As it's simple to guess, full parallelism is an unrealisable idea unless otherwise in rare mostly trivial cases. Since reality comes with interdependent tasks and shared resources and lack of workers. So concurrency is the reality. From manager's point of view this concurrency is best if it does not hinder him from fine controlling the tasks, and if positive it is called asynchronous. Also computer software engineering best practices, augmented by S in SOLID principle, historically made servers single step runners called micro-services, this returned back control to the clients. So current situation is concurrency from server point of view and asynchronicity from client point of view.

Parallel : It's a broad term that means that two pieces of code execute that "at the same time". It doesn't matter if it's "real" parallelism or if it's faked through some clever design pattern. The point is that you can start the "tasks" at the same time and then control them separately (with mutex and all the appropriate tricks). But usually you prefer to use the word "parallel" only for "true" parallelism, as in : you make it happen through non-cooperative multitasking (whether be throuch CPU/GPU cores, or only at software level by letting the OS managing it at a very low level). People are reluctant to say "parallel" just for complicated sequential code that fakes parallelism, like you would find in a browser window's javascript for example. Hence the reason why people in this thread say "asynchronous has nothing to do with parallelism". Well it does, but just don't confuse them.

并发:没有并行性就不可能有并发性(无论是模拟的还是真实的，正如我上面解释的那样)，但是这个术语特别关注的是两个系统将试图在某个时间点同时访问同一资源的事实。它强调了一个事实，那就是你必须要处理这个问题。

异步:每个人都说异步与并行无关，这是对的，但它为并行铺平了道路(让事情并行或不并行的负担在你身上——继续阅读)。

“异步”指的是并行性的一种表示形式，它形式化了并行性中通常涉及的三个基本内容:1)定义任务的初始化(比如它何时开始以及它获得哪些参数)，2)任务完成后必须做什么，3)代码在此期间应该继续做什么。

但它仍然只是语法(通常表示为回调方法)。在后台，底层系统可能简单地认为这些所谓的“任务”只是堆积起来的代码片段，直到它完成当前正在执行的代码。然后它一个接一个地把它们拆开，然后依次执行。与否。它还可以为每个任务创建一个线程，并并行运行它们。谁在乎呢?这部分不包括在概念中;)

我会让它简短而有趣，让你们能够理解这些概念。

并发与并行——任务执行的方式。

Take an example in real life: There’s a challenge that requires you to both eat a whole huge cake and sing a whole song. You’ll win if you’re the fastest who sings the whole song and finishes the cake. So the rule is that you sing and eat concurrently. How you do that does not belong to the rule. You can eat the whole cake, then sing the whole song, or you can eat half a cake, then sing half a song, then do that again, etc. Parallelism is a specific kind of concurrency where tasks are really executed simultaneously. In computer science, parallelism can only be achieved in multicore environments.

同步与异步——编程模型。

在同步中，您将代码编写为按顺序从上到下执行的步骤 底部。在异步编程模型中，你将代码作为任务编写， 然后并发执行。并发执行意味着 所有的任务都可能同时执行。

Concurrency Concurrency means that an application is making progress on more than one task at the same time (concurrently). Well, if the computer only has one CPU the application may not make progress on more than one task at exactly the same time, but more than one task is being processed at a time inside the application. It does not completely finish one task before it begins the next. Parallelism Parallelism means that an application splits its tasks up into smaller subtasks which can be processed in parallel, for instance on multiple CPUs at the exact same time. Concurrency vs. Parallelism In Detail As you can see, concurrency is related to how an application handles multiple tasks it works on. An application may process one task at at time (sequentially) or work on multiple tasks at the same time (concurrently). Parallelism on the other hand, is related to how an application handles each individual task. An application may process the task serially from start to end, or split the task up into subtasks which can be completed in parallel. As you can see, an application can be concurrent, but not parallel. This means that it processes more than one task at the same time, but the tasks are not broken down into subtasks. An application can also be parallel but not concurrent. This means that the application only works on one task at a time, and this task is broken down into subtasks which can be processed in parallel. Additionally, an application can be neither concurrent nor parallel. This means that it works on only one task at a time, and the task is never broken down into subtasks for parallel execution. Finally, an application can also be both concurrent and parallel, in that it both works on multiple tasks at the same time, and also breaks each task down into subtasks for parallel execution. However, some of the benefits of concurrency and parallelism may be lost in this scenario, as the CPUs in the computer are already kept reasonably busy with either concurrency or parallelism alone. Combining it may lead to only a small performance gain or even performance loss. Make sure you analyze and measure before you adopt a concurrent parallel model blindly.

从http://tutorials.jenkov.com/java-concurrency/concurrency-vs-parallelism.html

并发性可能发生在以下几种场景中:

异步——这意味着你的程序执行非阻塞操作。例如，它可以通过HTTP发起对远程资源的请求，然后在等待接收响应的同时继续执行一些其他任务。这有点像你发了一封电子邮件，然后继续你的生活，没有等待回复。

并行性——这意味着您的程序利用多核机器的硬件，通过将工作分解为任务，在同一时间执行任务，每个任务都在单独的核心上执行。这有点像洗澡时唱歌:你实际上是在同时做两件事。

多线程——这是一种允许不同线程并发执行的软件实现。多线程程序似乎同时在做几件事，即使它运行在单核机器上。这有点像通过各种IM窗口与不同的人聊天;虽然你实际上是在来回切换，但最终结果是你同时进行了多个对话。

“同步和异步是编程模型。并发和并行是任务执行的方式……” 来源:https://medium.com/better - programming/sync vs -异步并发- vs - vs - 5754 cdb60f66平行

换句话说，同步和异步描述了在进行函数调用时程序如何执行(它是等待还是继续执行?)，而并发和并行描述了函数(任务)将如何执行(并发=可能同时执行，并行=有效地同时执行)。