asynchronous and non-blocking calls? also between blocking and synchronous

What is the difference between asynchronous and non-blocking calls? Also between blocking and synchronous calls (with examples please)?

Solution 1:

In many circumstances they are different names for the same thing, but in some contexts they are quite different. So it depends. Terminology is not applied in a totally consistent way across the whole software industry.

For example in the classic sockets API, a non-blocking socket is one that simply returns immediately with a special "would block" error message, whereas a blocking socket would have blocked. You have to use a separate function such as select or poll to find out when is a good time to retry.

But asynchronous sockets (as supported by Windows sockets), or the asynchronous IO pattern used in .NET, are more convenient. You call a method to start an operation, and the framework calls you back when it's done. Even here, there are basic differences. Asynchronous Win32 sockets "marshal" their results onto a specific GUI thread by passing Window messages, whereas .NET asynchronous IO is free-threaded (you don't know what thread your callback will be called on).

So they don't always mean the same thing. To distil the socket example, we could say:

Blocking and synchronous mean the same thing: you call the API, it hangs up the thread until it has some kind of answer and returns it to you.
Non-blocking means that if an answer can't be returned rapidly, the API returns immediately with an error and does nothing else. So there must be some related way to query whether the API is ready to be called (that is, to simulate a wait in an efficient way, to avoid manual polling in a tight loop).
Asynchronous means that the API always returns immediately, having started a "background" effort to fulfil your request, so there must be some related way to obtain the result.

Solution 2:

synchronous / asynchronous is to describe the relation between two modules.
blocking / non-blocking is to describe the situation of one module.

An example:
Module X: "I".
Module Y: "bookstore".
X asks Y: do you have a book named "c++ primer"?

blocking: before Y answers X, X keeps waiting there for the answer. Now X (one module) is blocking. X and Y are two threads or two processes or one thread or one process? we DON'T know.
non-blocking: before Y answers X, X just leaves there and do other things. X may come back every two minutes to check if Y has finished its job? Or X won't come back until Y calls him? We don't know. We only know that X can do other things before Y finishes its job. Here X (one module) is non-blocking. X and Y are two threads or two processes or one process? we DON'T know. BUT we are sure that X and Y couldn't be one thread.
synchronous: before Y answers X, X keeps waiting there for the answer. It means that X can't continue until Y finishes its job. Now we say: X and Y (two modules) are synchronous. X and Y are two threads or two processes or one thread or one process? we DON'T know.
asynchronous: before Y answers X, X leaves there and X can do other jobs. X won't come back until Y calls him. Now we say: X and Y (two modules) are asynchronous. X and Y are two threads or two processes or one process? we DON'T know. BUT we are sure that X and Y couldn't be one thread.

Please pay attention on the two bold-sentences above. Why does the bold-sentence in the 2) contain two cases whereas the bold-sentence in the 4) contains only one case? This is a key of the difference between non-blocking and asynchronous.

Let me try to explain the four words with another way:

blocking: OMG, I'm frozen! I can't move! I have to wait for that specific event to happen. If that happens, I would be saved!
non-blocking: I was told that I had to wait for that specific event to happen. OK, I understand and I promise that I would wait for that. But while waiting, I can still do some other things, I'm not frozen, I'm still alive, I can jump, I can walk, I can sing a song etc.
synchronous: My mom is gonna cook, she sends me to buy some meat. I just said to my mom: We are synchronous! I'm so sorry but you have to wait even if I might need 100 years to get some meat back...
asynchronous: We will make a pizza, we need tomato and cheeze. Now I say: Let's go shopping. I'll buy some tomatoes and you will buy some cheeze. We needn't wait for each other because we are asynchronous.

Here is a typical example about non-blocking & synchronous:

// thread X
while (true)
{
    msg = recv(Y, NON_BLOCKING_FLAG);
    if (msg is not empty)
    {
        break;
    }
    else
    {
        sleep(2000); // 2 sec
    }
}

// thread Y
// prepare the book for X
send(X, book);

You can see that this design is non-blocking (you can say that most of time this loop does something nonsense but in CPU's eyes, X is running, which means that X is non-blocking. If you want you can replace sleep(2000) with any other code) whereas X and Y (two modules) are synchronous because X can't continue to do any other things (X can't jump out of the loop) until it gets the book from Y.
Normally in this case, make X blocking is much better because non-blocking spends much resource for a stupid loop. But this example is good to help you understand the fact: non-blocking doesn't mean asynchronous.

The four words do make us confused easily, what we should remember is that the four words serve for the design of architecture. Learning about how to design a good architecture is the only way to distinguish them.

For example, we may design such a kind of architecture:

// Module X = Module X1 + Module X2
// Module X1
while (true)
{
    msg = recv(many_other_modules, NON_BLOCKING_FLAG);
    if (msg is not null)
    {
        if (msg == "done")
        {
            break;
        }
        // create a thread to process msg
    }
    else
    {
        sleep(2000); // 2 sec
    }
}
// Module X2
broadcast("I got the book from Y");


// Module Y
// prepare the book for X
send(X, book);

In the example here, we can say that

X1 is non-blocking
X1 and X2 are synchronous
X and Y are asynchronous

If you need, you can also describe those threads created in X1 with the four words.

The more important things are: when do we use synchronous instead of asynchronous? when do we use blocking instead of non-blocking? Is making X1 blocking better than non-blocking? Is making X and Y synchronous better than asynchronous? Why is Nginx non-blocking? Why is Apache blocking? These questions are what you must figure out.

To make a good choice, you must analyze your need and test the performance of different architectures. There is no such an architecture that is suitable for various of needs.

Solution 3:

Asynchronous refers to something done in parallel, say is another thread.
Non-blocking often refers to polling, i.e. checking whether given condition holds (socket is readable, device has more data, etc.)

Solution 4:

Synchronous is defined as happening at the same time (in predictable timing, or in predictable ordering).

Asynchronous is defined as not happening at the same time. (with unpredictable timing or with unpredictable ordering).

This is what causes the first confusion, which is that asynchronous is some sort of synchronization scheme, and yes it is used to mean that, but in actuality it describes processes that are happening unpredictably with regards to when or in what order they run. And such events often need to be synchronized in order to make them behave correctly, where multiple synchronization schemes exists to do so, one of those called blocking, another called non-blocking, and yet another one confusingly called asynchronous.

So you see, the whole problem is about finding a way to synchronize an asynchronous behavior, because you've got some operation that needs the response of another before it can begin. Thus it's a coordination problem, how will you know that you can now start that operation?

The simplest solution is known as blocking.

Blocking is when you simply choose to wait for the other thing to be done and return you a response before moving on to the operation that needed it.

So if you need to put butter on toast, and thus you first need to toast the bred. The way you'd coordinate them is that you'd first toast the bred, then stare endlessly at the toaster until it pops the toast, and then you'd proceed to put butter on them.

It's the simplest solution, and works very well. There's no real reason not to use it, unless you happen to also have other things you need to be doing which don't require coordination with the operations. For example, doing some dishes. Why wait idle staring at the toaster constantly for the toast to pop, when you know it'll take a bit of time, and you could wash a whole dish while it finishes?

That's where two other solutions known respectively as non-blocking and asynchronous come into play.

Non-blocking is when you choose to do other unrelated things while you wait for the operation to be done. Checking back on the availability of the response as you see fit.

So instead of looking at the toaster for it to pop. You go and wash a whole dish. And then you peek at the toaster to see if the toasts have popped. If they haven't, you go wash another dish, checking back at the toaster between each dish. When you see the toasts have popped, you stop washing the dishes, and instead you take the toast and move on to putting butter on them.

Having to constantly check on the toasts can be annoying though, imagine the toaster is in another room. In between dishes you waste your time going to that other room to check on the toast.

Here comes asynchronous.

Asynchronous is when you choose to do other unrelated things while you wait for the operation to be done. Instead of checking on it though, you delegate the work of checking to something else, could be the operation itself or a watcher, and you have that thing notify and possibly interupt you when the response is availaible so you can proceed to the other operation that needed it.

Its a weird terminology. Doesn't make a whole lot of sense, since all these solutions are ways to create synchronous coordination of dependent tasks. That's why I prefer to call it evented.

So for this one, you decide to upgrade your toaster so it beeps when the toasts are done. You happen to be constantly listening, even while you are doing dishes. On hearing the beep, you queue up in your memory that as soon as you are done washing your current dish, you'll stop and go put the butter on the toast. Or you could choose to interrupt the washing of the current dish, and deal with the toast right away.

If you have trouble hearing the beep, you can have your partner watch the toaster for you, and come tell you when the toast is ready. Your partner can itself choose any of the above three strategies to coordinate its task of watching the toaster and telling you when they are ready.

On a final note, it's good to understand that while non-blocking and async (or what I prefer to call evented) do allow you to do other things while you wait, you don't have too. You can choose to constantly loop on checking the status of a non-blocking call, doing nothing else. That's often worse than blocking though (like looking at the toaster, then away, then back at it until it's done), so a lot of non-blocking APIs allow you to transition into a blocking mode from it. For evented, you can just wait idle until you are notified. The downside in that case is that adding the notification was complex and potentially costly to begin with. You had to buy a new toaster with beep functionality, or convince your partner to watch it for you.

And one more thing, you need to realize the trade offs all three provide. One is not obviously better than the others. Think of my example. If your toaster is so fast, you won't have time to wash a dish, not even begin washing it, that's how fast your toaster is. Getting started on something else in that case is just a waste of time and effort. Blocking will do. Similarly, if washing a dish will take 10 times longer then the toasting. You have to ask yourself what's more important to get done? The toast might get cold and hard by that time, not worth it, blocking will also do. Or you should pick faster things to do while you wait. There's more obviously, but my answer is already pretty long, my point is you need to think about all that, and the complexities of implementing each to decide if its worth it, and if it'll actually improve your throughput or performance.

Edit:

Even though this is already long, I also want it to be complete, so I'll add two more points.

There also commonly exists a fourth model known as multiplexed. This is when while you wait for one task, you start another, and while you wait for both, you start one more, and so on, until you've got many tasks all started and then, you wait idle, but on all of them. So as soon as any is done, you can proceed with handling its response, and then go back to waiting for the others. It's known as multiplexed, because while you wait, you need to check each task one after the other to see if they are done, ad vitam, until one is. It's a bit of an extension on top of normal non-blocking.

In our example it would be like starting the toaster, then the dishwasher, then the microwave, etc. And then waiting on any of them. Where you'd check the toaster to see if it's done, if not, you'd check the dishwasher, if not, the microwave, and around again.

Even though I believe it to be a big mistake, synchronous is often used to mean one thing at a time. And asynchronous many things at a time. Thus you'll see synchronous blocking and non-blocking used to refer to blocking and non-blocking. And asynchronous blocking and non-blocking used to refer to multiplexed and evented.

I don't really understand how we got there. But when it comes to IO and Computation, synchronous and asynchronous often refer to what is better known as non-overlapped and overlapped. That is, asynchronous means that IO and Computation are overlapped, aka, happening concurrently. While synchronous means they are not, thus happening sequentially. For synchronous non-blocking, that would mean you don't start other IO or Computation, you just busy wait and simulate a blocking call. I wish people stopped misusing synchronous and asynchronous like that. So I'm not encouraging it.

Edit2:

I think a lot of people got a bit confused by my definition of synchronous and asynchronous. Let me try and be a bit more clear.

Synchronous is defined as happening with predictable timing and/or ordering. That means you know when something will start and end.

Asynchronous is defined as not happening with predictable timing and/or ordering. That means you don't know when something will start and end.

Both of those can be happening in parallel or concurrently, or they can be happening sequentially. But in the synchronous case, you know exactly when things will happen, while in the asynchronous case you're not sure exactly when things will happen, but you can still put some coordination in place that at least guarantees some things will happen only after others have happened (by synchronizing some parts of it).

Thus when you have asynchronous processes, asynchronous programming lets you place some order guarantees so that some things happen in the right sequence, even though you don't know when things will start and end.

Here's an example, if we need to do A then B and C can happen at any time. In a sequential but asynchronous model you can have:

A -> B -> C
or
A -> C -> B
or
C -> A -> B

Every time you run the program, you could get a different one of those, seemingly at random. Now this is still sequential, nothing is parallel or concurrent, but you don't know when things will start and end, except you have made it so B always happens after A.

If you add concurrency only (no parallelism), you can also get things like:

A<start> -> C<start> -> A<end>   -> C<end>   -> B<start> -> B<end>
or
C<start> -> A<start> -> C<end>   -> A<end>   -> B<start> -> B<end>
or
A<start> -> A<end>   -> B<start> -> C<start> -> B<end>   -> C<end>
etc...

Once again, you don't really know when things will start and end, but you have made it so B is coordinated to always start after A ends, but that's not necessarily immediately after A ends, it's at some unknown time after A ends, and B could happen in-between fully or partially.

And if you add parallelism, now you have things like:

A<start> -> A<end>   -> B<start>       -> B<end>         ->
            C<start> -> C<keeps going> -> C<keeps going> -> C<end>
or
A<start> -> A<end>         -> B<start> -> B<end>
C<start> -> C<keeps going> -> C<end>
etc...

Now if we look at the synchronous case, in a sequential setting you would have:

A -> B -> C

And this is the order always, each time you run the program, you get A then B and then C, even though C conceptually from the requirements can happen at any time, in a synchronous model you still define exactly when it will start and end. Off course, you could specify it like:

C -> A -> B

instead, but since it is synchronous, then this order will be the ordering every time the program is ran, unless you changed the code again to change the order explicitly.

Now if you add concurrency to a synchronous model you can get:

C<start> -> A<start> -> C<end> -> A<end> -> B<start> -> B<end>

And once again, this would be the order no matter how many time you ran the program. And similarly, you could explicitly change it in your code, but it would be consistent across program execution.

Finally, if you add parallelism as well to a synchronous model you get:

A<start> -> A<end> -> B<start> -> B<end>
C<start> -> C<end>

Once again, this would be the case on every program run. An important aspect here is that to make it fully synchronous this way, it means B must start after both A and C ends. If C is an operation that can complete faster or slower say depending on the CPU power of the machine, or other performance consideration, to make it synchronous you still need to make it so B waits for it to end, otherwise you get an asynchronous behavior again, where not all timings are deterministic.

You'll get this kind of synchronous thing a lot in coordinating CPU operations with the CPU clock, and you have to make sure that you can complete each operation in time for the next clock cycle, otherwise you need to delay everything by one more clock to give room for this one to finish, if you don't, you mess up your synchronous behavior, and if things depended on that order they'd break.

Finally, lots of systems have synchronous and asynchronous behavior mixed in, so if you have any kind of inherently unpredictable events, like when a user will click a button, or when a remote API will return a response, but you need things to have guaranteed ordering, you will basically need a way to synchronize the asynchronous behavior so it guarantees order and timing as needed. Some strategies to synchronize those are what I talk about previously, you have blocking, non-blocking, async, multiplexed, etc. See the emphasis on "async", this is what I mean by the word being confusing. Somebody decided to call a strategy to synchronize asynchronous processes "async". This then wrongly made people think that asynchronous meant concurrent and synchronous meant sequential, or that somehow blocking was the opposite of asynchronous, where as I just explained, synchronous and asynchronous in reality is a different concept that relates to the timing of things as being in sync (in time with each other, either on some shared clock or in a predictable order) or out of sync (not on some shared clock or in an unpredictable order). Where as asynchronous programming is a strategy to synchronize two events that are themselves asynchronous (happening at an unpredictable time and/or order), and for which we need to add some guarantees of when they might happen or at least in what order.

So we're left with two things using the word "asynchronous" in them:

Asynchronous processes: processes that we don't know at what time they will start and end, and thus in what order they would end up running.
Asynchronous programming: a style of programming that lets you synchronize two asynchronous processes using callbacks or watchers that interrupt the executor in order to let them know something is done, so that you can add predictable ordering between the processes.

Solution 5:

A nonblocking call returns immediately with whatever data are available: the full number of bytes requested, fewer, or none at all.

An asynchronous call requests a transfer that will be performed in its whole(entirety) but will complete at some future time.