asynchronous vs non-blocking

Question

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

Answer 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.

Answer 2

As you can probably see from the multitude of different (and often mutually exclusive) answers, it depends on who you ask. In some arenas, the terms are synonymous. Or they might each refer to two similar concepts:

• One interpretation is that the call will do something in the background essentially unsupervised in order to allow the program to not be held up by a lengthy process that it does not need to control. Playing audio might be an example - a program could call a function to play (say) an mp3, and from that point on could continue on to other things while leaving it to the OS to manage the process of rendering the audio on the sound hardware.
• The alternative interpretation is that the call will do something that the program will need to monitor, but will allow most of the process to occur in the background only notifying the program at critical points in the process. For example, asynchronous file IO might be an example - the program supplies a buffer to the operating system to write to file, and the OS only notifies the program when the operation is complete or an error occurs.

In either case, the intention is to allow the program to not be blocked waiting for a slow process to complete - how the program is expected to respond is the only real difference. Which term refers to which also changes from programmer to programmer, language to language, or platform to platform. Or the terms may refer to completely different concepts (such as the use of synchronous/asynchronous in relation to thread programming).

Sorry, but I don't believe there is a single right answer that is globally true.

Answer 3

The blocking models require the initiating application to block when the I/O has started. This means that it isn't possible to overlap processing and I/O at the same time. The synchronous non-blocking model allows overlap of processing and I/O, but it requires that the application check the status of the I/O on a recurring basis. This leaves asynchronous non-blocking I/O, which permits overlap of processing and I/O, including notification of I/O completion.

Answer 4

• 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.)

Answer 5

Putting this question in the context of NIO and NIO.2 in java 7, async IO is one step more advanced than non-blocking. With java NIO non-blocking calls, one would set all channels (SocketChannel, ServerSocketChannel, FileChannel, etc) as such by calling AbstractSelectableChannel.configureBlocking(false). After those IO calls return, however, you will likely still need to control the checks such as if and when to read/write again, etc.
For instance,

while (!isDataEnough()) {
    socketchannel.read(inputBuffer);
    // do something else and then read again
}

With the asynchronous api in java 7, these controls can be made in more versatile ways. One of the 2 ways is to use CompletionHandler. Notice that both read calls are non-blocking.

asyncsocket.read(inputBuffer, 60, TimeUnit.SECONDS /* 60 secs for timeout */, 
    new CompletionHandler<Integer, Object>() {
        public void completed(Integer result, Object attachment) {...}  
        public void failed(Throwable e, Object attachment) {...}
    }
}

Answer 6

Blocking: control returns to invoking precess after processing of primitive(sync or async) completes

Non blocking: control returns to process immediately after invocation