This time, let's clear up: synchronous, asynchronous, blocking, non blocking

Hello everyone , I am a yes.

Following last article Speak thoroughly I/O Model after , Let's talk about the Internet I/O Several confusing concepts often accompany ： Sync 、 asynchronous 、 Blocking 、 Nonblocking differences .

After writing this article, the groundwork is almost done , Then it officially begins Netty In depth analysis of , Coming soon , Hee hee .

Don't talk much , Start ！

Sync & asynchronous

Synchronous and asynchronous refer to ： Whether the current thread needs to wait for the method call to complete .

For example, you call a method to carry 100 stones ：

• Synchronization refers to calling this method , Your thread needs to wait for these 100 stones to be lifted , Then get the result of moving , Then continue to execute the remaining code logic .

// Synchronization mode 

result =  Lift a hundred stones ();
// You need to wait for the result of moving , To execute the following logic 
if(result) {
    
 Pay when the stone is finished ();
}
 Calculate the next stone lifting task ();

• Asynchrony refers to calling this method , Immediately return directly , Don't wait for these 100 stones to be moved , You can immediately execute the following code logic , Then use callback or event notification to get the result that the stone has been moved .

// Asynchronous way 

 Lift a hundred stones ({
    
    // Callback 
	 Pay when the stone is finished ();
});
// Don't wait for the stone to be lifted , Immediately execute the following logic 
 Calculate the next stone lifting task ();

It can be seen directly , Synchronization and asynchrony are the differences in the calling methods , This makes our coding method different .

The code logic under asynchronous invocation is relatively less intuitive , You need to use callback or event notification , This requires high coding ability in complex logic . Synchronous calls are straightforward , Wait for the execution to finish, then get the result, and then execute the following logic , The requirements for coding ability are low , It's also less likely to make mistakes .

So you will find that there are many methods, which are called asynchronously , But the final use is asynchronous to synchronous .

For example, you submit a task to the thread pool , Get one future, At this time, it is asynchronous , Then you call in the code immediately future.get(), It becomes waiting for the task to be completed , This is called asynchronous to synchronous , image Dubbo RPC Call synchronization to get the return result, which is achieved in this way .

Blocking & Non blocking

Blocking and non blocking refer to ： When the current interface data is not ready , Whether the thread is blocked and suspended .

What is blocking pending ？ That is, the current thread is still in CPU In the time slice , Blocked method called , Because the data is not ready , Then give up before the time slice CPU.

So blocking and synchronization seem to be waiting , But essentially they are different , I didn't give up when synchronizing CPU.

Non blocking is when the current interface data is not ready , The thread will not be blocked and suspended , You can constantly poll the request interface , See if the data is ready .

So far, we can draw a conclusion ：

• Sync & Asynchronous refers to ： When the data has not been processed , The logical processing of the code is different .
• Blocking & Non blocking finger ： When the data has not been processed ( Not ready ), State of thread .

So synchronization & Asynchrony is actually viewed from the high-level dimension of framework , And jam & Non blocking is often chosen for the underlying system call , In other words, the two are considered from different dimensions .

combining I/O Look at

Premise ： There is an operating system between the program and the hardware , And for the sake of safety ,Linux The system is divided into ： User mode and kernel mode

Under this premise , Let's be clear I/O The operation has two steps ：

1. launch I/O request
2. actual I/O Reading and writing , That is, data is copied from kernel cache to user space

Blocking I/O And non blocking I/O. As above , It actually refers to whether the user thread is blocked , Here refers to the steps 1（ launch I/O request ）.

• Blocking I/O, It refers to that the user thread initiates I/O On request , If the data is not ready （ For example, there is no network data reception ）, Will block the current thread , Give up CPU.
• Non blocking I/O, It refers to that the user thread initiates I/O On request , If the data is not ready （ For example, there is no network data reception ）, It will not block the current thread , You can continue to perform subsequent tasks .

You can find , Blocking and non blocking here actually refer to whether the user thread will be blocked .

Sync I/O And asynchronous I/O. As above , We can know that this is the basis I/O Different response modes .

• Sync I/O, It refers to that the user thread initiates I/O On request , There are data , Then go to step 2（ actual I/O Reading and writing , That is, data is copied from kernel cache to user space ）, In this process, the user thread waits for the copy to complete .
• asynchronous I/O, It refers to that the user thread initiates I/O On request , There are data , Then go to step 2（ actual I/O Reading and writing , That is, data is copied from kernel cache to user space ）, There is no need for user threads to wait in the process of copying , User threads can execute other logic , Wait until the kernel copies the data from kernel space to user space , The user thread will get a “ notice ”.

Think again , stay I/O In this scenario, synchronization and asynchrony are actually the implementation of the kernel , Because the executor of the copy is the kernel , One is to synchronously copy data to user space , User threads need to wait . One is through asynchronous way , User threads don't have to wait , After copying , The kernel will call the specified callback function .

If you don't understand the above , Just remember ：

• Sync I/O： It means that the user thread will need to wait for the step 2 completion of enforcement .
• asynchronous I/O： It means that the user thread does not need to wait for the step 2 perform .

Okay , If you have understood the above concepts , So what we call synchronization blocking in ordinary days I/O, Synchronous nonblocking I/O Wait is actually to combine the above two steps , It shouldn't be hard to understand .

Let me briefly summarize , About I/O The block 、 Non blocking 、 Sync 、 asynchronous ：

• Blocking and non blocking refer to initiating I/O After the request , Different user thread states , Blocking I/O The current user thread will be blocked when the data is not ready , Instead of blocking I/O Will immediately return an error , Does not block the current user thread .
• Synchronous and asynchronous refer to , Kernel I/O Copy implementation , When the data is ready , You need to copy kernel space data to user space , If it's synchronization I/O Then the user thread will wait for the copy to complete , The asynchronous I/O Then the user thread of this copy process can do whatever it should do , When the kernel is copied, it will “ notice ” User threads .

Last

it is to be noted that , The same noun may have different meanings in different scenes . My article about synchronization 、 asynchronous 、 Blocking 、 The concepts of non blocking are based on I/O In the scene .

Limited personal ability , I don't know if I have made it clear , If you have any questions, you can leave a message in the message area .

Okay , About Netty The series is almost ready , The next chapter begins to deepen Netty On a journey ！

Links to the first two foreshadowing articles are as follows ：

Why the Internet I/O Will be blocked

Let us , From a to Z , Be transparent I/O Model

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I am a yes, From a little bit to a billion , See you next time ！