Detailed explanation of the difference between synchronous requests and asynchronous requests

First explain the concepts of synchronization and asynchronousness
Synchronization refers to the communication method in which the sender sends data and waits for the receiver to send back a response before sending the next data packet. Asynchronous refers to the communication method in which the sender sends data, waits for the receiver to send back a response, and then sends the next data packet.
Concepts of synchronous communication and asynchronous communication
Synchronous communication requires that both parties communicate at the same clock frequency and be accurately coordinated to ensure accurate synchronization of the sender and receiver by sharing a single clock or timing pulse source. High efficiency;
The asynchronous communication method does not require both parties to synchronize. The sender and receiver can use their own clock sources. Both parties follow an asynchronous communication protocol, using characters as data transmission units. The time interval for the sender to transmit characters is uncertain, and the sending efficiency Less efficient than synchronous transmission.
User's communication choice for calling WEB services: synchronous or asynchronous.
Users can implement service calls synchronously or asynchronously. From the user's point of view, the difference between these two methods is:
Synchronization - the user calls the service through a single thread; the thread sends the request, blocks while the service is running, and waits for the response.
Asynchronous - A consumer calls a service through two threads; one thread sends the request, while a separate thread receives the response.
The terms synchronization and asynchronous are often confused with sequence and concurrency. The latter two terms relate to the order in which individual tasks must be performed, while synchronous and asynchronous relate to the way a thread performs a single task (such as calling a single service). A good way to understand the difference between synchronous and asynchronous calls is to consider the consequences of crash recovery:
Synchronous - If a consumer crashes while blocking while the service is running, when it restarts, it will Unable to reconnect to the ongoing call, so the response is lost. The user must repeat the call process and hope it doesn't crash this time.
Asynchronous - If the user crashes while waiting for a response after sending a request, when it restarts, it can continue to wait for the response, so the response is not lost.
Crash recovery isn't the only difference between synchronous and asynchronous calls, but if you're trying to determine which way a call goes one way, consider how each handles crash recovery, which can often give you a good Answer.
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When using Ajax in Java Web development, it is important to fully understand the difference between synchronous and asynchronous communication models (see Resources). The lack of support for asynchronous communication models has implications for client-side development, integration with web frameworks, tag library usage, IDE usage, and thread behavior.
In the synchronous request/response communication model, it is always the browser (as opposed to a web server, application server, or web application) that initiates the request (by the web user). The web server, application server, or web application then responds to the incoming request. The user cannot continue to use the browser while the synchronous request/response pair is being processed.
In the asynchronous request/response communication model, communication from the browser (via the web user) to the web server, application server, or web application (and vice versa) is decoupled. In the processing of asynchronous request/response pairs, web users can continue to use the browser while the current asynchronous request is being processed. Once the asynchronous request processing is complete, the asynchronous response is communicated (from the web server, application server, or web application) back to the client page. Typically, the call has no impact on Web users during this process; they do not need to wait for a response.
Submitting data in Jquery ajax can be done synchronously or asynchronously. 1. What is the difference between synchronous and asynchronous? , 2. Under what circumstances is it used? Synchronous: Submit the request ->Wait for server processing ->After processing and return, the client browser cannot do anything during this period
Asynchronous: The request is triggered by an event ->Server processing (this means the browser can still do other things ) -> Processed 1. My understanding: 1. Single-mindedness: You can only do one thing at the moment, and other things must wait for the current thing to be completed before you can continue with the next thing. 2. Half-hearted: You can do multiple things at the same time: your left hand presses the space bar, your right hand can keep hitting the mouse, and your eyes have to look at the screen at the same time, which is very hard. 2. Please look at the following script: function

ValidateClick()
{ var areaNo=$("#<%=drpList.ClientID %>").val(); if(areaNo) { alert("请选择"); return false; } $.ajax({
cache:"False",
type:"POST", (A)//async: false, 【(A)】
url:"Validate.ashx",
data:$("#form1").serialize(), 
success:function(o)
{ 
if(o=="ERROR") { alert("输入错误！"); return false;} 
else if(o=="OK")
{ alert("正确！");return true; } 
else
{ alert("其他错误！"); return false;}
},
error:function(xhr, ajaxOptions, thrownError)
{ alert("网络异常，稍后重试！"); return false; }
}); // 【(B) 】 //alert("这里");return true;}

At (A): the default is not true, which means asynchronous transmission is used. When async: true: we use POST to submit the data to the url page. , the method or function at [(B)] will be executed at the same time, regardless of whether the result has been returned at (A) or (B) has been executed. When the result is returned in (A), a prompt will pop up according to the specified if , when async: false: At this time, you must wait for the result to be returned at (A), and then execute the if judgment statement. After the judgment is completed, then execute the function at (B).

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