9.简述同步和异步的区别

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同步是阻塞模式，异步是非阻塞模式。

同步就是指一个进程在执行某个请求的时候，若该请求需要一段时间才能返
回信息，那么这个进程将会一直等待下去，直到收到返回信息才继续执行下去

异步是指进程不需要一直等下去，而是继续执行下面的操作，不管其他进程的状
态。当有消息返回时系统会通知进程进行处理，这样可以提高执行的效率。

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Synchronous is blocking mode, asynchronous is non-blocking mode.

Synchronization is when a process executes a request that takes some time to return
Return the message, and the process will wait until it receives the return message

Asynchronous means that the process does not have to wait forever, but instead continues to perform the following operation, regardless of the appearance of other processes
State. When a message is returned, the system notifies the process to process it, which makes execution more efficient.