Asynchrony, in computer programming, refers to the occurrence of events independent of the main program flow and ways to deal with such events. These may be "outside" events such as the arrival of signals, or actions instigated by a program that take place concurrently with program execution, without the programblocking to wait for results. Asynchronous input/output is an example of the latter cause of asynchrony, and lets programs issue commands to storage or network devices that service these requests while the processor continues executing the program. Doing so provides a degree of parallelism. A common way for dealing with asynchrony in a programming interface is to provide subroutines that return to their caller an object, sometimes called a future or promise, that represents the ongoing events. Such an object will then typically come with a synchronizing operation that blocks until the operation is completed. Some programming languages, such as Cilk, have special syntax for expressing an asynchronous procedure call. Examples of asynchrony include the following:
Asynchronous method dispatch, a data communication method used when there is a need for the server side to handle a large number of long lasting client requests. Using synchronous method dispatch, this scenario may turn the server into an unavailable busy state resulting in a connection failure response caused by a network connection request timeout. The servicing of a client request is immediately dispatched to an available thread from a pool of threads and the client is put in a blocking state. Upon the completion of the task, the server is notified by a callback. The server unblocks the client and transmits the response back to the client. In case of thread starvation, clients are blocked waiting for threads to become available.
Scenarios for Async
1. I/O operations: I/O operations like file I/O, network I/O, database I/O,, and web service calls benefit from asynchrony. Anytime that your code is making a database call, or a web service call or a network call or talking to the file system, that can be done asynchronously while that operation is in progress. 2. Performing multiple operations in parallel: When you need to do different operations in parallel, for example, making a database call, web service call and any calculations, then we can use asynchrony. 3. Long-running event-driven requests: This is the idea where you have a request that comes in, and the request goes to sleep for some time waiting for some other event to take place when that event takes place, you want the request to continue and then send a response to client. So in this case, when request comes in, then thread is assigned to that request and as request goes to sleep, then thread is sent back to threadpool and as the task completes, then it generates the event and picks a thread from thread pool for sending response (the thread sent and picked from thread pool might or might not be the same.
