Asynchronous vs synchronous execution, what does it really mean? [closed]

Answer 1

Synchronous/Asynchronous HAS NOTHING TO DO WITH MULTI-THREADING.

Synchronous or Synchronized means "connected", or "dependent" in some way. In other words, two synchronous tasks must be aware of one another, and one task must execute in some way that is dependent on the other, such as wait to start until the other task has completed.
Asynchronous means they are totally independent and neither one must consider the other in any way, either in the initiation or in execution.

Synchronous (one thread):

1 thread ->   |<---A---->||<----B---------->||<------C----->|

Synchronous (multi-threaded):

thread A -> |<---A---->|   
                        \  
thread B ------------>   ->|<----B---------->|   
                                              \   
thread C ---------------------------------->   ->|<------C----->| 

Asynchronous (one thread):

         A-Start ------------------------------------------ A-End   
           | B-Start -----------------------------------------|--- B-End   
           |    |      C-Start ------------------- C-End      |      |   
           |    |       |                           |         |      |
           V    V       V                           V         V      V      
1 thread->|<-A-|<--B---|<-C-|-A-|-C-|--A--|-B-|--C-->|---A---->|--B-->| 

Asynchronous (multi-Threaded):

 thread A ->     |<---A---->|
 thread B ----->     |<----B---------->| 
 thread C --------->     |<------C--------->|

• Start and end points of tasks A, B, C represented by <, > characters.
• CPU time slices represented by vertical bars |

Technically, the concept of synchronous/asynchronous really does not have anything to do with threads. Although, in general, it is unusual to find asynchronous tasks running on the same thread, it is possible, (see below for examples) and it is common to find two or more tasks executing synchronously on separate threads... No, the concept of synchronous/asynchronous has to do solely with whether or not a second or subsequent task can be initiated before the other (first) task has completed, or whether it must wait. That is all. What thread (or threads), or processes, or CPUs, or indeed, what hardware, the task[s] are executed on is not relevant. Indeed, to make this point I have edited the graphics to show this.

---

ASYNCHRONOUS EXAMPLE:

In solving many engineering problems, the software is designed to split up the overall problem into multiple individual tasks and then execute them asynchronously. Inverting a matrix, or a finite element analysis problem, are good examples. In computing, sorting a list is an example. The quicksort routine, for example, splits the list into two lists and performs a quicksort on each of them, calling itself (quicksort) recursively. In both of the above examples, the two tasks can (and often were) executed asynchronously. They do not need to be on separate threads. Even a machine with one CPU and only one thread of execution can be coded to initiate processing of a second task before the first one has completed. The only criterion is that the results of one task are not necessary as inputs to the other task. As long as the start and end times of the tasks overlap, (possible only if the output of neither is needed as inputs to the other), they are being executed asynchronously, no matter how many threads are in use.

SYNCHRONOUS EXAMPLE:

Any process consisting of multiple tasks where the tasks must be executed in sequence, but one must be executed on another machine (Fetch and/or update data, get a stock quote from financial service, etc.). If it's on a separate machine it is on a separate thread, whether synchronous or asynchronous.

Answer 2

I think a good way to think of it is a classic running Relay Race

Synchronous: Processes like members of the same team, they won't execute until they receive baton (end of the execution of previous process/runner) and yet they are all acting in sync with each other.

Asynchronous: Where processes like members of different teams on the same relay race track, they will run and stop, async with each other, but within same race (overall program execution).

Does it make sense?

Answer 3

As a really simple example,

SYNCHRONOUS

Imagine 3 school students instructed to run a relay race on a road.

1st student runs her given distance, stops and passes the baton to the 2nd. No one else has started to run.

1------>
        2.
                3.

When the 2nd student retrieves the baton, she starts to run her given distance.

      1.
        2------>
                3.

The 2nd student got her shoelace untied. Now she has stopped and tying up again. Because of this, 2nd's end time has got extended and the 3rd's starting time has got delayed.

      1.
        --2.--->
                3.

This pattern continues on till the 3rd retrieves the baton from 2nd and finishes the race.

ASYNCHRONOUS

Just Imagine 10 random people walking on the same road. They're not on a queue of course, just randomly walking on different places on the road in different paces.

2nd person's shoelace got untied. She stopped to get it tied up again.

But nobody is waiting for her to get it tied up. Everyone else is still walking the same way they did before, in that same pace of theirs.

10-->    9-->
   8--> 7-->   6-->
 5-->     4-->
1-->   2.    3-->

Answer 4

A different english definition of Synchronize is Here

Coordinate; combine.

I think that is a better definition than of "Happening at the same time". That one is also a definition, but I don't think it is the one that fits the way it is used in Computer Science.

So an asynchronous task is not co-coordinated with other tasks, whereas a synchronous task IS co-coordinated with other tasks, so one finishes before another starts.

How that is achieved is a different question.

Answer 5

Synchronous execution means the execution happens in a single series. A->B->C->D. If you are calling those routines, A will run, then finish, then B will start, then finish, then C will start, etc.

With Asynchronous execution, you begin a routine, and let it run in the background while you start your next, then at some point, say "wait for this to finish". It's more like:

Start A->B->C->D->Wait for A to finish

The advantage is that you can execute B, C, and or D while A is still running (in the background, on a separate thread), so you can take better advantage of your resources and have fewer "hangs" or "waits".

Answer 6

In a nutshell, synchronization refers to two or more processes' start and end points, NOT their executions. In this example, Process A's endpoint is synchronized with Process B's start point:

SYNCHRONOUS
   |--------A--------|
                     |--------B--------|

Asynchronous processes, on the other hand, do not have their start and endpoints synchronized:

ASYNCHRONOUS
   |--------A--------|
         |--------B--------|

Where Process A overlaps Process B, they're running concurrently or synchronously (dictionary definition), hence the confusion.

UPDATE: Charles Bretana improved his answer, so this answer is now just a simple (potentially oversimplified) mnemonic.