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

What is the difference between asynchronous and synchronous execution?

In regards to the "at the same time" definition of synchronous execution (which is sometimes confusing), here's a good way to understand it:

Synchronous Execution: All tasks within a block of code are all executed at the same time.

Asynchronous Execution: All tasks within a block of code are not all executed at the same time.

Yes synchronous means at the same time, literally, it means doing work all together. multiple human/objects in the world can do multiple things at the same time but if we look at computer, it says synchronous means where the processes work together that means the processes are dependent on the return of one another and that's why they get executed one after another in proper sequence. Whereas asynchronous means where processes don't work together, they may work at the same time(if are on multithread), but work independently.

I think this is bit round-about explanation but still it clarifies using real life example.

Small Example:

Let's say playing an audio involves three steps:

1. Getting the compressed song from harddisk
2. Decompress the audio.
3. Play the uncompressed audio.

If your audio player does step 1,2,3 sequentially for every song then it is synchronous. You will have to wait for some time to hear the song till the song actually gets fetched and decompressed.

If your audio player does step 1,2,3 independent of each other, then it is asynchronous. ie. While playing audio 1 ( step 3), if it fetches audio 3 from harddisk in parallel (step 1) and it decompresses the audio 2 in parallel. (step 2 ) You will end up in hearing the song without waiting much for fetch and decompress.

Synchronous means queue way execution one by one task will be executed. Suppose there is only vehicle that need to be share among friend to reach their destination one by one vehicle will be share.
In asynchronous case each friend can get rented vehicle and reach its destination.

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.

You are confusing Synchronous with Parallel vs Series. Synchronous mean all at the same time. Syncronized means related to each othere which can mean in series or at a fixed interval. While the program is doing all, it it running in series. Get a dictionary...this is why we have unsweet tea. You have tea or sweetened tea.

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.

In simpler terms:

SYNCHRONOUS

You are in a queue to get a movie ticket. You cannot get one until everybody in front of you gets one, and the same applies to the people queued behind you.

ASYNCHRONOUS

You are in a restaurant with many other people. You order your food. Other people can also order their food, they don't have to wait for your food to be cooked and served to you before they can order. In the kitchen restaurant workers are continuously cooking, serving, and taking orders. People will get their food served as soon as it is cooked.

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.

Sync and async operations is about execution order a new task in relation to the current task. Two tasks on a table: current task and a new task

Synchronous(blocking) - implies that tasks will be executed one by one. A next task is started only after its previous task is finished. Task 2 is not started until Task 1 is finished

Asynchronous(non-blocking) – implies that task returns control immediately with a promise to execute a code and notify about result later(e.g. callback, feature). Task 2 is executed even if Task 1 is not finished

SO answer about sync and async: in iOS, in Android

[Paralel vs Concurrent]

Simple Explanation via analogy

Synchronous Execution

My boss is a busy man. He tells me to write the code. I tell him: Fine. I get started and he's watching me like a vulture, standing behind me, off my shoulder. I'm like "Dude, WTF: why don't you go and do something while I finish this?"

he's like: "No, I'm waiting right here until you finish." This is synchronous.

Asynchronous Execution

The boss tells me to do it, and rather than waiting right there for my work, the boss goes off and does other tasks. When I finish my job I simply report to my boss and say: "I'm DONE!" This is Asynchronous Execution.

(Take my advice: NEVER work with the boss behind you.)

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".

Simply said asynchronous execution is doing stuff in the background.

For example if you want to download a file from the internet you might use a synchronous function to do that but it will block your thread until the file finished downloading. This can make your application unresponsive to any user input.

Instead you could download the file in the background using asynchronous method. In this case the download function returns immediately and program execution continues normally. All the download operations are done in the background and your program will be notified when it's finished.

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?

I created a gif for explain this, hope to be helpful: look, line 3 is asynchronous and others are synchronous. all lines before line 3 should wait until before line finish its work, but because of line 3 is asynchronous, next line (line 4), don't wait for line 3, but line 5 should wait for line 4 to finish its work, and line 6 should wait for line 5 and 7 for 6, because line 4,5,6,7 are not asynchronous.

Use an example of instructions for making a breakfast

1. Pour a cup of coffee.
2. Heat up a pan, then fry two eggs.
3. Fry three slices of bacon.
4. Toast two pieces of bread.
5. Add butter and jam to the toast.
6. Pour a glass of orange juice.

If you have experience cooking, you'd execute those instructions asynchronously. you'd start warming the pan for eggs, then start the bacon. You'd put the bread in the toaster, then start the eggs. At each step of the process, you'd start a task, then turn your attention to tasks that are ready for your attention.

Cooking breakfast is a good example of asynchronous work that isn't parallel. One person (or thread) can handle all these tasks. Continuing the breakfast analogy, one person can make breakfast asynchronously by starting the next task before the first completes. The cooking progresses whether or not someone is watching it. As soon as you start warming the pan for the eggs, you can begin frying the bacon. Once the bacon starts, you can put the bread into the toaster.

For a parallel algorithm, you'd need multiple cooks (or threads). One would make the eggs, one the bacon, and so on. Each one would be focused on just that one task. Each cook (or thread) would be blocked synchronously waiting for bacon to be ready to flip, or the toast to pop.

Reference from Asynchronous programming concepts

When executing a sequence like: a>b>c>d>, if we get a failure in the middle of execution like:

a
b
c
fail

Then we re-start from the beginning:

a
b
c
d

this is synchronous

If, however, we have the same sequence to execute: a>b>c>d>, and we have a failure in the middle:

a
b
c
fail

...but instead of restarting from the beginning, we re-start from the point of failure:

c
d

...this is know as asynchronous.

Synchronous basically means that you can only execute one thing at a time. Asynchronous means that you can execute multiple things at a time and you don't have to finish executing the current thing in order to move on to next one.

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-->

A synchronous operation does its work before returning to the caller.

An asynchronous operation does (most or all of) its work after returning to the caller.

Synchronous means that the caller waits for the response or completion, asynchronous that the caller continues and a response comes later (if applicable).

As an example:

static void Main(string[] args)
{
    Console.WriteLine("Before call");
    doSomething();
    Console.WriteLine("After call");
}

private static void doSomething()
{
    Console.WriteLine("In call");
}

This will always ouput:

Before call
In call
After call

But if we were to make doSomething asynchronous (multiple ways to do it), then the output could become:

Before call
After call
In call

Because the method making the asynchronous call would immediately continue with the next line of code. I say "could", because order of execution can't be guaranteed with asynch operations. It could also execute as the original, depending on thread timings, etc.