Asynchronous vs synchronous execution what does it really mean

Question

What is the difference between asynchronous and synchronous execution

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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:

Getting the compressed song from harddisk
Decompress the audio.
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.

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When you execute something synchronously  you wait for it to finish before moving on to another task  When you execute something asynchronously  you can move on to another task before it finishes   That being said  in the context of computers this translates into executing a process or task on another  thread   A thread is a series of commands  a block of code  that exists as a unit of work  The operating system can manage multiple threads and assign a thread a piece   slice   of processor time before switching to another thread to give it a turn to do some work  At its core  pardon the pun   a processor can simply execute a command  it has no concept of doing two things at one time  The operating system simulates this by allocating slices of time to different threads   Now  if you introduce multiple cores processors into the mix  then things CAN actually happen at the same time  The operating system can allocate time to one thread on the first processor  then allocate the same block of time to another thread on a different processor   All of this is about allowing the operating system to manage the completion of your task while you can go on in your code and do other things   Asynchronous programming is a complicated topic because of the semantics of how things tie together when you can do them at the same time  There are numerous articles and books on the subject  have a look

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Synchronous execution means the execution happens in a single series   A- gt B- gt C- gt 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- gt B- gt C- gt D- gt 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

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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

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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

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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

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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

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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

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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

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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

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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

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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

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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

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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

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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

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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

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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------ gt          2                  3    When the 2nd student retrieves the baton  she starts to run her given distance         1          2------ gt                  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 --- gt                  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-- gt     9-- gt     8-- gt  7-- gt    6-- gt   5-- gt      4-- gt  1-- gt    2     3-- gt

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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

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Use an example of instructions for making a breakfast   Pour a cup of coffee  Heat up a pan  then fry two eggs  Fry three slices of bacon  Toast two pieces of bread  Add butter and jam to the toast  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

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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

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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 - gt      lt ---A---- gt    lt ----B---------- gt    lt ------C----- gt     Synchronous  multi-threaded    thread A - gt    lt ---A---- gt                                  thread B ------------ gt    - gt   lt ----B---------- gt                                                         thread C ---------------------------------- gt    - gt   lt ------C----- gt      Asynchronous  one thread                A-Start ------------------------------------------ A-End                 B-Start ----------------------------------------- --- B-End                           C-Start ------------------- C-End                                                                                                    V    V       V                           V         V      V       1 thread- gt   lt -A-  lt --B---  lt -C- -A- -C- --A-- -B- --C-- gt  ---A---- gt  --B-- gt      Asynchronous  multi-Threaded     thread A - gt        lt ---A---- gt    thread B ----- gt        lt ----B---------- gt     thread C --------- gt        lt ------C--------- gt      Start and end points of tasks A  B  C represented by  lt    gt  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

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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

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

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