Synchronous Vs Asynchronous related to web services

Question

I was trying to find a good explanation for the difference between synchronous communication vs asynchronous communication for web services, all over the internet. but it seems that even the people who is willing to provide an answer to the problem is also confused with it. and one answer is the complete vice versa for another answer.

If anybody can provide a good explanation about the difference of above matter with a clear idea, it would be helpful for everybody who would face the same problem in the future.

Answer 1

Asynchronous service

Say you have a long running web service (say it reads a large file from the file system and does some processing).

If you set this up as a 'synchronous' web service (using the WCF definition of that), then the calling client will have to wait until the processing completes, and typically this will block on one of the asp.net worker threads while processing completes. For a service with high traffic, this can become problematic.

If you set this up as an asynchronous web service, then what you are saying is that your code is going to delegate some of the long running processing to another thread, or use a non-blocking mechanism, and that this will return at some point in the future (if you are using c# 5.0, then you might want to look at examples of the async and await keywords).

For the example, reading a large file could be done using one of the async ReadFile methods.

This will not block one of the asp.net worker threads, allowing potentially greater throughput.

(There is often some confusion when people refer to making multiple simultaneous calls to the same service (often via AJAX from a web page) - while the calls from the page will typically be made using an asynchronous mechanism in javascript, this is not quite the same as what is described above - I like to keep a distinction between multiple parallel calls and asynchronous calls in my head)

Asynchronous calls

It's also worth noting that you can make an asynchronous call to a service even if that service is not set up to be 'asynchronous'. This is how AJAX calls in javascript will work, e.g.

var jqxhr = $.ajax( "AnyService.svc" )
    .done(function() { alert("success"); })
    .fail(function() { alert("error"); })
    .always(function() { alert("complete"); });

alert("Called");

For this example, you would expect to see 'Called' displayed before 'Success', as this will not wait for the service to return prior to proceeding. The service you are calling does not have to be 'asynchronous'.

Edit

As pointed out in the comments, you can also have a client that calls an 'asynchronous' service in a synchronous manner (i.e. the service will not block worker threads for further requests, but the client will block at that side).

Answer 2

First lets clear your doubt about Synchronous and asynchronous

Synchronous communication is direct communication where the communicators are time synchronized. This means that all parties involved in the communication are present at the same time. This includes, but is not limited to, a telephone conversation (not texting), a company board meeting, a chat room event and instant messaging.

Asynchronous communication does not require that all parties involved in the communication to be present at the same time. Some examples are e-mail messages, discussion boards, blogging, and text messaging over cell phones. In distance (specifically online) education asynchronous communication is the major (sometimes the only) method of communication. Usually, we use different discussion boards in each class with each having its own purpose.

e.g. synchronous

When I call you on the phone, I dial your number and WAIT until you pick up. Then you say something, and in the very same moment I listen to you. When you finished, I send you data (talk to you) and in the same moment you receive them (listen to me). At the end of our communication one of us says "END OF TRANSMISSION" (Good Bye), the other says "Acknoledged" (Good Bye) and then both ring off.

asynchronous

I write you a letter. I put it to the postoffice, and it will be sent to you. I the meantime I do NOT WAIT. I do many different other things. Then you receive the letter. You read it while I still do many different other things. Then you write me an answer and send it to me. In all those things I am not involved. At the next day I get a (synchronous) message (a signal) from the system (postman). It (he) says: "Here is a message for you". Alternatively I could poll my inbox every five minutes to check if a new letter is there. Then I pause my other work, receive your letter and read your answer. Then I do something according to this answer. But this are things you will not notice, because you are not involved in what I do with your asynchronous answer.

courtesy: How does Synchronous and Asynchronous communication work exactly