Asynchronous programming in .Net

The advancement in software technology is so rapid that it has necessitated the usage of high-end processors for executing the complex logic involved in it. It has also now become very common to execute code on multiple processors.
When all these have occurred in the hardware side, the software is also geared to provide efficient code that can harness the maximum possible performance of the processors. The concept of threading in software development is about this aspect of improving the speed and efficiency of the software. It helps to perform multiple operations simultaneously by utilizing the idle time of the processor(s) to a great extent.


Asynchronous programming is about executing part of code on separate threads. A thread is a sequence of execution in program. The .Net framework has implemented the feature of asynchronous programming through the Asynchronous Programming (APM) Model by allowing to executing tasks in a non-linear way. It provides rich set of classes implementing the asynchronous operations and prescribes a standardized mechanism that can allow executing them without directly working with the threads. With this, the application developed can perform faster and better, be more responsive and utilize system resources optimally.
Different types of APM in .Net

The APM involves executing an asynchronous operation by calling a method (starting with ‘Begin’) with the relevant data. To control the execution of this operation at its end stage, the terminating method (starting with ‘End’) is called in different ways.

For example, the method, BeginRead () and EndRead () of the FileStream class is used to read the bytes of a file asynchronously. While initiating the asynchronous call, an object of type, IASyncResult is returned with values filled with relevant information. The same IAysncResult object is also passed as parameter to the method called during the termination of the asynchronous method.

Based on the way in which the end of the asynchronous call is handled in the code, the .Net framework has classified three styles of APM as listed below:

Wait-until-Done model:

This model is used when the application cannot execute any additional work until the results of the asynchronous operation is received. The asynchronous operation is initiated by the call to ‘BeginOperation’.
By calling the method, ‘EndOperation’ (method corresponding to the asynchronous version of the termination of actual operation) in the main thread, further execution of application is blocked. The time between these two calls can be used to execute any other task. In this method, the IASyncResult instance returned from the call, ‘BeginOperation’ is passed as parameter to the ‘EndOperation’.

Polling model:

In this model, the status of execution of the asynchronous task is polled at regular time intervals (for example, in each iteration of a loop). During the time interval, some other tasks can get executed concurrently in the main thread. But, only after the completion of the execution of the asynchronous task, further execution takes place in the main thread. The completion status of execution of the asynchronous operation is updated in the IsCompleted property of the IAsyncResult object returned from the ‘EndOperation’ method. Hence, this model allows executing the tasks in main thread by polling on the status of completion of the asynchronous operation.

Callback model:

This model uses an AsyncCallback delegate that is created and passed as parameter to the ‘BeginOperation’ method. This will in turn allow the delegate to be called at the end of execution of the asynchronous operation executing on another thread. Also, any state information (like file handle, status flags, etc.) required during the termination of the asynchronous operation is passed through this callback method. This model allows the execution of asynchronous operation in a ‘fire and forget’ kind of state wherein the termination occurs in a different code.
Helper classes for APM

Some of the helper classes designed to aid APM are ThreadPool and Timer included in the System.Threading namespace. ThreadPool is used for getting a thread (already instantiated and maintained by the framework) from a pool of threads for performing asynchronous programming. The main aim of using ThreadPool is that it saves time for the application since it reuses existing threads without any setup tasks executed for the thread. Also, the code size and hence the developing effort is greatly reduced since the thread management tasks like creating, scheduling and termination are taken care of by the framework itself.

Timer is another important helper class used for creating periodically reoccurring routines. Object of this class will fire an asynchronous call to a method based on time. Options to specify the callback routine, time interval, etc. can be specified during the creation of the Timer object.

Another class called SynchronizationContext class is used usually to fire an asynchronous call. This class provides two methods for asynchronous execution which does not return any value or object. The Send method executes the asynchronous code in a separate thread and also blocks the execution of the caller until it returns. This is contrary to the method Post wherein the asynchronous code is executed without blocking the execution of caller.

Tips:

• Most of the classes supporting APM contain methods starting from ‘Begin’ and ‘End’. It is easier to identify such methods for asynchronous operations. Example: BeginRead, EndRead, etc.

• To handle exceptions (like InvalidOperationException) thrown during asynchronous processing of a request, it has to be done at the time of ‘End…’ call.

• Since it is very difficult to troubleshoot multithreaded program(with asynchronous code) due to the inconsistent behavior of the bug causing error, it is suggested to carefully design proper threading technique and incrementally develop the application so that it is more scalable and robust.

• Usage of Thread pool of the framework is highly recommended so as to reduce the memory overhead caused due to the additional storage of context information of the newly created thread.

While performance and responsiveness of the application is very vital for the success of an application, it is also equally important to analyze the pros and cons of usage of asynchronous programming in the development of the software. It is found that the effort and time spent for using asynchronous programming is worth for complex and time critical applications having long running tasks rather an application of lesser complexity.

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