Utilizing the Win32 Internet Functions Asynchronously

The amount of time it takes an application to access an Internet resource depends on a number of factors, such as the connection being used, the server on which the resource is located, and the number of users trying to access the resource. For applications that download multiple resources or handle multiple tasks (including one or more downloads), waiting for each download to complete before moving on to the next task can be extremely inefficient. To avoid waiting, many of the Microsoft® Win32® Internet functions provide a way to perform tasks asynchronously.

In asynchronous mode, an application can execute any Win32 Internet function that includes a context value as one of its parameters and can continue to execute other commands or functions while it waits for the Win32 Internet function to complete its task. While the task is completing, a status callback function provided by the application is notified about the progress of the task and when it has completed. At this time, the status callback function can call other functions or perform any other needed tasks that were dependent on the task's completion.

Benefits

So what type of benefits can you get by using the Win32 Internet functions asynchronously? You can, for example:

Start the download of multiple Internet resources simultaneously.
Your application can start connecting to multiple Internet resources at the same time and download them as they become available.
Increase performance of your application.
An application using the Win32 Internet functions asynchronously does not have to wait until the request is completed, so the application is free to do other tasks that are not dependent on the request, thus improving the application's overall performance.
Monitor the progress of the download.
The status callback function receives notifications while it is processing a request. If needed, your application can take the information provided by that status callback function to keep the user informed about the progress of the operation or to interrupt requests that are taking too long to complete.

Scenarios

Let's say your application needs to download coffee prices from the Downfall Coffee & Tea and the Fourth Coffee sites and compare prices. The Fourth Coffee site usually has a slower response time, so your application should download the information from Downfall Coffee & Tea first.

Two versions of the application are developed. One works synchronously, downloading the prices from the Downfall Coffee & Tea site first and then the prices from the Fourth Coffee site afterward. The second works asynchronously, sending requests to both sites and downloading the prices when they become available.

The following table illustrates what would happen if the Fourth Coffee site was faster on a particular day.

Event	Synchronous version	Asynchronous version
Start	Send request to Downfall Coffee & Tea	Send requests to Downfall Coffee & Tea and Fourth Coffee
Request from the asynchronous version to Fourth Coffee completed	Waiting	Download prices from Fourth Coffee
Request to Downfall Coffee & Tea completed	Download prices from Downfall Coffee & Tea	Download prices from Downfall Coffee & Tea
After Downfall Coffee & Tea's prices are downloaded	Send request to Fourth Coffee	Compare prices
Asynchronous version's comparison completed	Waiting	Operation complete
Request from the synchronous version to Fourth Coffee completed	Download prices from Fourth Coffee	n/a
After Fourth Coffee's prices are downloaded	Compare prices	n/a
Synchronous version's comparison completed	Operation complete	n/a

Another example would be a Web browser such as Microsoft® Internet Explorer. When the browser downloads a page, it often needs to download other resources, such as images and sound files. In asynchronous mode, the page and its associated resources can be requested simultaneously and downloaded as they become available, instead of requesting and downloading the page and each resource one at a time.

Utilizing the Win32 Internet Functions Asynchronously

Benefits

Scenarios

Related Topics