Adobe AIR HTTP Connection Limit

Question

I'm working on an Adobe AIR application which can upload files to a web server, which is running Apache and PHP. Several files can be uploaded at the same time and the application also calls the web server for various API requests.

The problem I'm having is that if I start two file uploads, while they are in progress any other HTTP requests will time out, which is causing a problem for the application and from a user point of view.

Are Adobe AIR applications limited to 2 HTTP connections, or is something else probably the issue? From searching about this issue I've not found much but one article did indicated that it wasn't limited to just two connections.

The file uploads are performed by calling the File classes upload method, and the API calls are done using the HTTPService class. The development web server I am using is a WAMP server, however when the application is released it will be talking to a LAMP server.

Thanks, Grant

Here is the code I'm using to upload the file:

protected function btnAddFile_clickHandler(event:MouseEvent):void
{
    // Create a new File object and display the browse file dialog
    var uploadFile:File = new File();
    uploadFile.browseForOpen("Select File to Upload");
    uploadFile.addEventListener(Event.SELECT, uploadFile_SelectedHandler);
}

private function uploadFile_SelectedHandler(event:Event):void
{
    // Get the File object which was used to select the file
    var uploadFile:File = event.target as File;
    uploadFile.addEventListener(ProgressEvent.PROGRESS, file_progressHandler);
    uploadFile.addEventListener(IOErrorEvent.IO_ERROR, file_ioErrorHandler);
    uploadFile.addEventListener(Event.COMPLETE, file_completeHandler);

    // Create the request URL based on the download URL
    var requestURL:URLRequest = new URLRequest(AppEnvironment.instance.serverHostname + "upload.php");
    requestURL.method = URLRequestMethod.POST;

    // Set the post parameters
    var params:URLVariables = new URLVariables(); 
    params.name = "filename.ext";
    requestURL.data = params;

    // Start uploading the file to the server
    uploadFile.upload(requestURL, "file");
}

Here is the code for the API calls:

private function sendHTTPPost(apiFile:String, postParams:Object, resultCallback:Function, initialCallerResultCallback:Function):void
{
    var httpService:mx.rpc.http.HTTPService = new mx.rpc.http.HTTPService();
    httpService.url = AppEnvironment.instance.serverHostname + apiFile;
    httpService.method = "POST";
    httpService.requestTimeout = 10;
    httpService.resultFormat = HTTPService.RESULT_FORMAT_TEXT;
    httpService.addEventListener("result", resultCallback);
    httpService.addEventListener("fault", httpFault);
    var token:AsyncToken = httpService.send(postParams);

    // Add the initial caller's result callback function to the token
    token.initialCallerResultCallback = initialCallerResultCallback;
}

I know of no functional limit to the number of HTTP requests that an AIR app can send. Can you post the code you are using to send the requests? Any chance you are modifying the "concurrency" property of the HTTPService object? — Jason Dean
@JasonDean - I've updated the post and added some code. I'm not modifying the concurrency property anywhere. I have tried increasing the requestTimeout but that only means it takes longer to fail with a timeout given the uploads can take quite a while if the file is very large. Thanks. — Grant

Jörn Horstmann Jörn Horstmann · Accepted Answer · 2011-10-06T15:47:50

If you are on a windows system, Adobe AIR is using Microsofts WinINet library to access the web. This library by default limits the number of concurrent connections to a single server to 2:

WinInet limits the number of simultaneous connections that it makes to a single HTTP server. If you exceed this limit, the requests block until one of the current connections has completed. This is by design and is in agreement with the HTTP specification and industry standards.

... Connections to a single HTTP 1.1 server are limited to two simultaneous connections

There is an API to change the value of this limit but I don't know if it is accessible from AIR.

Since this limit also affects page loading speed for web sites, some sites are using multiple DNS names for artifacts such as images, javascripts and stylesheets to allow a browser to open more parallel connections.

So if you are controlling the server part, a workaround could be to create DNS aliases like www.example.com for uploads and api.example.com for API requests.

Adobe AIR HTTP Connection Limit

3 Answers