Session state lost after HttpWebRequest within AJAX post - asp.net

Question

I have a bit of strange behaviour in an asp.net web application that involves the session state being lost.
Process
A user logs into the application and the session is set. They then fill out 1 field, and the application then does an AJAX POST to a .asmx web service. Within the web service, I am using a HttpWebRequest to grab data from another server.
This data is then output to the browser.
A few more fields are then filled in, and the data is then again Post to the same web service via an AJAX POST.
Problem
Straight after the HttpWebRequest, I grab the username from a session variable. This works.
On the next AJAX request however, the session no longer exists.
While testing this, I removed the stage at which the HttpWebRequest is called and my session is never lost. So for some reason, the session is removed AFTER my first AJAX POST and before the second AJAX POST only if I am running the HttpWebRequest code.
Code
I am not doing anything fancy in the code. Just doing a simple jQuery AJAX Post
$.ajax({
url: method,
data: params,
type: "POST",
contentType: "application/json; charset=utf-8", dataType: "json",
success: function (data) {
// handle data
},
error: function(xhr,status,error) { }
});
Creating a System.Net.HttpWebRequest and then getting the System.Net.HttpWebResponse out of that.
Then reading a session variable dim username as string = Session(_SESSION_USERNAME).ToString()
I have never noticed this behaviour before when using HttpWebRequest before (not using any AJAX though)
Function Backfill(value As String) As Details
Dim details As Details = Nothing
Dim appSettings As ConfigSettings.AppConfig = ConfigSettings.AppConfig.getConfig()
Dim url As String = appSettings.Settings.BackfillUrl
Dim username As String = appSettings.Settings.BackfillUser
Dim password As String = appSettings.Settings.BackfillPass
Dim expParameters As String = ""
Dim xml As XmlDocument = Nothing
Dim xmlHttp As XMLHTTP = Nothing
Dim nodeList As XmlNodeList = Nothing
Dim node As XmlNode = Nothing
Dim response As String = ""
Dim success As String = ""
'
' REMOVED TO HIDE INFO
expParameters = "<PARAMETERS>" & _
"</PARAMETERS>"
Try
xmlHttp = New XMLHTTP()
xmlHttp.open("POST", url)
xmlHttp.Send(expParameters)
response = xmlHttp.responseText()
xml = New XmlDocument
xml.LoadXml(response)
SaveExperianFile(xml, value)
nodeList = xml.DocumentElement.ChildNodes
node = nodeList.Item(0)
success = node.Attributes.GetNamedItem("success").Value.ToString.Trim
If success.ToLower.Trim = "y" Then
details = SetDetails(xml)
End If
Catch ex As Exception
Finally
If Not xmlHttp Is Nothing Then
xmlHttp.Dispose()
xmlHttp = Nothing
End If
End Try
Return details
End Function
edit
The XMLHTTP Class code can be seen here http://codepaste.net/ymnqsf
edit
Seems as though something strange is happening when I am saving the XMLDocument to my file system.
Private Sub SaveExperianFile(xml As XmlDocument, value As String)
Dim appConfig As ConfigSettings.AppConfig = ConfigSettings.AppConfig.getConfig()
Try
xml.Save(HttpContext.Current.Server.MapPath(appConfig.Settings.SavePath & value & "_backfill.xml"))
Catch ex As Exception
End Try
End Sub
If I don't call this method, then the session is always set.
Question
Do you know what is causing this behaviour?

Answer 1

Can you check if you just loose the session or the whole application is restarted. If you are saving the XML in the virtual directory / web application folder then it might case web application to restart. If many dozen files were added in short succession to each other, that would be a case to restart the App Pool.

Answer 2

just a hunch, but maybe you need to maintain cookies across HttpWebRequests.
have a look at this question for more help.
Http web request doesn't maintaining session