Static variables and long running thread on IIS 7.5 - asp.net

Question

Help me solve next problem.
I have ASP .NET MVC2 application. I run it on IIS 7.5. In one page user clicks button and handler for this button sends request to server (jquery.ajax). At server action in controller starts new thread (it makes long time import):
var thread = new Thread(RefreshCitiesInDatabase);
thread.Start();
State of import is available in static variable. New thread changes value of variable in the begin of work.
User can check state of import too with the help of this variable, which is used in view. And user sees import's state.
When I start this function few minutes everything is okey. On page I see right state of import, quantity of imported records is changed, I see changes in logs. But after few minutes begin troubles.
When I refresh page with import state sometimes I see that import is okey but sometimes I see page with default values about import (like application is just started), but after that again I can see page with normal import's state.
I tried to attach Visual Studio to IIS process and debug application. But when request comes to controller sometimes static variables have right values and sometimes they have default values (static int has 0, static string has "" etc.).
Tell me what I do wrong. May be I must start additional thread in other way?
Thanks in advance,
Dmitry
I add parts of code:
Controller:
public class ImportCitiesController : Controller
{
[Dependency]
public SaveCities SaveCities { get; set; }
//Start import
public JsonResult StartCitiesImport()
{
//Methos in core dll, which makes import
SaveCities.StartCitiesSaving();
return Json("ok");
}
//Get Information about import
public ActionResult GetImportState()
{
var model = new ImportCityStatusModel
{ NowImportProcessing = SaveCities.CitiesSaving };
return View(model);
}
}
Class in Core:
public class SaveCities
{
// Property equals true, when program are saving to database
public static bool CitiesSaving = false;
public void StartCitiesSaving()
{
var thread = new Thread(RefreshCitiesInDatabase);
thread.Start();
}
private static void RefreshCitiesInDatabase()
{
CitiesSaving = true;
//Processing......
CitiesSaving = false;
}
}
UPDATE
I think, I found problem, but still I don't know how solve it. My IIS uses application pool with parameter "Maximum Worker Processes" = 10. And all tasks in application are handled by few processes. And my request to controll about import's state always is handled by different processes. And they have different static variables. I guess it is right way for solving.
But I don't know how merge all static values in one place.

Answer 1

Without looking at the code, here are the obvious question. Are you sure your access is thread safe (that is do you properly use lock to update you value or even access it => C# thread safety with get/set) ?
A code sample could be nice.
thanks for the code, it seem that CitiesSaving is not locked properly before read/write you should hide the instance variable behind a property to handle all the locking. Marking this field as volatile could also help (see http://msdn.microsoft.com/en-us/library/aa645755(v=vs.71).aspx )